Question

82

setTimeout callback argument

rated 0 times [ 89] [ 7] / answers: 1 / hits: 98602 / 13 Years ago, sat, april 2, 2011, 12:00:00

Let's consider this piece of JavaScript:

function Person(name) {

    this.name = name;

}



Person.prototype.showName = function() {

    alert(this.name);

}





var mike = new Person(mike);

//mike.showName();  



window.name = window;

I don't understand the difference between the behavior of

setTimeout(mike.showName(), 5000);

and

setTimeout(function(){

    mike.showName();

}, 5000);

Why is the behavior different? It really confuses me. Thanks.

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andrewb

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answered 13 Years ago alejandro · Accepted Answer

Your question really has nothing at all to do with setTimeout. You simply need to understand the difference between a function call and a reference to a function.

Consider these four assignments:

var one = function() { mike.showName(); };

var two = mike.showName;

var three = mike.showName();

var four = (function() { mike.showName(); })();

The first two assign a reference to a function to their respective variables. The last two, however, call functions (that's what the parens are for) and assign their return values to the vars on the left-hand side.

How this relates to setTimeout:

The setTimeout function expects as its first argument a reference to a function, so either one or two above would be correct, but three and four would not. However, it is important to note that it is not, strictly speaking, a mistake to pass the return value of a function to setTimeout, although you'll frequently see that said.

This is perfectly fine, for example:

function makeTimeoutFunc(param) {

    return function() {

        // does something with param

    }

}



setTimeout(makeTimeoutFunc(), 5000);

It has nothing to do with how setTimeout receives a function as its argument, but that it does.