src/syscall/js/js.go - go - Git at Google

 // Copyright 2018 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // +build js,wasm

 // 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.
 package js

 import (
 	"unsafe"
 )

 // ref is used to identify a JavaScript value, since the value itself can not be passed to WebAssembly.
 //
 // The JavaScript value "undefined" is represented by the value 0.
 // A JavaScript number (64-bit float, except 0 and NaN) is represented by its IEEE 754 binary representation.
 // All other values are represented as an IEEE 754 binary representation of NaN with bits 0-31 used as
 // an ID and bits 32-33 used to differentiate between string, symbol, function and object.
 type ref uint64

 // nanHead are the upper 32 bits of a ref which are set if the value is not encoded as an IEEE 754 number (see above).
 const nanHead = 0x7FF80000

 // 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
 }

 // Value represents a JavaScript value. The zero value is the JavaScript value "undefined".
 type Value struct {
 	ref ref
 }

 // JSValue implements Wrapper interface.
 func (v Value) JSValue() Value {
 	return v
 }

 func makeValue(v ref) Value {
 	return Value{ref: v}
 }

 func predefValue(id uint32) Value {
 	return Value{ref: nanHead<<32 | ref(id)}
 }

 func floatValue(f float64) Value {
 	if f == 0 {
 		return valueZero
 	}
 	if f != f {
 		return valueNaN
 	}
 	return Value{ref: *(*ref)(unsafe.Pointer(&f))}
 }

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

 // Error implements the error interface.
 func (e Error) Error() string {
 	return "JavaScript error: " + e.Get("message").String()
 }

 var (
 	valueUndefined = Value{ref: 0}
 	valueNaN       = predefValue(0)
 	valueZero      = predefValue(1)
 	valueNull      = predefValue(2)
 	valueTrue      = predefValue(3)
 	valueFalse     = predefValue(4)
 	valueGlobal    = predefValue(5)
 	jsGo           = predefValue(6) // instance of the Go class in JavaScript

 	objectConstructor = valueGlobal.Get("Object")
 	arrayConstructor  = valueGlobal.Get("Array")
 )

 // Undefined returns the JavaScript value "undefined".
 func Undefined() Value {
 	return valueUndefined
 }

 // Null returns the JavaScript value "null".
 func Null() Value {
 	return valueNull
 }

 // Global returns the JavaScript global object, usually "window" or "global".
 func Global() Value {
 	return valueGlobal
 }

 // 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 ValueOf(x interface{}) Value {
 	switch x := x.(type) {
 	case Value: // should precede Wrapper to avoid a loop
 		return x
 	case Wrapper:
 		return x.JSValue()
 	case nil:
 		return valueNull
 	case bool:
 		if x {
 			return valueTrue
 		} else {
 			return valueFalse
 		}
 	case int:
 		return floatValue(float64(x))
 	case int8:
 		return floatValue(float64(x))
 	case int16:
 		return floatValue(float64(x))
 	case int32:
 		return floatValue(float64(x))
 	case int64:
 		return floatValue(float64(x))
 	case uint:
 		return floatValue(float64(x))
 	case uint8:
 		return floatValue(float64(x))
 	case uint16:
 		return floatValue(float64(x))
 	case uint32:
 		return floatValue(float64(x))
 	case uint64:
 		return floatValue(float64(x))
 	case uintptr:
 		return floatValue(float64(x))
 	case unsafe.Pointer:
 		return floatValue(float64(uintptr(x)))
 	case float32:
 		return floatValue(float64(x))
 	case float64:
 		return floatValue(x)
 	case string:
 		return makeValue(stringVal(x))
 	case []interface{}:
 		a := arrayConstructor.New(len(x))
 		for i, s := range x {
 			a.SetIndex(i, s)
 		}
 		return a
 	case map[string]interface{}:
 		o := objectConstructor.New()
 		for k, v := range x {
 			o.Set(k, v)
 		}
 		return o
 	default:
 		panic("ValueOf: invalid value")
 	}
 }

 func stringVal(x string) ref

 // Type represents the JavaScript type of a Value.
 type Type int

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

 func (t Type) String() string {
 	switch t {
 	case TypeUndefined:
 		return "undefined"
 	case TypeNull:
 		return "null"
 	case TypeBoolean:
 		return "boolean"
 	case TypeNumber:
 		return "number"
 	case TypeString:
 		return "string"
 	case TypeSymbol:
 		return "symbol"
 	case TypeObject:
 		return "object"
 	case TypeFunction:
 		return "function"
 	default:
 		panic("bad type")
 	}
 }

 func (t Type) isObject() bool {
 	return t == TypeObject || t == TypeFunction
 }

 // 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.
 func (v Value) Type() Type {
 	switch v.ref {
 	case valueUndefined.ref:
 		return TypeUndefined
 	case valueNull.ref:
 		return TypeNull
 	case valueTrue.ref, valueFalse.ref:
 		return TypeBoolean
 	}
 	if v.isNumber() {
 		return TypeNumber
 	}
 	typeFlag := v.ref >> 32 & 3
 	switch typeFlag {
 	case 1:
 		return TypeString
 	case 2:
 		return TypeSymbol
 	case 3:
 		return TypeFunction
 	default:
 		return TypeObject
 	}
 }

 // Get returns the JavaScript property p of value v.
 // It panics if v is not a JavaScript object.
 func (v Value) Get(p string) Value {
 	if vType := v.Type(); !vType.isObject() {
 		panic(&ValueError{"Value.Get", vType})
 	}
 	return makeValue(valueGet(v.ref, p))
 }

 func valueGet(v ref, p string) ref

 // Set sets the JavaScript property p of value v to ValueOf(x).
 // It panics if v is not a JavaScript object.
 func (v Value) Set(p string, x interface{}) {
 	if vType := v.Type(); !vType.isObject() {
 		panic(&ValueError{"Value.Set", vType})
 	}
 	valueSet(v.ref, p, ValueOf(x).ref)
 }

 func valueSet(v ref, p string, x ref)

 // Index returns JavaScript index i of value v.
 // It panics if v is not a JavaScript object.
 func (v Value) Index(i int) Value {
 	if vType := v.Type(); !vType.isObject() {
 		panic(&ValueError{"Value.Index", vType})
 	}
 	return makeValue(valueIndex(v.ref, i))
 }

 func valueIndex(v ref, i int) ref

 // SetIndex sets the JavaScript index i of value v to ValueOf(x).
 // It panics if v is not a JavaScript object.
 func (v Value) SetIndex(i int, x interface{}) {
 	if vType := v.Type(); !vType.isObject() {
 		panic(&ValueError{"Value.SetIndex", vType})
 	}
 	valueSetIndex(v.ref, i, ValueOf(x).ref)
 }

 func valueSetIndex(v ref, i int, x ref)

 func makeArgs(args []interface{}) []ref {
 	argVals := make([]ref, len(args))
 	for i, arg := range args {
 		argVals[i] = ValueOf(arg).ref
 	}
 	return argVals
 }

 // Length returns the JavaScript property "length" of v.
 // It panics if v is not a JavaScript object.
 func (v Value) Length() int {
 	if vType := v.Type(); !vType.isObject() {
 		panic(&ValueError{"Value.SetIndex", vType})
 	}
 	return valueLength(v.ref)
 }

 func valueLength(v ref) int

 // 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 (v Value) Call(m string, args ...interface{}) Value {
 	res, ok := valueCall(v.ref, m, makeArgs(args))
 	if !ok {
 		if vType := v.Type(); !vType.isObject() { // check here to avoid overhead in success case
 			panic(&ValueError{"Value.Call", vType})
 		}
 		if propType := v.Get(m).Type(); propType != TypeFunction {
 			panic("syscall/js: Value.Call: property " + m + " is not a function, got " + propType.String())
 		}
 		panic(Error{makeValue(res)})
 	}
 	return makeValue(res)
 }

 func valueCall(v ref, m string, args []ref) (ref, bool)

 // 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 (v Value) Invoke(args ...interface{}) Value {
 	res, ok := valueInvoke(v.ref, makeArgs(args))
 	if !ok {
 		if vType := v.Type(); vType != TypeFunction { // check here to avoid overhead in success case
 			panic(&ValueError{"Value.Invoke", vType})
 		}
 		panic(Error{makeValue(res)})
 	}
 	return makeValue(res)
 }

 func valueInvoke(v ref, args []ref) (ref, bool)

 // 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 (v Value) New(args ...interface{}) Value {
 	res, ok := valueNew(v.ref, makeArgs(args))
 	if !ok {
 		if vType := v.Type(); vType != TypeFunction { // check here to avoid overhead in success case
 			panic(&ValueError{"Value.Invoke", vType})
 		}
 		panic(Error{makeValue(res)})
 	}
 	return makeValue(res)
 }

 func valueNew(v ref, args []ref) (ref, bool)

 func (v Value) isNumber() bool {
 	return v.ref == valueZero.ref ||
 		v.ref == valueNaN.ref ||
 		(v.ref != valueUndefined.ref && v.ref>>32&nanHead != nanHead)
 }

 func (v Value) float(method string) float64 {
 	if !v.isNumber() {
 		panic(&ValueError{method, v.Type()})
 	}
 	if v.ref == valueZero.ref {
 		return 0
 	}
 	return *(*float64)(unsafe.Pointer(&v.ref))
 }

 // Float returns the value v as a float64.
 // It panics if v is not a JavaScript number.
 func (v Value) Float() float64 {
 	return v.float("Value.Float")
 }

 // Int returns the value v truncated to an int.
 // It panics if v is not a JavaScript number.
 func (v Value) Int() int {
 	return int(v.float("Value.Int"))
 }

 // Bool returns the value v as a bool.
 // It panics if v is not a JavaScript boolean.
 func (v Value) Bool() bool {
 	switch v.ref {
 	case valueTrue.ref:
 		return true
 	case valueFalse.ref:
 		return false
 	default:
 		panic(&ValueError{"Value.Bool", v.Type()})
 	}
 }

 // 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 (v Value) Truthy() bool {
 	switch v.Type() {
 	case TypeUndefined, TypeNull:
 		return false
 	case TypeBoolean:
 		return v.Bool()
 	case TypeNumber:
 		return v.ref != valueNaN.ref && v.ref != valueZero.ref
 	case TypeString:
 		return v.String() != ""
 	case TypeSymbol, TypeFunction, TypeObject:
 		return true
 	default:
 		panic("bad type")
 	}
 }

 // 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 (v Value) String() string {
 	switch v.Type() {
 	case TypeString:
 		return jsString(v.ref)
 	case TypeUndefined:
 		return "<undefined>"
 	case TypeNull:
 		return "<null>"
 	case TypeBoolean:
 		return "<boolean: " + jsString(v.ref) + ">"
 	case TypeNumber:
 		return "<number: " + jsString(v.ref) + ">"
 	case TypeSymbol:
 		return "<symbol>"
 	case TypeObject:
 		return "<object>"
 	case TypeFunction:
 		return "<function>"
 	default:
 		panic("bad type")
 	}
 }

 func jsString(v ref) string {
 	str, length := valuePrepareString(v)
 	b := make([]byte, length)
 	valueLoadString(str, b)
 	return string(b)
 }

 func valuePrepareString(v ref) (ref, int)

 func valueLoadString(v ref, b []byte)

 // InstanceOf reports whether v is an instance of type t according to JavaScript's instanceof operator.
 func (v Value) InstanceOf(t Value) bool {
 	return valueInstanceOf(v.ref, t.ref)
 }

 func valueInstanceOf(v ref, t ref) bool

 // 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 (e *ValueError) Error() string {
 	return "syscall/js: call of " + e.Method + " on " + e.Type.String()
 }

 // CopyBytesToGo copies bytes from the Uint8Array src to dst.
 // It returns the number of bytes copied, which will be the minimum of the lengths of src and dst.
 // CopyBytesToGo panics if src is not an Uint8Array.
 func CopyBytesToGo(dst []byte, src Value) int {
 	n, ok := copyBytesToGo(dst, src.ref)
 	if !ok {
 		panic("syscall/js: CopyBytesToGo: expected src to be an Uint8Array")
 	}
 	return n
 }

 func copyBytesToGo(dst []byte, src ref) (int, bool)

 // CopyBytesToJS copies bytes from src to the Uint8Array dst.
 // It returns the number of bytes copied, which will be the minimum of the lengths of src and dst.
 // CopyBytesToJS panics if dst is not an Uint8Array.
 func CopyBytesToJS(dst Value, src []byte) int {
 	n, ok := copyBytesToJS(dst.ref, src)
 	if !ok {
 		panic("syscall/js: CopyBytesToJS: expected dst to be an Uint8Array")
 	}
 	return n
 }

 func copyBytesToJS(dst ref, src []byte) (int, bool)
	// Copyright 2018 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// +build js,wasm

	// 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.
	package js

	import (
	"unsafe"
	)

	// ref is used to identify a JavaScript value, since the value itself can not be passed to WebAssembly.
	//
	// The JavaScript value "undefined" is represented by the value 0.
	// A JavaScript number (64-bit float, except 0 and NaN) is represented by its IEEE 754 binary representation.
	// All other values are represented as an IEEE 754 binary representation of NaN with bits 0-31 used as
	// an ID and bits 32-33 used to differentiate between string, symbol, function and object.
	type ref uint64

	// nanHead are the upper 32 bits of a ref which are set if the value is not encoded as an IEEE 754 number (see above).
	const nanHead = 0x7FF80000

	// 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
	}

	// Value represents a JavaScript value. The zero value is the JavaScript value "undefined".
	type Value struct {
	ref ref
	}

	// JSValue implements Wrapper interface.
	func (v Value) JSValue() Value {
	return v
	}

	func makeValue(v ref) Value {
	return Value{ref: v}
	}

	func predefValue(id uint32) Value {
	return Value{ref: nanHead<<32 \| ref(id)}
	}

	func floatValue(f float64) Value {
	if f == 0 {
	return valueZero
	}
	if f != f {
	return valueNaN
	}
	return Value{ref: (ref)(unsafe.Pointer(&f))}
	}

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

	// Error implements the error interface.
	func (e Error) Error() string {
	return "JavaScript error: " + e.Get("message").String()
	}

	var (
	valueUndefined = Value{ref: 0}
	valueNaN = predefValue(0)
	valueZero = predefValue(1)
	valueNull = predefValue(2)
	valueTrue = predefValue(3)
	valueFalse = predefValue(4)
	valueGlobal = predefValue(5)
	jsGo = predefValue(6) // instance of the Go class in JavaScript

	objectConstructor = valueGlobal.Get("Object")
	arrayConstructor = valueGlobal.Get("Array")
	)

	// Undefined returns the JavaScript value "undefined".
	func Undefined() Value {
	return valueUndefined
	}

	// Null returns the JavaScript value "null".
	func Null() Value {
	return valueNull
	}

	// Global returns the JavaScript global object, usually "window" or "global".
	func Global() Value {
	return valueGlobal
	}

	// 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 ValueOf(x interface{}) Value {
	switch x := x.(type) {
	case Value: // should precede Wrapper to avoid a loop
	return x
	case Wrapper:
	return x.JSValue()
	case nil:
	return valueNull
	case bool:
	if x {
	return valueTrue
	} else {
	return valueFalse
	}
	case int:
	return floatValue(float64(x))
	case int8:
	return floatValue(float64(x))
	case int16:
	return floatValue(float64(x))
	case int32:
	return floatValue(float64(x))
	case int64:
	return floatValue(float64(x))
	case uint:
	return floatValue(float64(x))
	case uint8:
	return floatValue(float64(x))
	case uint16:
	return floatValue(float64(x))
	case uint32:
	return floatValue(float64(x))
	case uint64:
	return floatValue(float64(x))
	case uintptr:
	return floatValue(float64(x))
	case unsafe.Pointer:
	return floatValue(float64(uintptr(x)))
	case float32:
	return floatValue(float64(x))
	case float64:
	return floatValue(x)
	case string:
	return makeValue(stringVal(x))
	case []interface{}:
	a := arrayConstructor.New(len(x))
	for i, s := range x {
	a.SetIndex(i, s)
	}
	return a
	case map[string]interface{}:
	o := objectConstructor.New()
	for k, v := range x {
	o.Set(k, v)
	}
	return o
	default:
	panic("ValueOf: invalid value")
	}
	}

	func stringVal(x string) ref

	// Type represents the JavaScript type of a Value.
	type Type int

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

	func (t Type) String() string {
	switch t {
	case TypeUndefined:
	return "undefined"
	case TypeNull:
	return "null"
	case TypeBoolean:
	return "boolean"
	case TypeNumber:
	return "number"
	case TypeString:
	return "string"
	case TypeSymbol:
	return "symbol"
	case TypeObject:
	return "object"
	case TypeFunction:
	return "function"
	default:
	panic("bad type")
	}
	}

	func (t Type) isObject() bool {
	return t == TypeObject \|\| t == TypeFunction
	}

	// 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.
	func (v Value) Type() Type {
	switch v.ref {
	case valueUndefined.ref:
	return TypeUndefined
	case valueNull.ref:
	return TypeNull
	case valueTrue.ref, valueFalse.ref:
	return TypeBoolean
	}
	if v.isNumber() {
	return TypeNumber
	}
	typeFlag := v.ref >> 32 & 3
	switch typeFlag {
	case 1:
	return TypeString
	case 2:
	return TypeSymbol
	case 3:
	return TypeFunction
	default:
	return TypeObject
	}
	}

	// Get returns the JavaScript property p of value v.
	// It panics if v is not a JavaScript object.
	func (v Value) Get(p string) Value {
	if vType := v.Type(); !vType.isObject() {
	panic(&ValueError{"Value.Get", vType})
	}
	return makeValue(valueGet(v.ref, p))
	}

	func valueGet(v ref, p string) ref

	// Set sets the JavaScript property p of value v to ValueOf(x).
	// It panics if v is not a JavaScript object.
	func (v Value) Set(p string, x interface{}) {
	if vType := v.Type(); !vType.isObject() {
	panic(&ValueError{"Value.Set", vType})
	}
	valueSet(v.ref, p, ValueOf(x).ref)
	}

	func valueSet(v ref, p string, x ref)

	// Index returns JavaScript index i of value v.
	// It panics if v is not a JavaScript object.
	func (v Value) Index(i int) Value {
	if vType := v.Type(); !vType.isObject() {
	panic(&ValueError{"Value.Index", vType})
	}
	return makeValue(valueIndex(v.ref, i))
	}

	func valueIndex(v ref, i int) ref

	// SetIndex sets the JavaScript index i of value v to ValueOf(x).
	// It panics if v is not a JavaScript object.
	func (v Value) SetIndex(i int, x interface{}) {
	if vType := v.Type(); !vType.isObject() {
	panic(&ValueError{"Value.SetIndex", vType})
	}
	valueSetIndex(v.ref, i, ValueOf(x).ref)
	}

	func valueSetIndex(v ref, i int, x ref)

	func makeArgs(args []interface{}) []ref {
	argVals := make([]ref, len(args))
	for i, arg := range args {
	argVals[i] = ValueOf(arg).ref
	}
	return argVals
	}

	// Length returns the JavaScript property "length" of v.
	// It panics if v is not a JavaScript object.
	func (v Value) Length() int {
	if vType := v.Type(); !vType.isObject() {
	panic(&ValueError{"Value.SetIndex", vType})
	}
	return valueLength(v.ref)
	}

	func valueLength(v ref) int

	// 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 (v Value) Call(m string, args ...interface{}) Value {
	res, ok := valueCall(v.ref, m, makeArgs(args))
	if !ok {
	if vType := v.Type(); !vType.isObject() { // check here to avoid overhead in success case
	panic(&ValueError{"Value.Call", vType})
	}
	if propType := v.Get(m).Type(); propType != TypeFunction {
	panic("syscall/js: Value.Call: property " + m + " is not a function, got " + propType.String())
	}
	panic(Error{makeValue(res)})
	}
	return makeValue(res)
	}

	func valueCall(v ref, m string, args []ref) (ref, bool)

	// 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 (v Value) Invoke(args ...interface{}) Value {
	res, ok := valueInvoke(v.ref, makeArgs(args))
	if !ok {
	if vType := v.Type(); vType != TypeFunction { // check here to avoid overhead in success case
	panic(&ValueError{"Value.Invoke", vType})
	}
	panic(Error{makeValue(res)})
	}
	return makeValue(res)
	}

	func valueInvoke(v ref, args []ref) (ref, bool)

	// 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 (v Value) New(args ...interface{}) Value {
	res, ok := valueNew(v.ref, makeArgs(args))
	if !ok {
	if vType := v.Type(); vType != TypeFunction { // check here to avoid overhead in success case
	panic(&ValueError{"Value.Invoke", vType})
	}
	panic(Error{makeValue(res)})
	}
	return makeValue(res)
	}

	func valueNew(v ref, args []ref) (ref, bool)

	func (v Value) isNumber() bool {
	return v.ref == valueZero.ref \|\|
	v.ref == valueNaN.ref \|\|
	(v.ref != valueUndefined.ref && v.ref>>32&nanHead != nanHead)
	}

	func (v Value) float(method string) float64 {
	if !v.isNumber() {
	panic(&ValueError{method, v.Type()})
	}
	if v.ref == valueZero.ref {
	return 0
	}
	return (float64)(unsafe.Pointer(&v.ref))
	}

	// Float returns the value v as a float64.
	// It panics if v is not a JavaScript number.
	func (v Value) Float() float64 {
	return v.float("Value.Float")
	}

	// Int returns the value v truncated to an int.
	// It panics if v is not a JavaScript number.
	func (v Value) Int() int {
	return int(v.float("Value.Int"))
	}

	// Bool returns the value v as a bool.
	// It panics if v is not a JavaScript boolean.
	func (v Value) Bool() bool {
	switch v.ref {
	case valueTrue.ref:
	return true
	case valueFalse.ref:
	return false
	default:
	panic(&ValueError{"Value.Bool", v.Type()})
	}
	}

	// 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 (v Value) Truthy() bool {
	switch v.Type() {
	case TypeUndefined, TypeNull:
	return false
	case TypeBoolean:
	return v.Bool()
	case TypeNumber:
	return v.ref != valueNaN.ref && v.ref != valueZero.ref
	case TypeString:
	return v.String() != ""
	case TypeSymbol, TypeFunction, TypeObject:
	return true
	default:
	panic("bad type")
	}
	}

	// 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 (v Value) String() string {
	switch v.Type() {
	case TypeString:
	return jsString(v.ref)
	case TypeUndefined:
	return "<undefined>"
	case TypeNull:
	return "<null>"
	case TypeBoolean:
	return "<boolean: " + jsString(v.ref) + ">"
	case TypeNumber:
	return "<number: " + jsString(v.ref) + ">"
	case TypeSymbol:
	return "<symbol>"
	case TypeObject:
	return "<object>"
	case TypeFunction:
	return "<function>"
	default:
	panic("bad type")
	}
	}

	func jsString(v ref) string {
	str, length := valuePrepareString(v)
	b := make([]byte, length)
	valueLoadString(str, b)
	return string(b)
	}

	func valuePrepareString(v ref) (ref, int)

	func valueLoadString(v ref, b []byte)

	// InstanceOf reports whether v is an instance of type t according to JavaScript's instanceof operator.
	func (v Value) InstanceOf(t Value) bool {
	return valueInstanceOf(v.ref, t.ref)
	}

	func valueInstanceOf(v ref, t ref) bool

	// 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 (e *ValueError) Error() string {
	return "syscall/js: call of " + e.Method + " on " + e.Type.String()
	}

	// CopyBytesToGo copies bytes from the Uint8Array src to dst.
	// It returns the number of bytes copied, which will be the minimum of the lengths of src and dst.
	// CopyBytesToGo panics if src is not an Uint8Array.
	func CopyBytesToGo(dst []byte, src Value) int {
	n, ok := copyBytesToGo(dst, src.ref)
	if !ok {
	panic("syscall/js: CopyBytesToGo: expected src to be an Uint8Array")
	}
	return n
	}

	func copyBytesToGo(dst []byte, src ref) (int, bool)

	// CopyBytesToJS copies bytes from src to the Uint8Array dst.
	// It returns the number of bytes copied, which will be the minimum of the lengths of src and dst.
	// CopyBytesToJS panics if dst is not an Uint8Array.
	func CopyBytesToJS(dst Value, src []byte) int {
	n, ok := copyBytesToJS(dst.ref, src)
	if !ok {
	panic("syscall/js: CopyBytesToJS: expected dst to be an Uint8Array")
	}
	return n
	}

	func copyBytesToJS(dst ref, src []byte) (int, bool)