Package js

import "syscall/js"
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Overview ▾

Package js gives access to the WebAssembly host environment when using the js/wasm architecture. Its API is based on JavaScript semantics.

This package is EXPERIMENTAL. Its current scope is only to allow tests to run, but not yet to provide a comprehensive API for users. It is exempt from the Go compatibility promise.

type Error

Error wraps a JavaScript error.

type Error struct {
        // Value is the underlying JavaScript error value.
        Value
}

func (Error) Error

func (e Error) Error() string

Error implements the error interface.

type Func

Func is a wrapped Go function to be called by JavaScript.

type Func struct {
        Value // the JavaScript function that invokes the Go function
        // contains filtered or unexported fields
}

func FuncOf

func FuncOf(fn func(this Value, args []Value) interface{}) Func

FuncOf returns a wrapped function.

Invoking the JavaScript function will synchronously call the Go function fn with the value of JavaScript's "this" keyword and the arguments of the invocation. The return value of the invocation is the result of the Go function mapped back to JavaScript according to ValueOf.

A wrapped function triggered during a call from Go to JavaScript gets executed on the same goroutine. A wrapped function triggered by JavaScript's event loop gets executed on an extra goroutine. Blocking operations in the wrapped function will block the event loop. As a consequence, if one wrapped function blocks, other wrapped funcs will not be processed. A blocking function should therefore explicitly start a new goroutine.

Func.Release must be called to free up resources when the function will not be used any more.

Example

Code:

var cb js.Func
cb = js.FuncOf(func(this js.Value, args []js.Value) interface{} {
        fmt.Println("button clicked")
        cb.Release() // release the function if the button will not be clicked again
        return nil
})
js.Global().Get("document").Call("getElementById", "myButton").Call("addEventListener", "click", cb)

func (Func) Release

func (c Func) Release()

Release frees up resources allocated for the function. The function must not be invoked after calling Release.

type Type

Type represents the JavaScript type of a Value.

type Type int
const (
        TypeUndefined Type = iota
        TypeNull
        TypeBoolean
        TypeNumber
        TypeString
        TypeSymbol
        TypeObject
        TypeFunction
)

func (Type) String

func (t Type) String() string

type TypedArray

TypedArray represents a JavaScript typed array.

type TypedArray struct {
        Value
}

func TypedArrayOf

func TypedArrayOf(slice interface{}) TypedArray

TypedArrayOf returns a JavaScript typed array backed by the slice's underlying array.

The supported types are []int8, []int16, []int32, []uint8, []uint16, []uint32, []float32 and []float64. Passing an unsupported value causes a panic.

TypedArray.Release must be called to free up resources when the typed array will not be used any more.

func (TypedArray) Release

func (a TypedArray) Release()

Release frees up resources allocated for the typed array. The typed array and its buffer must not be accessed after calling Release.

type Value

Value represents a JavaScript value. The zero value is the JavaScript value "undefined".

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

func Global

func Global() Value

Global returns the JavaScript global object, usually "window" or "global".

func Null

func Null() Value

Null returns the JavaScript value "null".

func Undefined

func Undefined() Value

Undefined returns the JavaScript value "undefined".

func ValueOf

func ValueOf(x interface{}) Value

ValueOf returns x as a JavaScript value:

| Go                     | JavaScript             |
| ---------------------- | ---------------------- |
| js.Value               | [its value]            |
| js.TypedArray          | typed array            |
| js.Func                | function               |
| nil                    | null                   |
| bool                   | boolean                |
| integers and floats    | number                 |
| string                 | string                 |
| []interface{}          | new array              |
| map[string]interface{} | new object             |

Panics if x is not one of the expected types.

func (Value) Bool

func (v Value) Bool() bool

Bool returns the value v as a bool. It panics if v is not a JavaScript boolean.

func (Value) Call

func (v Value) Call(m string, args ...interface{}) Value

Call does a JavaScript call to the method m of value v with the given arguments. It panics if v has no method m. The arguments get mapped to JavaScript values according to the ValueOf function.

func (Value) Float

func (v Value) Float() float64

Float returns the value v as a float64. It panics if v is not a JavaScript number.

func (Value) Get

func (v Value) Get(p string) Value

Get returns the JavaScript property p of value v.

func (Value) Index

func (v Value) Index(i int) Value

Index returns JavaScript index i of value v.

func (Value) InstanceOf

func (v Value) InstanceOf(t Value) bool

InstanceOf reports whether v is an instance of type t according to JavaScript's instanceof operator.

func (Value) Int

func (v Value) Int() int

Int returns the value v truncated to an int. It panics if v is not a JavaScript number.

func (Value) Invoke

func (v Value) Invoke(args ...interface{}) Value

Invoke does a JavaScript call of the value v with the given arguments. It panics if v is not a function. The arguments get mapped to JavaScript values according to the ValueOf function.

func (Value) JSValue

func (v Value) JSValue() Value

JSValue implements Wrapper interface.

func (Value) Length

func (v Value) Length() int

Length returns the JavaScript property "length" of v.

func (Value) New

func (v Value) New(args ...interface{}) Value

New uses JavaScript's "new" operator with value v as constructor and the given arguments. It panics if v is not a function. The arguments get mapped to JavaScript values according to the ValueOf function.

func (Value) Set

func (v Value) Set(p string, x interface{})

Set sets the JavaScript property p of value v to ValueOf(x).

func (Value) SetIndex

func (v Value) SetIndex(i int, x interface{})

SetIndex sets the JavaScript index i of value v to ValueOf(x).

func (Value) String

func (v Value) String() string

String returns the value v converted to string according to JavaScript type conversions.

func (Value) Truthy

func (v Value) Truthy() bool

Truthy returns the JavaScript "truthiness" of the value v. In JavaScript, false, 0, "", null, undefined, and NaN are "falsy", and everything else is "truthy". See https://developer.mozilla.org/en-US/docs/Glossary/Truthy.

func (Value) Type

func (v Value) Type() Type

Type returns the JavaScript type of the value v. It is similar to JavaScript's typeof operator, except that it returns TypeNull instead of TypeObject for null.

type ValueError

A ValueError occurs when a Value method is invoked on a Value that does not support it. Such cases are documented in the description of each method.

type ValueError struct {
        Method string
        Type   Type
}

func (*ValueError) Error

func (e *ValueError) Error() string

type Wrapper

Wrapper is implemented by types that are backed by a JavaScript value.

type Wrapper interface {
        // JSValue returns a JavaScript value associated with an object.
        JSValue() Value
}