syscall/js: add the Value.Truthy method

Truthy returns the JavaScript "truthiness" of the given value.  In
JavaScript, false, 0, "", null, undefined, and NaN are "falsy", and
everything else is "truthy".

Fixes #28264

Change-Id: I4586f98646c05a4147d06a7c4a5d9c61d956fc83
GitHub-Last-Rev: 649b353ebc
GitHub-Pull-Request: golang/go#28358
Reviewed-on: https://go-review.googlesource.com/c/144384
Reviewed-by: Richard Musiol <neelance@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/js/js.go

@@ -370,6 +370,26 @@ func (v Value) Bool() 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 (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 converted to string according to JavaScript type conversions.
 func (v Value) String() string {
 	str, length := valuePrepareString(v.ref)

src/syscall/js/js_test.go

@@ -4,6 +4,15 @@
 
 // +build js,wasm
 
+// To run these tests:
+//
+// - Install Node
+// - Add /path/to/go/misc/wasm to your $PATH (so that "go test" can find
+//   "go_js_wasm_exec").
+// - GOOS=js GOARCH=wasm go test
+//
+// See -exec in "go help test", and "go help run" for details.
+
 package js_test
 
 import (
@@ -19,11 +28,19 @@ var dummys = js.Global().Call("eval", `({
 	someInt: 42,
 	someFloat: 42.123,
 	someArray: [41, 42, 43],
+	someDate: new Date(),
 	add: function(a, b) {
 		return a + b;
 	},
 	zero: 0,
+	stringZero: "0",
 	NaN: NaN,
+	emptyObj: {},
+	emptyArray: [],
+	Infinity: Infinity,
+	NegInfinity: -Infinity,
+	objNumber0: new Number(0),
+	objBooleanFalse: new Boolean(false),
 })`)
 
 func TestBool(t *testing.T) {
@@ -331,3 +348,40 @@ func ExampleNewCallback() {
 	})
 	js.Global().Get("document").Call("getElementById", "myButton").Call("addEventListener", "click", cb)
 }
+
+// See
+// - https://developer.mozilla.org/en-US/docs/Glossary/Truthy
+// - https://stackoverflow.com/questions/19839952/all-falsey-values-in-javascript/19839953#19839953
+// - http://www.ecma-international.org/ecma-262/5.1/#sec-9.2
+func TestTruthy(t *testing.T) {
+	want := true
+	for _, key := range []string{
+		"someBool", "someString", "someInt", "someFloat", "someArray", "someDate",
+		"stringZero", // "0" is truthy
+		"add",        // functions are truthy
+		"emptyObj", "emptyArray", "Infinity", "NegInfinity",
+		// All objects are truthy, even if they're Number(0) or Boolean(false).
+		"objNumber0", "objBooleanFalse",
+	} {
+		if got := dummys.Get(key).Truthy(); got != want {
+			t.Errorf("%s: got %#v, want %#v", key, got, want)
+		}
+	}
+
+	want = false
+	if got := dummys.Get("zero").Truthy(); got != want {
+		t.Errorf("got %#v, want %#v", got, want)
+	}
+	if got := dummys.Get("NaN").Truthy(); got != want {
+		t.Errorf("got %#v, want %#v", got, want)
+	}
+	if got := js.ValueOf("").Truthy(); got != want {
+		t.Errorf("got %#v, want %#v", got, want)
+	}
+	if got := js.Null().Truthy(); got != want {
+		t.Errorf("got %#v, want %#v", got, want)
+	}
+	if got := js.Undefined().Truthy(); got != want {
+		t.Errorf("got %#v, want %#v", got, want)
+	}
+}