syscall/js: turn constant package vars into functions

This is so the values can not be changed and the type is easy to see.

Requested on https://go-review.googlesource.com/c/go/+/120561.

Change-Id: If2ed48ca3ba8874074687bfb2375d2f5592e8e0d
Reviewed-on: https://go-review.googlesource.com/120564
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

3 files changed, 38 insertions, 32 deletions

diff --git a/src/syscall/js/callback.go b/src/syscall/js/callback.go
index 2c693240fa..cfcce693cb 100644
--- a/src/syscall/js/callback.go
+++ b/src/syscall/js/callback.go
@@ -8,9 +8,9 @@ package js
 
 import "sync"
 
-var pendingCallbacks = Global.Get("Array").New()
+var pendingCallbacks = Global().Get("Array").New()
 
-var makeCallbackHelper = Global.Call("eval", `
+var makeCallbackHelper = Global().Call("eval", `
 	(function(id, pendingCallbacks, resolveCallbackPromise) {
 		return function() {
 			pendingCallbacks.push({ id: id, args: arguments });
@@ -19,7 +19,7 @@ var makeCallbackHelper = Global.Call("eval", `
 	})
 `)
 
-var makeEventCallbackHelper = Global.Call("eval", `
+var makeEventCallbackHelper = Global().Call("eval", `
 	(function(preventDefault, stopPropagation, stopImmediatePropagation, fn) {
 		return function(event) {
 			if (preventDefault) {
@@ -118,7 +118,7 @@ func callbackLoop() {
 		sleepUntilCallback()
 		for {
 			cb := pendingCallbacks.Call("shift")
-			if cb == Undefined {
+			if cb == Undefined() {
 				break
 			}
 
@@ -127,7 +127,7 @@ func callbackLoop() {
 			f, ok := callbacks[id]
 			callbacksMu.Unlock()
 			if !ok {
-				Global.Get("console").Call("error", "call to closed callback")
+				Global().Get("console").Call("error", "call to closed callback")
 				continue
 			}
 
diff --git a/src/syscall/js/js.go b/src/syscall/js/js.go
index cbd0730c64..93c3965246 100644
--- a/src/syscall/js/js.go
+++ b/src/syscall/js/js.go
@@ -39,23 +39,29 @@ func (e Error) Error() string {
 }
 
 var (
-	// Undefined is the JavaScript value "undefined". The zero Value equals to Undefined.
-	Undefined = makeValue(0)
-
-	// Null is the JavaScript value "null".
-	Null = makeValue(1)
+	valueUndefined         = makeValue(0)
+	valueNull              = makeValue(1)
+	valueGlobal            = makeValue(2)
+	memory                 = makeValue(3) // WebAssembly linear memory
+	resolveCallbackPromise = makeValue(4) // function that the callback helper uses to resume the execution of Go's WebAssembly code
+)
 
-	// Global is the JavaScript global object, usually "window" or "global".
-	Global = makeValue(2)
+// Undefined returns the JavaScript value "undefined".
+func Undefined() Value {
+	return valueUndefined
+}
 
-	// memory is the WebAssembly linear memory.
-	memory = makeValue(3)
+// Null returns the JavaScript value "null".
+func Null() Value {
+	return valueNull
+}
 
-	// resolveCallbackPromise is a function that the callback helper uses to resume the execution of Go's WebAssembly code.
-	resolveCallbackPromise = makeValue(4)
-)
+// Global returns the JavaScript global object, usually "window" or "global".
+func Global() Value {
+	return valueGlobal
+}
 
-var uint8Array = Global.Get("Uint8Array")
+var uint8Array = valueGlobal.Get("Uint8Array")
 
 // ValueOf returns x as a JavaScript value.
 func ValueOf(x interface{}) Value {
@@ -65,7 +71,7 @@ func ValueOf(x interface{}) Value {
 	case Callback:
 		return x.enqueueFn
 	case nil:
-		return Null
+		return valueNull
 	case bool:
 		return makeValue(boolVal(x))
 	case int:
diff --git a/src/syscall/js/js_test.go b/src/syscall/js/js_test.go
index 53d21a3f4f..e5e950f3a3 100644
--- a/src/syscall/js/js_test.go
+++ b/src/syscall/js/js_test.go
@@ -12,7 +12,7 @@ import (
 	"testing"
 )
 
-var dummys = js.Global.Call("eval", `({
+var dummys = js.Global().Call("eval", `({
 	someBool: true,
 	someString: "abc\u1234",
 	someInt: 42,
@@ -90,16 +90,16 @@ func TestFloat(t *testing.T) {
 }
 
 func TestUndefined(t *testing.T) {
-	dummys.Set("test", js.Undefined)
-	if dummys == js.Undefined || dummys.Get("test") != js.Undefined || dummys.Get("xyz") != js.Undefined {
+	dummys.Set("test", js.Undefined())
+	if dummys == js.Undefined() || dummys.Get("test") != js.Undefined() || dummys.Get("xyz") != js.Undefined() {
 		t.Errorf("js.Undefined expected")
 	}
 }
 
 func TestNull(t *testing.T) {
 	dummys.Set("test1", nil)
-	dummys.Set("test2", js.Null)
-	if dummys == js.Null || dummys.Get("test1") != js.Null || dummys.Get("test2") != js.Null {
+	dummys.Set("test2", js.Null())
+	if dummys == js.Null() || dummys.Get("test1") != js.Null() || dummys.Get("test2") != js.Null() {
 		t.Errorf("js.Null expected")
 	}
 }
@@ -128,7 +128,7 @@ func TestCall(t *testing.T) {
 	if got := dummys.Call("add", i, 2).Int(); got != 42 {
 		t.Errorf("got %#v, want %#v", got, 42)
 	}
-	if got := dummys.Call("add", js.Global.Call("eval", "40"), 2).Int(); got != 42 {
+	if got := dummys.Call("add", js.Global().Call("eval", "40"), 2).Int(); got != 42 {
 		t.Errorf("got %#v, want %#v", got, 42)
 	}
 }
@@ -141,17 +141,17 @@ func TestInvoke(t *testing.T) {
 }
 
 func TestNew(t *testing.T) {
-	if got := js.Global.Get("Array").New(42).Length(); got != 42 {
+	if got := js.Global().Get("Array").New(42).Length(); got != 42 {
 		t.Errorf("got %#v, want %#v", got, 42)
 	}
 }
 
 func TestInstanceOf(t *testing.T) {
-	someArray := js.Global.Get("Array").New()
-	if got, want := someArray.InstanceOf(js.Global.Get("Array")), true; got != want {
+	someArray := js.Global().Get("Array").New()
+	if got, want := someArray.InstanceOf(js.Global().Get("Array")), true; got != want {
 		t.Errorf("got %#v, want %#v", got, want)
 	}
-	if got, want := someArray.InstanceOf(js.Global.Get("Function")), false; got != want {
+	if got, want := someArray.InstanceOf(js.Global().Get("Function")), false; got != want {
 		t.Errorf("got %#v, want %#v", got, want)
 	}
 }
@@ -165,7 +165,7 @@ func TestCallback(t *testing.T) {
 		c <- struct{}{}
 	})
 	defer cb.Close()
-	js.Global.Call("setTimeout", cb, 0, 42)
+	js.Global().Call("setTimeout", cb, 0, 42)
 	<-c
 }
 
@@ -186,7 +186,7 @@ func TestEventCallback(t *testing.T) {
 		})
 		defer cb.Close()
 
-		event := js.Global.Call("eval", fmt.Sprintf("({ called: false, %s: function() { this.called = true; } })", name))
+		event := js.Global().Call("eval", fmt.Sprintf("({ called: false, %s: function() { this.called = true; } })", name))
 		js.ValueOf(cb).Invoke(event)
 		if !event.Get("called").Bool() {
 			t.Errorf("%s not called", name)
@@ -202,5 +202,5 @@ func ExampleNewCallback() {
 		fmt.Println("button clicked")
 		cb.Close() // close the callback if the button will not be clicked again
 	})
-	js.Global.Get("document").Call("getElementById", "myButton").Call("addEventListener", "click", cb)
+	js.Global().Get("document").Call("getElementById", "myButton").Call("addEventListener", "click", cb)
 }