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: 649b353ebc23b09d840faf927a2eeca41ee164bf
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>

diff --git a/src/syscall/js/js.go b/src/syscall/js/js.go
index dc741e79a7e42199466bd8748e78277cf1bf2136..19cdedc309ad6e63204df2b4d5e3975af4bb83b0 100644 (file)
--- a/src/syscall/js/js.go
+++ b/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)

diff --git a/src/syscall/js/js_test.go b/src/syscall/js/js_test.go
index ed39fe3714187a394e04673846e2c28ff877d8e3..73d112a2e8f51c23912a7f373c73c57f74536856 100644 (file)
--- a/src/syscall/js/js_test.go
+++ b/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)
+       }
+}