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goja

ECMAScript 5.1(+) implementation in Go.

It is not a replacement for V8 or SpiderMonkey or any other general-purpose JavaScript engine as it is much slower. It can be used as an embedded scripting language where the cost of making a lot of cgo calls may outweight the benefits of a faster JavaScript engine or as a way to avoid having non-Go dependencies.

This project was largely inspired by otto.

Features

• Full ECMAScript 5.1 support (yes, including regex and strict mode).
• Passes nearly all tc39 tests tagged with es5id. The goal is to pass all of them. Note, the last working commit is https://github.com/tc39/test262/commit/1ba3a7c4a93fc93b3d0d7e4146f59934a896837d. The next commit made use of template strings which goja does not support.
• On average 6-7 times faster than otto. Also uses considerably less memory.

Current Status

• API is still work in progress and is subject to change.
• Some of the AnnexB functionality is missing.
• No typed arrays yet.

Basic Example

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. Primitive types (ints and uints, floats, string, bool) are converted to the corresponding JavaScript primitives.

func(FunctionCall) Value is treated as a native JavaScript function.

func(ConstructorCall) *Object is treated as a JavaScript constructor (see Native Constructors).

map[string]interface{} is converted into a host object that largely behaves like a JavaScript Object.

[]interface{} is converted into a host object that behaves largely like a JavaScript Array, however it's not extensible because extending it can change the pointer so it becomes detached from the original.

*[]interface{} is same as above, but the array becomes extensible.

A function is wrapped within a native JavaScript function. When called the arguments are automatically converted to the appropriate Go types. If conversion is not possible, a TypeError is thrown.

A slice type is converted into a generic reflect based host object that behaves similar to an unexpandable Array.

A map type with numeric or string keys and no methods is converted into a host object where properties are map keys.

A map type with methods is converted into a host object where properties are method names, the map values are not accessible. This is to avoid ambiguity between m["Property"] and m.Property.

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 these conversions wrap the original value which means any changes made inside JS are reflected on the value and calling Export() returns the original value. This applies to all reflect based types.

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.

Native Constructors

In order to implement a constructor function in Go:

func MyObject(call goja.ConstructorCall) *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 runtime.ToValue(), however in this case
    // instanceof will not work as expected.
    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

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.