How do I ensure that Javascript's “this” will refer to the object when using setTimeout?
Question
<!doctype html>
<html>
<head>
<title>What is 'this'?</title>
<script>
function Obj(){
log('Obj instantiated');
}
Obj.prototype.foo = function (){
log('foo() says:');
log(this);
}
Obj.prototype.bar = function (){
log('bar() was triggered');
setTimeout(this.foo,300);
}
function log(v){console.log(v)}
var obj = new Obj();
</script>
</head>
<body>
<button onclick="obj.foo()">Foo</button>
<button onclick="obj.bar()">Bar</button>
</body>
</html>
And here is the console output:
Obj instantiated
foo() says:
Obj {foo: function, bar: function}
bar() was triggered
foo() says:
Window {top: Window, window: Window, location: Location, external: Object, chrome: Object…}
So when it calls obj.foo from the setTimeout, 'this' changes ownership to the Window. How do I prevent that or properly work with that behavior?
Thanks
3 answers
solution1
7 ACCPTED 2014-01-01 21:45:54
Use a .bind call.
setTimeout(this.foo.bind(this),300);
Not all browsers support it, but there is a shim on MDN and Underscore has _.bind(...) as well
solution2
3 2014-01-01 21:56:41
The answers using bind are the best, most modern way, of handling this. But if you have to support older environments and don't want to shim Function.prototype.bind , then you could also do it like this:
Obj.prototype.bar = function (){
log('bar() was triggered');
var ob = this;
setTimeout(function() {ob.foo();}, 300);
}
solution3
0 2014-01-01 21:47:16
使用像这样的bind方法
setTimeout(this.foo.bind(this),300);
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