syscall/js: extend ValueOf to support arrays and objects

This commits adds []interface{} and map[string]interface{} as quick
ways to create JavaScript arrays and objects. They correspond to the
JavaScript notations [...] and {...}. A type alias can be used for
a concise notation.

Change-Id: I98bb08dbef1e0f3bd3d65c732d6b09e1520026ba
Reviewed-on: https://go-review.googlesource.com/126855
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/js/js.go

@@ -64,6 +64,9 @@ var (
	valueGlobal    = predefValue(5)
	memory         = predefValue(6) // WebAssembly linear memory
	jsGo           = predefValue(7) // instance of the Go class in JavaScript

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

// Undefined returns the JavaScript value "undefined".
@@ -83,15 +86,17 @@ func Global() Value {

// ValueOf returns x as a JavaScript value:
//
//  | Go                    | JavaScript            |
//  | --------------------- | --------------------- |
//  | js.Value              | [its value]           |
//  | js.TypedArray         | [typed array]         |
//  | js.Callback           | function              |
//  | nil                   | null                  |
//  | bool                  | boolean               |
//  | integers and floats   | number                |
//  | string                | string                |
//  | Go                     | JavaScript             |
//  | ---------------------- | ---------------------- |
//  | js.Value               | [its value]            |
//  | js.TypedArray          | typed array            |
//  | js.Callback            | function               |
//  | nil                    | null                   |
//  | bool                   | boolean                |
//  | integers and floats    | number                 |
//  | string                 | string                 |
//  | []interface{}          | new array              |
//  | map[string]interface{} | new object             |
func ValueOf(x interface{}) Value {
	switch x := x.(type) {
	case Value:
@@ -138,6 +143,18 @@ func ValueOf(x interface{}) Value {
		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")
	}

src/syscall/js/js_test.go

@@ -254,6 +254,21 @@ func TestType(t *testing.T) {
	}
}

type object = map[string]interface{}
type array = []interface{}

func TestValueOf(t *testing.T) {
	a := js.ValueOf(array{0, array{0, 42, 0}, 0})
	if got := a.Index(1).Index(1).Int(); got != 42 {
		t.Errorf("got %v, want %v", got, 42)
	}

	o := js.ValueOf(object{"x": object{"y": 42}})
	if got := o.Get("x").Get("y").Int(); got != 42 {
		t.Errorf("got %v, want %v", got, 42)
	}
}

func TestCallback(t *testing.T) {
	c := make(chan struct{})
	cb := js.NewCallback(func(args []js.Value) {