Execution context of setTimeout (JavaScript) - Stack Overflow

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1) Can someone illustrate how setTimeout works in terms of execution threads.

Consider:

function foo() { alert('foo'); }
function bar() { alert('bar'); }  
setTimeout(foo,1000);
bar();

or

function foo() { alert('foo'); setTimeout(foo,1000); }
function bar() { alert('bar'); }  
setTimeout(foo,1000);
bar();

or

function foo() { alert('foo'); setTimeout(foo,1000); }
function bar() { /* an execution that runs with unknown time */ }  
setTimeout(foo,1000);
bar();

or

function foo() { alert('foo'); setTimeout(foo,1000); }
function bar() { /* some ajax call that reply with unknown time */ }  
setTimeout(foo,1000);
bar();

or

function foo() { alert('foo'); setTimeout(foo,1000); }
function bar() { alert('foo'); setTimeout(bar,1000); }  
setTimeout(foo,1000);
setTimeout(bar,1000);

2) Can someone explain how why "this" object doesn't work in setTimeout and what we can do to get around that problem?

1) Can someone illustrate how setTimeout works in terms of execution threads.

Consider:

function foo() { alert('foo'); }
function bar() { alert('bar'); }  
setTimeout(foo,1000);
bar();

or

function foo() { alert('foo'); setTimeout(foo,1000); }
function bar() { alert('bar'); }  
setTimeout(foo,1000);
bar();

or

function foo() { alert('foo'); setTimeout(foo,1000); }
function bar() { /* an execution that runs with unknown time */ }  
setTimeout(foo,1000);
bar();

or

function foo() { alert('foo'); setTimeout(foo,1000); }
function bar() { /* some ajax call that reply with unknown time */ }  
setTimeout(foo,1000);
bar();

or

function foo() { alert('foo'); setTimeout(foo,1000); }
function bar() { alert('foo'); setTimeout(bar,1000); }  
setTimeout(foo,1000);
setTimeout(bar,1000);

2) Can someone explain how why "this" object doesn't work in setTimeout and what we can do to get around that problem?

• javascript
• settimeout

Share Improve this question Follow edited Dec 13, 2012 at 23:28 Tony Stark asked Dec 13, 2012 at 23:01 Tony StarkTony Stark 2111 silver badge33 bronze badges 4

• 4 All of those calls to setTimeout will do nothing at all. You're passing in "foo" as a string, which will be interpreted as meaning you want a function whose body looks like the string contents. – Pointy Commented Dec 13, 2012 at 23:04
• 1 Take a look at the MDN: developer.mozilla/en-US/docs/DOM/… – Blender Commented Dec 13, 2012 at 23:04
• The context of setTimeout is window, to use this from the function cache it before. – elclanrs Commented Dec 13, 2012 at 23:05
• 1) You are alerted bar, then foo. 2) You are alerted bar, then foo every second. However setInterval(foo, 100) would do the same thing better. 3) bar fully executes, and after 10 seconds foo executes. A timer pushes something to the bottom of the priority queue. 4) Again, foo will wait until bar has finished execution. – Liam Bigelow Commented Dec 13, 2012 at 23:05

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2 Answers 2

Sorted by: Reset to default 9

Do read the article suggested by @DaveAnderson.

As to the other stuff, the setTimeout/setInterval has two forms:

setTimeout(arg, timeout)

If arg is a string, then it's treated as the source code to be executed. This is as bad as eval(). Avoid it.

If arg is a function, then it's executed in the global context:

var Test = function () {
    this.x = 1;
    setTimeout(function () {
        console.log('x: ' + this.x);
    }, 10);
};

var t = new Test();

Prints x: undefined.

So what you wanted to do is:

function foo() { alert('foo'); }
setTimeout('foo()', 1000);

or better:

setTimeout(foo, 1000);

To fix the context of the function, use the bind method:

var Test = function () {
    this.x = 1;
    var f = function () {
        console.log('x: ' + this.x);
    };
    setTimeout(f.bind(this), 10);         // use this as the context
};

var t = new Test();

or do it manually:

var Test = function () {
    this.x = 1;
    var that = this;
    setTimeout(function () {
        console.log('x: ' + that.x);     // closure: closing over that
    }, 10);
};

var t = new Test();

As far as I could recall:

var me = this;
me.f();

(this may change its meaning in another context).

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