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Package js

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.

Index

Examples

FuncOf

Package files

func.go js.go

func CopyBytesToGoSource

func CopyBytesToGo(dst []byte, src Value) int

CopyBytesToGo copies bytes from src to dst. It panics if src is not an Uint8Array or Uint8ClampedArray. It returns the number of bytes copied, which will be the minimum of the lengths of src and dst.

func CopyBytesToJSSource

func CopyBytesToJS(dst Value, src []byte) int

CopyBytesToJS copies bytes from src to dst. It panics if dst is not an Uint8Array or Uint8ClampedArray. It returns the number of bytes copied, which will be the minimum of the lengths of src and dst.

type ErrorSource

Error wraps a JavaScript error.

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

func (Error) ErrorSource

func (e Error) Error() string

Error implements the error interface.

type FuncSource

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 FuncOfSource

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

FuncOf returns a function to be used by JavaScript.

The Go function fn is called 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.

Invoking the wrapped Go function from JavaScript will pause the event loop and spawn a new goroutine. Other wrapped functions which are triggered during a call from Go to JavaScript get executed on the same goroutine.

As a consequence, if one wrapped function blocks, JavaScript's event loop is blocked until that function returns. Hence, calling any async JavaScript API, which requires the event loop, like fetch (http.Client), will cause an immediate deadlock. Therefore a blocking function should explicitly start a new goroutine.

Func.Release must be called to free up resources when the function will not be invoked 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) ReleaseSource

func (c Func) Release()

Release frees up resources allocated for the function. The function must not be invoked after calling Release. It is allowed to call Release while the function is still running.

type TypeSource

Type represents the JavaScript type of a Value.

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

func (Type) StringSource

func (t Type) String() string

type ValueSource

Value represents a JavaScript value. The zero value is the JavaScript value "undefined". Values can be checked for equality with the Equal method.

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

func GlobalSource

func Global() Value

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

func NullSource

func Null() Value

Null returns the JavaScript value "null".

func UndefinedSource

func Undefined() Value

Undefined returns the JavaScript value "undefined".

func ValueOfSource

func ValueOf(x interface{}) Value

ValueOf returns x as a JavaScript value:

| Go                     | JavaScript             |
| ---------------------- | ---------------------- |
| js.Value               | [its value]            |
| 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) BoolSource

func (v Value) Bool() bool

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

func (Value) CallSource

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) DeleteSource

func (v Value) Delete(p string)

Delete deletes the JavaScript property p of value v. It panics if v is not a JavaScript object.

func (Value) EqualSource

func (v Value) Equal(w Value) bool

Equal reports whether v and w are equal according to JavaScript's === operator.

func (Value) FloatSource

func (v Value) Float() float64

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

func (Value) GetSource

func (v Value) Get(p string) Value

Get returns the JavaScript property p of value v. It panics if v is not a JavaScript object.

func (Value) IndexSource

func (v Value) Index(i int) Value

Index returns JavaScript index i of value v. It panics if v is not a JavaScript object.

func (Value) InstanceOfSource

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) IntSource

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) InvokeSource

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 JavaScript function. The arguments get mapped to JavaScript values according to the ValueOf function.

func (Value) IsNaNSource

func (v Value) IsNaN() bool

IsNaN reports whether v is the JavaScript value "NaN".

func (Value) IsNullSource

func (v Value) IsNull() bool

IsNull reports whether v is the JavaScript value "null".

func (Value) IsUndefinedSource

func (v Value) IsUndefined() bool

IsUndefined reports whether v is the JavaScript value "undefined".

func (Value) JSValueSource

func (v Value) JSValue() Value

JSValue implements Wrapper interface.

func (Value) LengthSource

func (v Value) Length() int

Length returns the JavaScript property "length" of v. It panics if v is not a JavaScript object.

func (Value) NewSource

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 JavaScript function. The arguments get mapped to JavaScript values according to the ValueOf function.

func (Value) SetSource

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

Set sets the JavaScript property p of value v to ValueOf(x). It panics if v is not a JavaScript object.

func (Value) SetIndexSource

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

SetIndex sets the JavaScript index i of value v to ValueOf(x). It panics if v is not a JavaScript object.

func (Value) StringSource

func (v Value) String() string

String returns the value v as a string. String is a special case because of Go's String method convention. Unlike the other getters, it does not panic if v's Type is not TypeString. Instead, it returns a string of the form "<T>" or "<T: V>" where T is v's type and V is a string representation of v's value.

func (Value) TruthySource

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) TypeSource

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 ValueErrorSource

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) ErrorSource

func (e *ValueError) Error() string

type WrapperSource

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
}

© Google, Inc.
Licensed under the Creative Commons Attribution License 3.0.
https://golang.org/pkg/syscall/js/