Accessing javascript function properties before they are defined?

Question

I was reading through this thread: Hidden Features of JavaScript? and found this post: Hidden Features of JavaScript?

I was playing around with the code in firebug and I found that this bit of code seems to work fine:

var fn = function(x) {
   console.log(this.foo);
}
fn.foo = 1;

How come I can access the property of the function before it's assigned?

score 2 · Accepted Answer · answered Mar 03 '10 at 14:27

2

The return value 1 is not from console.log(this.foo),

Its from fn.foo = 1, firebug just return last value in its console

Try this, you will see also 1 too

fn=function(){}
fn.foo = 1;

answered Mar 03 '10 at 14:27

YOU

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Oh I see. So is this a firebug glitch or is this javascript doing something funky? – Yansky Mar 03 '10 at 14:32
@Yansky: Neither. It's feature-by-design. The last return value is always printed. – kennytm Mar 03 '10 at 14:32

score 1 · Answer 2 · answered Mar 03 '10 at 14:30

Because that function isn't executed when you assign it to fn. A variable is resolved in execution time.

Even without the fn.foo = 1; line, getting an undefined property from an object returns undefined anyway. That's not an error.

Also, this.foo will not print 1 when you run fn(), because this inside the function does not point to the function fn, but window, or the instance that receives a new fn().

score 0 · Answer 3 · answered Mar 03 '10 at 14:26

0

if it was not defined you could access it anyway, it would just have the value undefined.

I guess you call fn(x) after the fn.foo = 1; statement. if you call it before, you will log undefined after, you log 1.

answered Mar 03 '10 at 14:26

Mathieu

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Accessing javascript function properties before they are defined?

3 Answers3