goja

package module
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 Go to latest Published: Mar 2, 2023 License: MIT Imports: 33 Imported by: 6

README

goja

ECMAScript 5.1(+) implementation in Go.

Go Reference

Goja is an implementation of ECMAScript 5.1 in pure Go with emphasis on standard compliance and performance.

This project was largely inspired by otto.

Minimum required Go version is 1.16.

Features

  • Full ECMAScript 5.1 support (including regex and strict mode).
  • Passes nearly all tc39 tests for the features implemented so far. The goal is to pass all of them. See .tc39_test262_checkout.sh for the latest working commit id.
  • Capable of running Babel, Typescript compiler and pretty much anything written in ES5.
  • Sourcemaps.
  • Most of ES6 functionality, still work in progress, see https://github.com/dop251/goja/milestone/1?closed=1

Known incompatibilities and caveats

WeakMap

WeakMap is implemented by embedding references to the values into the keys. This means that as long as the key is reachable all values associated with it in any weak maps also remain reachable and therefore cannot be garbage collected even if they are not otherwise referenced, even after the WeakMap is gone. The reference to the value is dropped either when the key is explicitly removed from the WeakMap or when the key becomes unreachable.

To illustrate this:

var m = new WeakMap();
var key = {};
var value = {/* a very large object */};
m.set(key, value);
value = undefined;
m = undefined; // The value does NOT become garbage-collectable at this point
key = undefined; // Now it does
// m.delete(key); // This would work too

The reason for it is the limitation of the Go runtime. At the time of writing (version 1.15) having a finalizer set on an object which is part of a reference cycle makes the whole cycle non-garbage-collectable. The solution above is the only reasonable way I can think of without involving finalizers. This is the third attempt (see https://github.com/dop251/goja/issues/250 and https://github.com/dop251/goja/issues/199 for more details).

Note, this does not have any effect on the application logic, but may cause a higher-than-expected memory usage.

WeakRef and FinalizationRegistry

For the reason mentioned above implementing WeakRef and FinalizationRegistry does not seem to be possible at this stage.

JSON

JSON.parse() uses the standard Go library which operates in UTF-8. Therefore, it cannot correctly parse broken UTF-16 surrogate pairs, for example:

JSON.parse(`"\\uD800"`).charCodeAt(0).toString(16) // returns "fffd" instead of "d800"
Date

Conversion from calendar date to epoch timestamp uses the standard Go library which uses int, rather than float as per ECMAScript specification. This means if you pass arguments that overflow int to the Date() constructor or if there is an integer overflow, the result will be incorrect, for example:

Date.UTC(1970, 0, 1, 80063993375, 29, 1, -288230376151711740) // returns 29256 instead of 29312

FAQ

How fast is it?

Although it's faster than many scripting language implementations in Go I have seen (for example it's 6-7 times faster than otto on average) it is not a replacement for V8 or SpiderMonkey or any other general-purpose JavaScript engine. You can find some benchmarks here.

Why would I want to use it over a V8 wrapper?

It greatly depends on your usage scenario. If most of the work is done in javascript (for example crypto or any other heavy calculations) you are definitely better off with V8.

If you need a scripting language that drives an engine written in Go so that you need to make frequent calls between Go and javascript passing complex data structures then the cgo overhead may outweigh the benefits of having a faster javascript engine.

Because it's written in pure Go there are no cgo dependencies, it's very easy to build and it should run on any platform supported by Go.

It gives you a much better control over execution environment so can be useful for research.

Is it goroutine-safe?

No. An instance of goja.Runtime can only be used by a single goroutine at a time. You can create as many instances of Runtime as you like but it's not possible to pass object values between runtimes.

Where is setTimeout()?

setTimeout() assumes concurrent execution of code which requires an execution environment, for example an event loop similar to nodejs or a browser. There is a separate project aimed at providing some NodeJS functionality, and it includes an event loop.

Can you implement (feature X from ES6 or higher)?

I will be adding features in their dependency order and as quickly as time permits. Please do not ask for ETAs. Features that are open in the milestone are either in progress or will be worked on next.

The ongoing work is done in separate feature branches which are merged into master when appropriate. Every commit in these branches represents a relatively stable state (i.e. it compiles and passes all enabled tc39 tests), however because the version of tc39 tests I use is quite old, it may be not as well tested as the ES5.1 functionality. Because there are (usually) no major breaking changes between ECMAScript revisions it should not break your existing code. You are encouraged to give it a try and report any bugs found. Please do not submit fixes though without discussing it first, as the code could be changed in the meantime.

How do I contribute?

Before submitting a pull request please make sure that:

  • You followed ECMA standard as close as possible. If adding a new feature make sure you've read the specification, do not just base it on a couple of examples that work fine.
  • Your change does not have a significant negative impact on performance (unless it's a bugfix and it's unavoidable)
  • It passes all relevant tc39 tests.

Current Status

  • There should be no breaking changes in the API, however it may be extended.
  • Some of the AnnexB functionality is missing.

Basic Example

Run JavaScript and get the result value.

vm := goja.New()
v, err := vm.RunString("2 + 2")
if err != nil {
    panic(err)
}
if num := v.Export().(int64); num != 4 {
    panic(num)
}

Passing Values to JS

Any Go value can be passed to JS using Runtime.ToValue() method. See the method's documentation for more details.

Exporting Values from JS

A JS value can be exported into its default Go representation using Value.Export() method.

Alternatively it can be exported into a specific Go variable using Runtime.ExportTo() method.

Within a single export operation the same Object will be represented by the same Go value (either the same map, slice or a pointer to the same struct). This includes circular objects and makes it possible to export them.

Calling JS functions from Go

There are 2 approaches:

vm := New()
_, err := vm.RunString(`
function sum(a, b) {
    return a+b;
}
`)
if err != nil {
    panic(err)
}
sum, ok := AssertFunction(vm.Get("sum"))
if !ok {
    panic("Not a function")
}

res, err := sum(Undefined(), vm.ToValue(40), vm.ToValue(2))
if err != nil {
    panic(err)
}
fmt.Println(res)
// Output: 42

const SCRIPT = `
function f(param) {
    return +param + 2;
}
`

vm := New()
_, err := vm.RunString(SCRIPT)
if err != nil {
    panic(err)
}

var fn func(string) string
err = vm.ExportTo(vm.Get("f"), &fn)
if err != nil {
    panic(err)
}

fmt.Println(fn("40")) // note, _this_ value in the function will be undefined.
// Output: 42

The first one is more low level and allows specifying this value, whereas the second one makes the function look like a normal Go function.

Mapping struct field and method names

By default, the names are passed through as is which means they are capitalised. This does not match the standard JavaScript naming convention, so if you need to make your JS code look more natural or if you are dealing with a 3rd party library, you can use a FieldNameMapper:

vm := New()
vm.SetFieldNameMapper(TagFieldNameMapper("json", true))
type S struct {
    Field int `json:"field"`
}
vm.Set("s", S{Field: 42})
res, _ := vm.RunString(`s.field`) // without the mapper it would have been s.Field
fmt.Println(res.Export())
// Output: 42

There are two standard mappers: TagFieldNameMapper and UncapFieldNameMapper, or you can use your own implementation.

Native Constructors

In order to implement a constructor function in Go use func (goja.ConstructorCall) *goja.Object. See Runtime.ToValue() documentation for more details.

Regular Expressions

Goja uses the embedded Go regexp library where possible, otherwise it falls back to regexp2.

Exceptions

Any exception thrown in JavaScript is returned as an error of type *Exception. It is possible to extract the value thrown by using the Value() method:

vm := New()
_, err := vm.RunString(`

throw("Test");

`)

if jserr, ok := err.(*Exception); ok {
    if jserr.Value().Export() != "Test" {
        panic("wrong value")
    }
} else {
    panic("wrong type")
}

If a native Go function panics with a Value, it is thrown as a Javascript exception (and therefore can be caught):

var vm *Runtime

func Test() {
    panic(vm.ToValue("Error"))
}

vm = New()
vm.Set("Test", Test)
_, err := vm.RunString(`

try {
    Test();
} catch(e) {
    if (e !== "Error") {
        throw e;
    }
}

`)

if err != nil {
    panic(err)
}

Interrupting

func TestInterrupt(t *testing.T) {
    const SCRIPT = `
    var i = 0;
    for (;;) {
        i++;
    }
    `

    vm := New()
    time.AfterFunc(200 * time.Millisecond, func() {
        vm.Interrupt("halt")
    })

    _, err := vm.RunString(SCRIPT)
    if err == nil {
        t.Fatal("Err is nil")
    }
    // err is of type *InterruptError and its Value() method returns whatever has been passed to vm.Interrupt()
}

NodeJS Compatibility

There is a separate project aimed at providing some of the NodeJS functionality.

Documentation

Index

Examples

Constants

This section is empty.

Variables

View Source 
var (
	SymHasInstance        = newSymbol(asciiString("Symbol.hasInstance"))
	SymIsConcatSpreadable = newSymbol(asciiString("Symbol.isConcatSpreadable"))
	SymIterator           = newSymbol(asciiString("Symbol.iterator"))
	SymMatch              = newSymbol(asciiString("Symbol.match"))
	SymMatchAll           = newSymbol(asciiString("Symbol.matchAll"))
	SymReplace            = newSymbol(asciiString("Symbol.replace"))
	SymSearch             = newSymbol(asciiString("Symbol.search"))
	SymSpecies            = newSymbol(asciiString("Symbol.species"))
	SymSplit              = newSymbol(asciiString("Symbol.split"))
	SymToPrimitive        = newSymbol(asciiString("Symbol.toPrimitive"))
	SymToStringTag        = newSymbol(asciiString("Symbol.toStringTag"))
	SymUnscopables        = newSymbol(asciiString("Symbol.unscopables"))
)
View Source 
var (
	InvalidRuneError = errors.New("invalid rune")
)

Functions

func IsInfinity 

func IsInfinity(v Value) bool

IsInfinity returns true if the supplied is (+/-)Infinity

func IsNaN 

func IsNaN(v Value) bool

IsNaN returns true if the supplied value is NaN.

func IsNull 

func IsNull(v Value) bool

IsNull returns true if the supplied Value is null.

func IsUndefined 

func IsUndefined(v Value) bool

IsUndefined returns true if the supplied Value is undefined. Note, it checks against the real undefined, not against the global object's 'undefined' property.

func Parse 

func Parse(name, src string, options ...parser.Option) (prg *js_ast.Program, err error)

Parse takes a source string and produces a parsed AST. Use this function if you want to pass options to the parser, e.g.: 

p, err := Parse("test.js", "var a = true", parser.WithDisableSourceMaps)
if err != nil { /* ... */ }
prg, err := CompileAST(p, true)
// ...

Otherwise use Compile which combines both steps.

Types

type ArrayBuffer 

type ArrayBuffer struct {
	// contains filtered or unexported fields
}

ArrayBuffer is a Go wrapper around ECMAScript ArrayBuffer. Calling Runtime.ToValue() on it returns the underlying ArrayBuffer. Calling Export() on an ECMAScript ArrayBuffer returns a wrapper. Use Runtime.NewArrayBuffer([]byte) to create one.

func (ArrayBuffer) Bytes 

func (a ArrayBuffer) Bytes() []byte

Bytes returns the underlying []byte for this ArrayBuffer. For detached ArrayBuffers returns nil.

func (ArrayBuffer) Detach 

func (a ArrayBuffer) Detach() bool

Detach the ArrayBuffer. After this, the underlying []byte becomes unreferenced and any attempt to use this ArrayBuffer results in a TypeError. Returns false if it was already detached, true otherwise. Note, this method may only be called from the goroutine that 'owns' the Runtime, it may not be called concurrently.

func (ArrayBuffer) Detached 

func (a ArrayBuffer) Detached() bool

Detached returns true if the ArrayBuffer is detached.

type AsyncContextTracker 

type AsyncContextTracker interface {
	Grab() (trackingObject interface{})
	Resumed(trackingObject interface{})
	Exited()
}

AsyncContextTracker is a handler that allows to track an async execution context to ensure it remains consistent across all callback invocations. Whenever a Promise reaction job is scheduled the Grab method is called. It is supposed to return the current context. The same context will be supplied to the Resumed method before the reaction job is executed. The Exited method is called after the reaction job is finished. This means that for each invocation of the Grab method there will be exactly one subsequent invocation of Resumed and then Exited methods (assuming the Promise is fulfilled or rejected). Also, the Resumed/Exited calls cannot be nested, so Exited can simply clear the current context instead of popping from a stack. Note, this works for both async functions and regular Promise.then()/Promise.catch() callbacks. See TestAsyncContextTracker for more insight.

To register it call Runtime.SetAsyncContextTracker().

type Callable 

type Callable func(this Value, args ...Value) (Value, error)

Callable represents a JavaScript function that can be called from Go.

func AssertFunction 

func AssertFunction(v Value) (Callable, bool)

AssertFunction checks if the Value is a function and returns a Callable. Note, for classes this returns a callable and a 'true', however calling it will always result in a TypeError. For classes use AssertConstructor().

Example 
vm := New()
_, err := vm.RunString(`
	function sum(a, b) {
		return a+b;
	}
	`)
if err != nil {
	panic(err)
}
sum, ok := AssertFunction(vm.Get("sum"))
if !ok {
	panic("Not a function")
}

res, err := sum(Undefined(), vm.ToValue(40), vm.ToValue(2))
if err != nil {
	panic(err)
}
fmt.Println(res)

Output:

42

type CompilerError 

type CompilerError struct {
	Message string
	File    *file.File
	Offset  int
}

type CompilerReferenceError 

type CompilerReferenceError struct {
	CompilerError
}

func (*CompilerReferenceError) Error 

func (e *CompilerReferenceError) Error() string

type CompilerSyntaxError 

type CompilerSyntaxError struct {
	CompilerError
}

func (*CompilerSyntaxError) Error 

func (e *CompilerSyntaxError) Error() string

type Constructor 

type Constructor func(newTarget *Object, args ...Value) (*Object, error)

Constructor is a type that can be used to call constructors. The first argument (newTarget) can be nil which sets it to the constructor function itself.

func AssertConstructor 

func AssertConstructor(v Value) (Constructor, bool)

AssertConstructor checks if the Value is a constructor and returns a Constructor.

Example 
vm := New()
res, err := vm.RunString(`
		(class C {
			constructor(x) {
				this.x = x;
			}
		})
	`)
if err != nil {
	panic(err)
}
if ctor, ok := AssertConstructor(res); ok {
	obj, err := ctor(nil, vm.ToValue("Test"))
	if err != nil {
		panic(err)
	}
	fmt.Print(obj.Get("x"))
} else {
	panic("Not a constructor")
}

Output:

Test

type ConstructorCall 

type ConstructorCall struct {
	This      *Object
	Arguments []Value
	NewTarget *Object
}

func (ConstructorCall) Argument 

func (f ConstructorCall) Argument(idx int) Value

type DynamicArray 

type DynamicArray interface {
	// Len returns the current array length.
	Len() int
	// Get an item at index idx. Note that idx may be any integer, negative or beyond the current length.
	Get(idx int) Value
	// Set an item at index idx. Note that idx may be any integer, negative or beyond the current length.
	// The expected behaviour when it's beyond length is that the array's length is increased to accommodate
	// the item. All elements in the 'new' section of the array should be zeroed.
	Set(idx int, val Value) bool
	// SetLen is called when the array's 'length' property is changed. If the length is increased all elements in the
	// 'new' section of the array should be zeroed.
	SetLen(int) bool
}

DynamicArray is an interface representing a handler for a dynamic array Object. Such an object can be created using the Runtime.NewDynamicArray() method.

Any integer property key or a string property key that can be parsed into an int value (including negative ones) is treated as an index and passed to the trap methods of the DynamicArray. Note this is different from the regular ECMAScript arrays which only support positive indexes up to 2^32-1.

DynamicArray cannot be sparse, i.e. hasOwnProperty(num) will return true for num >= 0 && num < Len(). Deleting such a property is equivalent to setting it to undefined. Note that this creates a slight peculiarity because hasOwnProperty() will still return true, even after deletion.

Note that Runtime.ToValue() does not have any special treatment for DynamicArray. The only way to create a dynamic array is by using the Runtime.NewDynamicArray() method. This is done deliberately to avoid silent code breaks when this interface changes.

type DynamicObject 

type DynamicObject interface {
	// Get a property value for the key. May return nil if the property does not exist.
	Get(key string) Value
	// Set a property value for the key. Return true if success, false otherwise.
	Set(key string, val Value) bool
	// Has should return true if and only if the property exists.
	Has(key string) bool
	// Delete the property for the key. Returns true on success (note, that includes missing property).
	Delete(key string) bool
	// Keys returns a list of all existing property keys. There are no checks for duplicates or to make sure
	// that the order conforms to https://262.ecma-international.org/#sec-ordinaryownpropertykeys
	Keys() []string
}

DynamicObject is an interface representing a handler for a dynamic Object. Such an object can be created using the Runtime.NewDynamicObject() method.

Note that Runtime.ToValue() does not have any special treatment for DynamicObject. The only way to create a dynamic object is by using the Runtime.NewDynamicObject() method. This is done deliberately to avoid silent code breaks when this interface changes.

type Exception 

type Exception struct {
	// contains filtered or unexported fields
}

func (*Exception) Error 

func (e *Exception) Error() string

func (*Exception) String 

func (e *Exception) String() string

func (*Exception) Value 

func (e *Exception) Value() Value

type FieldNameMapper 

type FieldNameMapper interface {
	// FieldName returns a JavaScript name for the given struct field in the given type.
	// If this method returns "" the field becomes hidden.
	FieldName(t reflect.Type, f reflect.StructField) string

	// MethodName returns a JavaScript name for the given method in the given type.
	// If this method returns "" the method becomes hidden.
	MethodName(t reflect.Type, m reflect.Method) string
}

FieldNameMapper provides custom mapping between Go and JavaScript property names.

func TagFieldNameMapper 

func TagFieldNameMapper(tagName string, uncapMethods bool) FieldNameMapper

TagFieldNameMapper returns a FieldNameMapper that uses the given tagName for struct fields and optionally uncapitalises (making the first letter lower case) method names. The common tag value syntax is supported (name[,options]), however options are ignored. Setting name to anything other than a valid ECMAScript identifier makes the field hidden.

Example 
vm := New()
vm.SetFieldNameMapper(TagFieldNameMapper("json", true))
type S struct {
	Field int `json:"field"`
}
vm.Set("s", S{Field: 42})
res, _ := vm.RunString(`s.field`)
fmt.Println(res.Export())

Output:

42

func UncapFieldNameMapper 

func UncapFieldNameMapper() FieldNameMapper

UncapFieldNameMapper returns a FieldNameMapper that uncapitalises struct field and method names making the first letter lower case.

Example 
vm := New()
s := testGoReflectMethod_O{
	Test: "passed",
}
vm.SetFieldNameMapper(UncapFieldNameMapper())
vm.Set("s", s)
res, _ := vm.RunString(`s.test + " and " + s.method("passed too")`)
fmt.Println(res.Export())

Output:

passed and passed too

type Flag 

type Flag int
const (
	FLAG_NOT_SET Flag = iota
	FLAG_FALSE
	FLAG_TRUE
)

func ToFlag 

func ToFlag(b bool) Flag

func (Flag) Bool 

func (f Flag) Bool() bool

type FunctionCall 

type FunctionCall struct {
	This      Value
	Arguments []Value
}

func (FunctionCall) Argument 

func (f FunctionCall) Argument(idx int) Value

type InterruptedError 

type InterruptedError struct {
	// contains filtered or unexported fields
}

func (*InterruptedError) Error 

func (e *InterruptedError) Error() string

func (*InterruptedError) String 

func (e *InterruptedError) String() string

func (*InterruptedError) Unwrap 

func (e *InterruptedError) Unwrap() error

func (*InterruptedError) Value 

func (e *InterruptedError) Value() interface{}

type JsonEncodable 

type JsonEncodable interface {
	JsonEncodable() interface{}
}

JsonEncodable allows custom JSON encoding by JSON.stringify() Note that if the returned value itself also implements JsonEncodable, it won't have any effect.

type Now 

type Now func() time.Time

type Object 

type Object struct {
	// contains filtered or unexported fields
}

func NewSharedDynamicArray 

func NewSharedDynamicArray(a DynamicArray) *Object

NewSharedDynamicArray is similar to Runtime.NewDynamicArray but the resulting Object can be shared across multiple Runtimes. The Object's prototype will be null. If you need to run Array's methods on it, use Array.prototype.[...].call(a, ...). The provided DynamicArray must be goroutine-safe.

func NewSharedDynamicObject 

func NewSharedDynamicObject(d DynamicObject) *Object

NewSharedDynamicObject is similar to Runtime.NewDynamicObject but the resulting Object can be shared across multiple Runtimes. The Object's prototype will be null. The provided DynamicObject must be goroutine-safe.

func (*Object) ClassName 

func (o *Object) ClassName() string

ClassName returns the class name

func (*Object) DefineAccessorProperty 

func (o *Object) DefineAccessorProperty(name string, getter, setter Value, configurable, enumerable Flag) error

DefineAccessorProperty is a Go equivalent of Object.defineProperty(o, name, {get: getter, set: setter, configurable: configurable, enumerable: enumerable})

func (*Object) DefineAccessorPropertySymbol 

func (o *Object) DefineAccessorPropertySymbol(name *Symbol, getter, setter Value, configurable, enumerable Flag) error

DefineAccessorPropertySymbol is a Go equivalent of Object.defineProperty(o, name, {get: getter, set: setter, configurable: configurable, enumerable: enumerable})

func (*Object) DefineDataProperty 

func (o *Object) DefineDataProperty(name string, value Value, writable, configurable, enumerable Flag) error

DefineDataProperty is a Go equivalent of Object.defineProperty(o, name, {value: value, writable: writable, configurable: configurable, enumerable: enumerable})

func (*Object) DefineDataPropertySymbol 

func (o *Object) DefineDataPropertySymbol(name *Symbol, value Value, writable, configurable, enumerable Flag) error

DefineDataPropertySymbol is a Go equivalent of Object.defineProperty(o, name, {value: value, writable: writable, configurable: configurable, enumerable: enumerable})

func (*Object) Delete 

func (o *Object) Delete(name string) error
Example 
vm := New()
obj := vm.NewObject()
_ = obj.Set("test", true)
before := obj.Get("test")
_ = obj.Delete("test")
after := obj.Get("test")
fmt.Printf("before: %v, after: %v", before, after)

Output:

before: true, after: <nil>

func (*Object) DeleteSymbol 

func (o *Object) DeleteSymbol(name *Symbol) error

func (*Object) Equals 

func (o *Object) Equals(other Value) bool

func (*Object) Export 

func (o *Object) Export() (ret interface{})

Export the Object to a plain Go type. If the Object is a wrapped Go value (created using ToValue()) returns the original value.

If the Object is a function, returns func(FunctionCall) Value. Note that exceptions thrown inside the function result in panics, which can also leave the Runtime in an unusable state. Therefore, these values should only be used inside another ES function implemented in Go. For calling a function from Go, use AssertFunction() or Runtime.ExportTo() as described in the README.

For a Map, returns the list of entries as [][2]interface{}.

For a Set, returns the list of elements as []interface{}.

For a Proxy, returns Proxy.

For a Promise, returns Promise.

For a DynamicObject or a DynamicArray, returns the underlying handler.

For an array, returns its items as []interface{}.

In all other cases returns own enumerable non-symbol properties as map[string]interface{}.

This method will panic with an *Exception if a JavaScript exception is thrown in the process.

Example (Map) 
vm := New()
m, err := vm.RunString(`
	new Map([[1, true], [2, false]]);
	`)
if err != nil {
	panic(err)
}
exp := m.Export()
fmt.Printf("%T, %v\n", exp, exp)

Output:

[][2]interface {}, [[1 true] [2 false]]

func (*Object) ExportType 

func (o *Object) ExportType() reflect.Type

ExportType returns the type of the value that is returned by Export().

func (*Object) Get 

func (o *Object) Get(name string) Value

Get an object's property by name. This method will panic with an *Exception if a JavaScript exception is thrown in the process.

func (*Object) GetSymbol 

func (o *Object) GetSymbol(sym *Symbol) Value

GetSymbol returns the value of a symbol property. Use one of the Sym* values for well-known symbols (such as SymIterator, SymToStringTag, etc...). This method will panic with an *Exception if a JavaScript exception is thrown in the process.

func (*Object) Keys 

func (o *Object) Keys() (keys []string)

Keys returns a list of Object's enumerable keys. This method will panic with an *Exception if a JavaScript exception is thrown in the process.

func (*Object) MarshalJSON 

func (o *Object) MarshalJSON() ([]byte, error)

MarshalJSON returns JSON representation of the Object. It is equivalent to JSON.stringify(o). Note, this implements json.Marshaler so that json.Marshal() can be used without the need to Export().

func (*Object) Prototype 

func (o *Object) Prototype() *Object

Prototype returns the Object's prototype, same as Object.getPrototypeOf(). If the prototype is null returns nil.

func (*Object) SameAs 

func (o *Object) SameAs(other Value) bool

func (*Object) Set 

func (o *Object) Set(name string, value interface{}) error

func (*Object) SetPrototype 

func (o *Object) SetPrototype(proto *Object) error

SetPrototype sets the Object's prototype, same as Object.setPrototypeOf(). Setting proto to nil is an equivalent of Object.setPrototypeOf(null).

func (*Object) SetSymbol 

func (o *Object) SetSymbol(name *Symbol, value interface{}) error
Example 
type IterResult struct {
	Done  bool
	Value Value
}

vm := New()
vm.SetFieldNameMapper(UncapFieldNameMapper()) // to use IterResult

o := vm.NewObject()
o.SetSymbol(SymIterator, func() *Object {
	count := 0
	iter := vm.NewObject()
	iter.Set("next", func() IterResult {
		if count < 10 {
			count++
			return IterResult{
				Value: vm.ToValue(count),
			}
		}
		return IterResult{
			Done: true,
		}
	})
	return iter
})
vm.Set("o", o)

res, err := vm.RunString(`
	var acc = "";
	for (var v of o) {
		acc += v + " ";
	}
	acc;
	`)
if err != nil {
	panic(err)
}
fmt.Println(res)

Output:

1 2 3 4 5 6 7 8 9 10

func (*Object) StrictEquals 

func (o *Object) StrictEquals(other Value) bool

func (*Object) String 

func (o *Object) String() string

func (*Object) Symbols 

func (o *Object) Symbols() []*Symbol

Symbols returns a list of Object's enumerable symbol properties. This method will panic with an *Exception if a JavaScript exception is thrown in the process.

func (*Object) ToBoolean 

func (o *Object) ToBoolean() bool

func (*Object) ToFloat 

func (o *Object) ToFloat() float64

func (*Object) ToInteger 

func (o *Object) ToInteger() int64

func (*Object) ToNumber 

func (o *Object) ToNumber() Value

func (*Object) ToObject 

func (o *Object) ToObject(*Runtime) *Object

func (*Object) ToString 

func (o *Object) ToString() Value

func (*Object) UnmarshalJSON 

func (o *Object) UnmarshalJSON([]byte) error

UnmarshalJSON implements the json.Unmarshaler interface. It is added to compliment MarshalJSON, because some alternative JSON encoders refuse to use MarshalJSON unless UnmarshalJSON is also present. It is a no-op and always returns nil.

type Program 

type Program struct {
	// contains filtered or unexported fields
}

func Compile 

func Compile(name, src string, strict bool) (*Program, error)

Compile creates an internal representation of the JavaScript code that can be later run using the Runtime.RunProgram() method. This representation is not linked to a runtime in any way and can be run in multiple runtimes (possibly at the same time).

func CompileAST 

func CompileAST(prg *js_ast.Program, strict bool) (*Program, error)

CompileAST creates an internal representation of the JavaScript code that can be later run using the Runtime.RunProgram() method. This representation is not linked to a runtime in any way and can be run in multiple runtimes (possibly at the same time).

func MustCompile 

func MustCompile(name, src string, strict bool) *Program

MustCompile is like Compile but panics if the code cannot be compiled. It simplifies safe initialization of global variables holding compiled JavaScript code.

type Promise 

type Promise struct {
	// contains filtered or unexported fields
}

Promise is a Go wrapper around ECMAScript Promise. Calling Runtime.ToValue() on it returns the underlying Object. Calling Export() on a Promise Object returns a Promise.

Use Runtime.NewPromise() to create one. Calling Runtime.ToValue() on a zero object or nil returns null Value.

WARNING: Instances of Promise are not goroutine-safe. See Runtime.NewPromise() for more details.

func (*Promise) Result 

func (p *Promise) Result() Value

func (*Promise) State 

func (p *Promise) State() PromiseState

type PromiseRejectionOperation 

type PromiseRejectionOperation int
const (
	PromiseRejectionReject PromiseRejectionOperation = iota
	PromiseRejectionHandle
)

type PromiseRejectionTracker 

type PromiseRejectionTracker func(p *Promise, operation PromiseRejectionOperation)

type PromiseState 

type PromiseState int
const (
	PromiseStatePending PromiseState = iota
	PromiseStateFulfilled
	PromiseStateRejected
)

type PropertyDescriptor 

type PropertyDescriptor struct {
	Value Value

	Writable, Configurable, Enumerable Flag

	Getter, Setter Value
	// contains filtered or unexported fields
}

func (*PropertyDescriptor) Empty 

func (p *PropertyDescriptor) Empty() bool

func (*PropertyDescriptor) IsAccessor 

func (p *PropertyDescriptor) IsAccessor() bool

func (*PropertyDescriptor) IsData 

func (p *PropertyDescriptor) IsData() bool

func (*PropertyDescriptor) IsGeneric 

func (p *PropertyDescriptor) IsGeneric() bool

type Proxy 

type Proxy struct {
	// contains filtered or unexported fields
}

Proxy is a Go wrapper around ECMAScript Proxy. Calling Runtime.ToValue() on it returns the underlying Proxy. Calling Export() on an ECMAScript Proxy returns a wrapper. Use Runtime.NewProxy() to create one.

func (Proxy) Handler 

func (p Proxy) Handler() *Object

func (Proxy) Revoke 

func (p Proxy) Revoke()

func (Proxy) Target 

func (p Proxy) Target() *Object

type ProxyTrapConfig 

type ProxyTrapConfig struct {
	// A trap for Object.getPrototypeOf, Reflect.getPrototypeOf, __proto__, Object.prototype.isPrototypeOf, instanceof
	GetPrototypeOf func(target *Object) (prototype *Object)

	// A trap for Object.setPrototypeOf, Reflect.setPrototypeOf
	SetPrototypeOf func(target *Object, prototype *Object) (success bool)

	// A trap for Object.isExtensible, Reflect.isExtensible
	IsExtensible func(target *Object) (success bool)

	// A trap for Object.preventExtensions, Reflect.preventExtensions
	PreventExtensions func(target *Object) (success bool)

	// A trap for Object.getOwnPropertyDescriptor, Reflect.getOwnPropertyDescriptor (string properties)
	GetOwnPropertyDescriptor func(target *Object, prop string) (propertyDescriptor PropertyDescriptor)

	// A trap for Object.getOwnPropertyDescriptor, Reflect.getOwnPropertyDescriptor (integer properties)
	GetOwnPropertyDescriptorIdx func(target *Object, prop int) (propertyDescriptor PropertyDescriptor)

	// A trap for Object.getOwnPropertyDescriptor, Reflect.getOwnPropertyDescriptor (Symbol properties)
	GetOwnPropertyDescriptorSym func(target *Object, prop *Symbol) (propertyDescriptor PropertyDescriptor)

	// A trap for Object.defineProperty, Reflect.defineProperty (string properties)
	DefineProperty func(target *Object, key string, propertyDescriptor PropertyDescriptor) (success bool)

	// A trap for Object.defineProperty, Reflect.defineProperty (integer properties)
	DefinePropertyIdx func(target *Object, key int, propertyDescriptor PropertyDescriptor) (success bool)

	// A trap for Object.defineProperty, Reflect.defineProperty (Symbol properties)
	DefinePropertySym func(target *Object, key *Symbol, propertyDescriptor PropertyDescriptor) (success bool)

	// A trap for the in operator, with operator, Reflect.has (string properties)
	Has func(target *Object, property string) (available bool)

	// A trap for the in operator, with operator, Reflect.has (integer properties)
	HasIdx func(target *Object, property int) (available bool)

	// A trap for the in operator, with operator, Reflect.has (Symbol properties)
	HasSym func(target *Object, property *Symbol) (available bool)

	// A trap for getting property values, Reflect.get (string properties)
	Get func(target *Object, property string, receiver Value) (value Value)

	// A trap for getting property values, Reflect.get (integer properties)
	GetIdx func(target *Object, property int, receiver Value) (value Value)

	// A trap for getting property values, Reflect.get (Symbol properties)
	GetSym func(target *Object, property *Symbol, receiver Value) (value Value)

	// A trap for setting property values, Reflect.set (string properties)
	Set func(target *Object, property string, value Value, receiver Value) (success bool)

	// A trap for setting property values, Reflect.set (integer properties)
	SetIdx func(target *Object, property int, value Value, receiver Value) (success bool)

	// A trap for setting property values, Reflect.set (Symbol properties)
	SetSym func(target *Object, property *Symbol, value Value, receiver Value) (success bool)

	// A trap for the delete operator, Reflect.deleteProperty (string properties)
	DeleteProperty func(target *Object, property string) (success bool)

	// A trap for the delete operator, Reflect.deleteProperty (integer properties)
	DeletePropertyIdx func(target *Object, property int) (success bool)

	// A trap for the delete operator, Reflect.deleteProperty (Symbol properties)
	DeletePropertySym func(target *Object, property *Symbol) (success bool)

	// A trap for Object.getOwnPropertyNames, Object.getOwnPropertySymbols, Object.keys, Reflect.ownKeys
	OwnKeys func(target *Object) (object *Object)

	// A trap for a function call, Function.prototype.apply, Function.prototype.call, Reflect.apply
	Apply func(target *Object, this Value, argumentsList []Value) (value Value)

	// A trap for the new operator, Reflect.construct
	Construct func(target *Object, argumentsList []Value, newTarget *Object) (value *Object)
}

ProxyTrapConfig provides a simplified Go-friendly API for implementing Proxy traps. If an *Idx trap is defined it gets called for integer property keys, including negative ones. Note that this only includes string property keys that represent a canonical integer (i.e. "0", "123", but not "00", "01", " 1" or "-0"). For efficiency strings representing integers exceeding 2^53 are not checked to see if they are canonical, i.e. the *Idx traps will receive "9007199254740993" as well as "9007199254740994", even though the former is not a canonical representation in ECMAScript (Number("9007199254740993") === 9007199254740992). See https://262.ecma-international.org/#sec-canonicalnumericindexstring If an *Idx trap is not set, the corresponding string one is used.

type RandSource 

type RandSource func() float64

type Runtime 

type Runtime struct {
	// contains filtered or unexported fields
}

func New 

func New() *Runtime

New creates an instance of a Javascript runtime that can be used to run code. Multiple instances may be created and used simultaneously, however it is not possible to pass JS values across runtimes.

func (*Runtime) CaptureCallStack 

func (r *Runtime) CaptureCallStack(depth int, stack []StackFrame) []StackFrame

CaptureCallStack appends the current call stack frames to the stack slice (which may be nil) up to the specified depth. The most recent frame will be the first one. If depth <= 0 or more than the number of available frames, returns the entire stack. This method is not safe for concurrent use and should only be called by a Go function that is called from a running script.

func (*Runtime) ClearInterrupt 

func (r *Runtime) ClearInterrupt()

ClearInterrupt resets the interrupt flag. Typically this needs to be called before the runtime is made available for re-use if there is a chance it could have been interrupted with Interrupt(). Otherwise if Interrupt() was called when runtime was not running (e.g. if it had already finished) so that Interrupt() didn't actually trigger, an attempt to use the runtime will immediately cause an interruption. It is up to the user to ensure proper synchronisation so that ClearInterrupt() is only called when the runtime has finished and there is no chance of a concurrent Interrupt() call.

func (*Runtime) CreateObject 

func (r *Runtime) CreateObject(proto *Object) *Object

CreateObject creates an object with given prototype. Equivalent of Object.create(proto).

func (*Runtime) ExportTo 

func (r *Runtime) ExportTo(v Value, target interface{}) error

ExportTo converts a JavaScript value into the specified Go value. The second parameter must be a non-nil pointer. Returns error if conversion is not possible.

Notes on specific cases:

Empty interface

Exporting to an interface{} results in a value of the same type as Value.Export() would produce.

Numeric types

Exporting to numeric types uses the standard ECMAScript conversion operations, same as used when assigning values to non-clamped typed array items, e.g. https://262.ecma-international.org/#sec-toint32.

Functions

Exporting to a 'func' creates a strictly typed 'gateway' into an ES function which can be called from Go. The arguments are converted into ES values using Runtime.ToValue(). If the func has no return values, the return value is ignored. If the func has exactly one return value, it is converted to the appropriate type using ExportTo(). If the last return value is 'error', exceptions are caught and returned as *Exception (instances of GoError are unwrapped, i.e. their 'value' is returned instead). In all other cases exceptions result in a panic. Any extra return values are zeroed.

'this' value will always be set to 'undefined'.

For a more low-level mechanism see AssertFunction().

Map types

An ES Map can be exported into a Go map type. If any exported key value is non-hashable, the operation panics (as reflect.Value.SetMapIndex() would). Symbol.iterator is ignored.

Exporting an ES Set into a map type results in the map being populated with (element) -> (zero value) key/value pairs. If any value is non-hashable, the operation panics (as reflect.Value.SetMapIndex() would). Symbol.iterator is ignored.

Any other Object populates the map with own enumerable non-symbol properties.

Slice types

Exporting an ES Set into a slice type results in its elements being exported.

Exporting any Object that implements the iterable protocol (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#the_iterable_protocol) into a slice type results in the slice being populated with the results of the iteration.

Array is treated as iterable (i.e. overwriting Symbol.iterator affects the result).

If an object has a 'length' property and is not a function it is treated as array-like. The resulting slice will contain obj[0], ... obj[length-1].

For any other Object an error is returned.

Array types

Anything that can be exported to a slice type can also be exported to an array type, as long as the lengths match. If they do not, an error is returned.

Proxy

Proxy objects are treated the same way as if they were accessed from ES code in regard to their properties (such as 'length' or [Symbol.iterator]). This means exporting them to slice types works, however exporting a proxied Map into a map type does not produce its contents, because the Proxy is not recognised as a Map. Same applies to a proxied Set.

Example (ArrayLikeToSlice) 
vm := New()
v, err := vm.RunString(`
	({
		length: 3,
		0: 1,
		1: 2,
		2: 3
	});
	`)
if err != nil {
	panic(err)
}

var arr []int
err = vm.ExportTo(v, &arr)
if err != nil {
	panic(err)
}

fmt.Println(arr)

Output:

[1 2 3]
Example (Func) 
const SCRIPT = `
	function f(param) {
		return +param + 2;
	}
	`

vm := New()
_, err := vm.RunString(SCRIPT)
if err != nil {
	panic(err)
}

var fn func(string) string
err = vm.ExportTo(vm.Get("f"), &fn)
if err != nil {
	panic(err)
}

fmt.Println(fn("40")) // note, _this_ value in the function will be undefined.

Output:

42
Example (FuncThrow) 
const SCRIPT = `
	function f(param) {
		throw new Error("testing");
	}
	`

vm := New()
_, err := vm.RunString(SCRIPT)
if err != nil {
	panic(err)
}

var fn func(string) (string, error)
err = vm.ExportTo(vm.Get("f"), &fn)
if err != nil {
	panic(err)
}
_, err = fn("")

fmt.Println(err)

Output:

Error: testing at f (<eval>:3:9(3))
Example (FuncVariadic) 
const SCRIPT = `
	function f() {
		return Array.prototype.join.call(arguments, ",");
	}
	`
vm := New()
_, err := vm.RunString(SCRIPT)
if err != nil {
	panic(err)
}

var fn func(args ...interface{}) string
err = vm.ExportTo(vm.Get("f"), &fn)
if err != nil {
	panic(err)
}
fmt.Println(fn("a", "b", 42))

Output:

a,b,42
Example (IterableToSlice) 
vm := New()
v, err := vm.RunString(`
	function reverseIterator() {
	    const arr = this;
	    let idx = arr.length;
	    return {
			next: () => idx > 0 ? {value: arr[--idx]} : {done: true}
	    }
	}
	const arr = [1,2,3];
	arr[Symbol.iterator] = reverseIterator;
	arr;
	`)
if err != nil {
	panic(err)
}

var arr []int
err = vm.ExportTo(v, &arr)
if err != nil {
	panic(err)
}

fmt.Println(arr)

Output:

[3 2 1]
Example (MapToMap) 
vm := New()
m, err := vm.RunString(`
	new Map([[1, true], [2, false]]);
	`)
if err != nil {
	panic(err)
}
exp := make(map[int]bool)
err = vm.ExportTo(m, &exp)
if err != nil {
	panic(err)
}
fmt.Println(exp)

Output:

map[1:true 2:false]
Example (MapToSlice) 
vm := New()
m, err := vm.RunString(`
	new Map([[1, true], [2, false]]);
	`)
if err != nil {
	panic(err)
}
exp := make([][]interface{}, 0)
err = vm.ExportTo(m, &exp)
if err != nil {
	panic(err)
}
fmt.Println(exp)

Output:

[[1 true] [2 false]]
Example (MapToTypedSlice) 
vm := New()
m, err := vm.RunString(`
	new Map([[1, true], [2, false]]);
	`)
if err != nil {
	panic(err)
}
exp := make([][2]interface{}, 0)
err = vm.ExportTo(m, &exp)
if err != nil {
	panic(err)
}
fmt.Println(exp)

Output:

[[1 true] [2 false]]
Example (SetToMap) 
vm := New()
s, err := vm.RunString(`
	new Set([1, 2, 3])
	`)
if err != nil {
	panic(err)
}
m := make(map[int]struct{})
err = vm.ExportTo(s, &m)
if err != nil {
	panic(err)
}
fmt.Println(m)

Output:

map[1:{} 2:{} 3:{}]
Example (SetToSlice) 
vm := New()
s, err := vm.RunString(`
	new Set([1, 2, 3])
	`)
if err != nil {
	panic(err)
}
var a []int
err = vm.ExportTo(s, &a)
if err != nil {
	panic(err)
}
fmt.Println(a)

Output:

[1 2 3]

func (*Runtime) Get 

func (r *Runtime) Get(name string) (ret Value)

Get the specified variable in the global context. Equivalent to dereferencing a variable by name in non-strict mode. If variable is not defined returns nil. Note, this is not the same as GlobalObject().Get(name), because if a global lexical binding (let or const) exists, it is used instead. This method will panic with an *Exception if a JavaScript exception is thrown in the process.

func (*Runtime) GlobalObject 

func (r *Runtime) GlobalObject() *Object

GlobalObject returns the global object.

func (*Runtime) Interrupt 

func (r *Runtime) Interrupt(v interface{})

Interrupt a running JavaScript. The corresponding Go call will return an *InterruptedError containing v. If the interrupt propagates until the stack is empty the currently queued promise resolve/reject jobs will be cleared without being executed. This is the same time they would be executed otherwise. Note, it only works while in JavaScript code, it does not interrupt native Go functions (which includes all built-ins). If the runtime is currently not running, it will be immediately interrupted on the next Run*() call. To avoid that use ClearInterrupt()

func (*Runtime) New 

func (r *Runtime) New(construct Value, args ...Value) (o *Object, err error)

New is an equivalent of the 'new' operator allowing to call it directly from Go.

func (*Runtime) NewArray 

func (r *Runtime) NewArray(items ...interface{}) *Object
Example 
vm := New()
array := vm.NewArray(1, 2, true)
vm.Set("array", array)
res, err := vm.RunString(`
	var acc = "";
	for (var v of array) {
		acc += v + " ";
	}
	acc;
	`)
if err != nil {
	panic(err)
}
fmt.Println(res)

Output:

1 2 true

func (*Runtime) NewArrayBuffer 

func (r *Runtime) NewArrayBuffer(data []byte) ArrayBuffer

func (*Runtime) NewDynamicArray 

func (r *Runtime) NewDynamicArray(a DynamicArray) *Object

NewDynamicArray creates an array Object backed by the provided DynamicArray handler. It is similar to NewDynamicObject, the differences are:

- the Object is an array (i.e. Array.isArray() will return true and it will have the length property).

- the prototype will be initially set to Array.prototype.

- the Object cannot have any own string properties except for the 'length'.

func (*Runtime) NewDynamicObject 

func (r *Runtime) NewDynamicObject(d DynamicObject) *Object

NewDynamicObject creates an Object backed by the provided DynamicObject handler.

All properties of this Object are Writable, Enumerable and Configurable data properties. Any attempt to define a property that does not conform to this will fail.

The Object is always extensible and cannot be made non-extensible. Object.preventExtensions() will fail.

The Object's prototype is initially set to Object.prototype, but can be changed using regular mechanisms (Object.SetPrototype() in Go or Object.setPrototypeOf() in JS).

The Object cannot have own Symbol properties, however its prototype can. If you need an iterator support for example, you could create a regular object, set Symbol.iterator on that object and then use it as a prototype. See TestDynamicObjectCustomProto for more details.

Export() returns the original DynamicObject.

This mechanism is similar to ECMAScript Proxy, however because all properties are enumerable and the object is always extensible there is no need for invariant checks which removes the need to have a target object and makes it a lot more efficient.

func (*Runtime) NewGoError 

func (r *Runtime) NewGoError(err error) *Object

func (*Runtime) NewObject 

func (r *Runtime) NewObject() (v *Object)

func (*Runtime) NewPromise 

func (r *Runtime) NewPromise() (promise *Promise, resolve func(result interface{}), reject func(reason interface{}))

NewPromise creates and returns a Promise and resolving functions for it.

WARNING: The returned values are not goroutine-safe and must not be called in parallel with VM running. In order to make use of this method you need an event loop such as the one in goja_nodejs (https://github.com/link-duan/goja_nodejs) where it can be used like this: 

loop := NewEventLoop()
loop.Start()
defer loop.Stop()
loop.RunOnLoop(func(vm *goja.Runtime) {
    p, resolve, _ := vm.NewPromise()
    vm.Set("p", p)
    go func() {
        time.Sleep(500 * time.Millisecond)   // or perform any other blocking operation
        loop.RunOnLoop(func(*goja.Runtime) { // resolve() must be called on the loop, cannot call it here
            resolve(result)
        })
    }()
}

func (*Runtime) NewProxy 

func (r *Runtime) NewProxy(target *Object, nativeHandler *ProxyTrapConfig) Proxy

func (*Runtime) NewTypeError 

func (r *Runtime) NewTypeError(args ...interface{}) *Object

func (*Runtime) RunProgram 

func (r *Runtime) RunProgram(p *Program) (result Value, err error)

RunProgram executes a pre-compiled (see Compile()) code in the global context.

func (*Runtime) RunScript 

func (r *Runtime) RunScript(name, src string) (Value, error)

RunScript executes the given string in the global context.

func (*Runtime) RunString 

func (r *Runtime) RunString(str string) (Value, error)

RunString executes the given string in the global context.

func (*Runtime) Set 

func (r *Runtime) Set(name string, value interface{}) error

Set the specified variable in the global context. Equivalent to running "name = value" in non-strict mode. The value is first converted using ToValue(). Note, this is not the same as GlobalObject().Set(name, value), because if a global lexical binding (let or const) exists, it is set instead.

Example (Lexical) 
r := New()
_, err := r.RunString("let x")
if err != nil {
	panic(err)
}
err = r.Set("x", 1)
if err != nil {
	panic(err)
}
fmt.Print(r.Get("x"), r.GlobalObject().Get("x"))

Output:

1 <nil>

func (*Runtime) SetAsyncContextTracker 

func (r *Runtime) SetAsyncContextTracker(tracker AsyncContextTracker)

SetAsyncContextTracker registers a handler that allows to track async execution contexts. See AsyncContextTracker documentation for more details. Setting it to nil disables the functionality. This method (as Runtime in general) is not goroutine-safe.

func (*Runtime) SetFieldNameMapper 

func (r *Runtime) SetFieldNameMapper(mapper FieldNameMapper)

SetFieldNameMapper sets a custom field name mapper for Go types. It can be called at any time, however the mapping for any given value is fixed at the point of creation. Setting this to nil restores the default behaviour which is all exported fields and methods are mapped to their original unchanged names.

func (*Runtime) SetMaxCallStackSize 

func (r *Runtime) SetMaxCallStackSize(size int)

SetMaxCallStackSize sets the maximum function call depth. When exceeded, a *StackOverflowError is thrown and returned by RunProgram or by a Callable call. This is useful to prevent memory exhaustion caused by an infinite recursion. The default value is math.MaxInt32. This method (as the rest of the Set* methods) is not safe for concurrent use and may only be called from the vm goroutine or when the vm is not running.

func (*Runtime) SetParserOptions 

func (r *Runtime) SetParserOptions(opts ...parser.Option)

SetParserOptions sets parser options to be used by RunString, RunScript and eval() within the code.

Example 
vm := New()
vm.SetParserOptions(parser.WithDisableSourceMaps)

res, err := vm.RunString(`
	"I did not hang!";
//# sourceMappingURL=/dev/zero`)

if err != nil {
	panic(err)
}
fmt.Println(res.String())

Output:

I did not hang!

func (*Runtime) SetPromiseRejectionTracker 

func (r *Runtime) SetPromiseRejectionTracker(tracker PromiseRejectionTracker)

SetPromiseRejectionTracker registers a function that will be called in two scenarios: when a promise is rejected without any handlers (with operation argument set to PromiseRejectionReject), and when a handler is added to a rejected promise for the first time (with operation argument set to PromiseRejectionHandle).

Setting a tracker replaces any existing one. Setting it to nil disables the functionality.

See https://tc39.es/ecma262/#sec-host-promise-rejection-tracker for more details.

func (*Runtime) SetRandSource 

func (r *Runtime) SetRandSource(source RandSource)

SetRandSource sets random source for this Runtime. If not called, the default math/rand is used.

func (*Runtime) SetTimeSource 

func (r *Runtime) SetTimeSource(now Now)

SetTimeSource sets the current time source for this Runtime. If not called, the default time.Now() is used.

func (*Runtime) ToValue 

func (r *Runtime) ToValue(i interface{}) Value

ToValue converts a Go value into a JavaScript value of a most appropriate type. Structural types (such as structs, maps and slices) are wrapped so that changes are reflected on the original value which can be retrieved using Value.Export().

WARNING! These wrapped Go values do not behave in the same way as native ECMAScript values. If you plan to modify them in ECMAScript, bear in mind the following caveats:

1. If a regular JavaScript Object is assigned as an element of a wrapped Go struct, map or array, it is Export()'ed and therefore copied. This may result in an unexpected behaviour in JavaScript: 

m := map[string]interface{}{}
vm.Set("m", m)
vm.RunString(`
var obj = {test: false};
m.obj = obj; // obj gets Export()'ed, i.e. copied to a new map[string]interface{} and then this map is set as m["obj"]
obj.test = true; // note, m.obj.test is still false
`)
fmt.Println(m["obj"].(map[string]interface{})["test"]) // prints "false"

2. Be careful with nested non-pointer compound types (structs, slices and arrays) if you modify them in ECMAScript. Better avoid it at all if possible. One of the fundamental differences between ECMAScript and Go is in the former all Objects are references whereas in Go you can have a literal struct or array. Consider the following example: 

type S struct {
    Field int
}

a := []S{{1}, {2}} // slice of literal structs
vm.Set("a", &a)
vm.RunString(`
    let tmp = {Field: 1};
    a[0] = tmp;
    a[1] = tmp;
    tmp.Field = 2;
`)

In ECMAScript one would expect a[0].Field and a[1].Field to be equal to 2, but this is really not possible (or at least non-trivial without some complex reference tracking).

To cover the most common use cases and to avoid excessive memory allocation, the following 'copy-on-change' mechanism is implemented (for both arrays and structs):

* When a nested compound value is accessed, the returned ES value becomes a reference to the literal value. This ensures that things like 'a[0].Field = 1' work as expected and simple access to 'a[0].Field' does not result in copying of a[0].

* The original container ('a' in our case) keeps track of the returned reference value and if a[0] is reassigned (e.g. by direct assignment, deletion or shrinking the array) the old a[0] is copied and the earlier returned value becomes a reference to the copy: 

let tmp = a[0];                      // no copy, tmp is a reference to a[0]
tmp.Field = 1;                       // a[0].Field === 1 after this
a[0] = {Field: 2};                   // tmp is now a reference to a copy of the old value (with Field === 1)
a[0].Field === 2 && tmp.Field === 1; // true

* Array value swaps caused by in-place sort (using Array.prototype.sort()) do not count as re-assignments, instead the references are adjusted to point to the new indices.

* Assignment to an inner compound value always does a copy (and sometimes type conversion): 

a[1] = tmp;    // a[1] is now a copy of tmp
tmp.Field = 3; // does not affect a[1].Field

3. Non-addressable structs, slices and arrays get copied. This sometimes may lead to a confusion as assigning to inner fields does not appear to work: 

a1 := []interface{}{S{1}, S{2}}
vm.Set("a1", &a1)
vm.RunString(`
   a1[0].Field === 1; // true
   a1[0].Field = 2;
   a1[0].Field === 2; // FALSE, because what it really did was copy a1[0] set its Field to 2 and immediately drop it
`)

An alternative would be making a1[0].Field a non-writable property which would probably be more in line with ECMAScript, however it would require to manually copy the value if it does need to be modified which may be impractical.

Note, the same applies to slices. If a slice is passed by value (not as a pointer), resizing the slice does not reflect on the original value. Moreover, extending the slice may result in the underlying array being re-allocated and copied. For example: 

a := []interface{}{1}
vm.Set("a", a)
vm.RunString(`a.push(2); a[0] = 0;`)
fmt.Println(a[0]) // prints "1"

Notes on individual types:

Primitive types

Primitive types (numbers, string, bool) are converted to the corresponding JavaScript primitives.

Strings

Because of the difference in internal string representation between ECMAScript (which uses UTF-16) and Go (which uses UTF-8) conversion from JS to Go may be lossy. In particular, code points that can be part of UTF-16 surrogate pairs (0xD800-0xDFFF) cannot be represented in UTF-8 unless they form a valid surrogate pair and are replaced with utf8.RuneError.

The string value must be a valid UTF-8. If it is not, invalid characters are replaced with utf8.RuneError, but the behaviour of a subsequent Export() is unspecified (it may return the original value, or a value with replaced invalid characters).

Nil

Nil is converted to null.

Functions

func(FunctionCall) Value is treated as a native JavaScript function. This increases performance because there are no automatic argument and return value type conversions (which involves reflect). Attempting to use the function as a constructor will result in a TypeError.

func(FunctionCall, *Runtime) Value is treated as above, except the *Runtime is also passed as a parameter.

func(ConstructorCall) *Object is treated as a native constructor, allowing to use it with the new operator: 

func MyObject(call goja.ConstructorCall) *goja.Object {
   // call.This contains the newly created object as per http://www.ecma-international.org/ecma-262/5.1/index.html#sec-13.2.2
   // call.Arguments contain arguments passed to the function

   call.This.Set("method", method)

   //...

   // If return value is a non-nil *Object, it will be used instead of call.This
   // This way it is possible to return a Go struct or a map converted
   // into goja.Value using ToValue(), however in this case
   // instanceof will not work as expected, unless you set the prototype:
   //
   // instance := &myCustomStruct{}
   // instanceValue := vm.ToValue(instance).(*Object)
   // instanceValue.SetPrototype(call.This.Prototype())
   // return instanceValue
   return nil
}

runtime.Set("MyObject", MyObject)

Then it can be used in JS as follows: 

var o = new MyObject(arg);
var o1 = MyObject(arg); // same thing
o instanceof MyObject && o1 instanceof MyObject; // true

When a native constructor is called directly (without the new operator) its behavior depends on this value: if it's an Object, it is passed through, otherwise a new one is created exactly as if it was called with the new operator. In either case call.NewTarget will be nil.

func(ConstructorCall, *Runtime) *Object is treated as above, except the *Runtime is also passed as a parameter.

Any other Go function is wrapped so that the arguments are automatically converted into the required Go types and the return value is converted to a JavaScript value (using this method). If conversion is not possible, a TypeError is thrown.

Functions with multiple return values return an Array. If the last return value is an `error` it is not returned but converted into a JS exception. If the error is *Exception, it is thrown as is, otherwise it's wrapped in a GoEerror. Note that if there are exactly two return values and the last is an `error`, the function returns the first value as is, not an Array.

Structs

Structs are converted to Object-like values. Fields and methods are available as properties, their values are results of this method (ToValue()) applied to the corresponding Go value.

Field properties are writable and non-configurable. Method properties are non-writable and non-configurable.

Attempt to define a new property or delete an existing property will fail (throw in strict mode) unless it's a Symbol property. Symbol properties only exist in the wrapper and do not affect the underlying Go value. Note that because a wrapper is created every time a property is accessed it may lead to unexpected results such as this: 

 type Field struct{
 }
 type S struct {
	Field *Field
 }
 var s = S{
	Field: &Field{},
 }
 vm := New()
 vm.Set("s", &s)
 res, err := vm.RunString(`
 var sym = Symbol(66);
 var field1 = s.Field;
 field1[sym] = true;
 var field2 = s.Field;
 field1 === field2; // true, because the equality operation compares the wrapped values, not the wrappers
 field1[sym] === true; // true
 field2[sym] === undefined; // also true
 `)

The same applies to values from maps and slices as well.

Handling of time.Time

time.Time does not get special treatment and therefore is converted just like any other `struct` providing access to all its methods. This is done deliberately instead of converting it to a `Date` because these two types are not fully compatible: `time.Time` includes zone, whereas JS `Date` doesn't. Doing the conversion implicitly therefore would result in a loss of information.

If you need to convert it to a `Date`, it can be done either in JS: 

var d = new Date(goval.UnixNano()/1e6);

... or in Go: 

 now := time.Now()
 vm := New()
 val, err := vm.New(vm.Get("Date").ToObject(vm), vm.ToValue(now.UnixNano()/1e6))
 if err != nil {
	...
 }
 vm.Set("d", val)

Note that Value.Export() for a `Date` value returns time.Time in local timezone.

Maps

Maps with string or integer key type are converted into host objects that largely behave like a JavaScript Object.

Maps with methods

If a map type has at least one method defined, the properties of the resulting Object represent methods, not map keys. This is because in JavaScript there is no distinction between 'object.key` and `object[key]`, unlike Go. If access to the map values is required, it can be achieved by defining another method or, if it's not possible, by defining an external getter function.

Slices

Slices are converted into host objects that behave largely like JavaScript Array. It has the appropriate prototype and all the usual methods should work. There is, however, a caveat: converted Arrays may not contain holes (because Go slices cannot). This means that hasOwnProperty(n) always returns `true` if n < length. Deleting an item with an index < length will set it to a zero value (but the property will remain). Nil slice elements are be converted to `null`. Accessing an element beyond `length` returns `undefined`. Also see the warning above about passing slices as values (as opposed to pointers).

Arrays

Arrays are converted similarly to slices, except the resulting Arrays are not resizable (and therefore the 'length' property is non-writable).

Any other type is converted to a generic reflect based host object. Depending on the underlying type it behaves similar to a Number, String, Boolean or Object.

Note that the underlying type is not lost, calling Export() returns the original Go value. This applies to all reflect based types.

type StackFrame 

type StackFrame struct {
	// contains filtered or unexported fields
}

func (*StackFrame) FuncName 

func (f *StackFrame) FuncName() string

func (*StackFrame) Position 

func (f *StackFrame) Position() file.Position

func (*StackFrame) SrcName 

func (f *StackFrame) SrcName() string

func (*StackFrame) Write 

func (f *StackFrame) Write(b *bytes.Buffer)

func (*StackFrame) WriteToValueBuilder 

func (f *StackFrame) WriteToValueBuilder(b *valueStringBuilder)

type StackOverflowError 

type StackOverflowError struct {
	// contains filtered or unexported fields
}

type Symbol 

type Symbol struct {
	// contains filtered or unexported fields
}

*Symbol is a Value containing ECMAScript Symbol primitive. Symbols must only be created using NewSymbol(). Zero values and copying of values (i.e. *s1 = *s2) are not permitted. Well-known Symbols can be accessed using Sym* package variables (SymIterator, etc...) Symbols can be shared by multiple Runtimes.

func NewSymbol 

func NewSymbol(s string) *Symbol
Example 
sym1 := NewSymbol("66")
sym2 := NewSymbol("66")
fmt.Printf("%s %s %v", sym1, sym2, sym1.Equals(sym2))

Output:

66 66 false

func (*Symbol) Equals 

func (s *Symbol) Equals(o Value) bool

func (*Symbol) Export 

func (s *Symbol) Export() interface{}

func (*Symbol) ExportType 

func (s *Symbol) ExportType() reflect.Type

func (*Symbol) SameAs 

func (s *Symbol) SameAs(other Value) bool

func (*Symbol) StrictEquals 

func (s *Symbol) StrictEquals(o Value) bool

func (*Symbol) String 

func (s *Symbol) String() string

func (*Symbol) ToBoolean 

func (s *Symbol) ToBoolean() bool

func (*Symbol) ToFloat 

func (s *Symbol) ToFloat() float64

func (*Symbol) ToInteger 

func (s *Symbol) ToInteger() int64

func (*Symbol) ToNumber 

func (s *Symbol) ToNumber() Value

func (*Symbol) ToObject 

func (s *Symbol) ToObject(r *Runtime) *Object

func (*Symbol) ToString 

func (s *Symbol) ToString() Value

type Value 

type Value interface {
	ToInteger() int64

	ToString() Value
	String() string
	ToFloat() float64
	ToNumber() Value
	ToBoolean() bool
	ToObject(*Runtime) *Object
	SameAs(Value) bool
	Equals(Value) bool
	StrictEquals(Value) bool
	Export() interface{}
	ExportType() reflect.Type
	// contains filtered or unexported methods
}

Value represents an ECMAScript value.

Export returns a "plain" Go value which type depends on the type of the Value.

For integer numbers it's int64.

For any other numbers (including Infinities, NaN and negative zero) it's float64.

For string it's a string. Note that unicode strings are converted into UTF-8 with invalid code points replaced with utf8.RuneError.

For boolean it's bool.

For null and undefined it's nil.

For Object it depends on the Object type, see Object.Export() for more details.

func NaN 

func NaN() Value

NaN returns a JS NaN value.

func NegativeInf 

func NegativeInf() Value

NegativeInf returns a JS -Inf value.

func Null 

func Null() Value

Null returns JS null value.

func PositiveInf 

func PositiveInf() Value

PositiveInf returns a JS +Inf value.

func Undefined 

func Undefined() Value

Undefined returns JS undefined value. Note if global 'undefined' property is changed this still returns the original value.

Source Files

View all Source files

Directories

Path Synopsis
Package ast declares types representing a JavaScript AST.
 Package ast declares types representing a JavaScript AST.
Package file encapsulates the file abstractions used by the ast & parser.
 Package file encapsulates the file abstractions used by the ast & parser.
Package ftoa provides ECMAScript-compliant floating point number conversion to string.
 Package ftoa provides ECMAScript-compliant floating point number conversion to string.
internal/fast
Package fast contains code ported from V8 (https://github.com/v8/v8/blob/master/src/numbers/fast-dtoa.cc)
 Package fast contains code ported from V8 (https://github.com/v8/v8/blob/master/src/numbers/fast-dtoa.cc)
Package parser implements a parser for JavaScript.
 Package parser implements a parser for JavaScript.
Package token defines constants representing the lexical tokens of JavaScript (ECMA5).
 Package token defines constants representing the lexical tokens of JavaScript (ECMA5).
Package unistring contains an implementation of a hybrid ASCII/UTF-16 string.
 Package unistring contains an implementation of a hybrid ASCII/UTF-16 string.

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