function declaration faster than function expression?

Question

The difference b/w function declaration & function expression is beautifully described in var functionName = function() {} vs function functionName() {}
In this it's mentioned that function declaration is evaluated during parse-time, & function expression is evaluated in the execution-phase

In bytes.com it's mentioned that function declaration is faster than function expression.

I created a basic test case for this: http://jsperf.com/function-declaration-vs-function-expression

Function Declaration:

function myfunc() {
 alert("yo");
}
myfunc();

Function Expression:

var myfunc = function() {
 alert("yo");
}
myfunc();

The test showed that function expression is 90% slower than function declaration.

Why such a difference in speed?

Edit:
From the results in http://jsperf.com/function-declaration-vs-function-expression

In Chrome, IE9, Opera & Safari -> Function Declaration is faster than Function Expression

In Firefox, IE7, IE8 -> Function Expression is faster than Function Declaration

In IE9 Function declaration is faster, whereas in IE 7 & 8 function expression is faster. Is it because of change in JavaScript engine in IE9, or was this move intentional?

Those results are pretty interesting, while it is 90% slower in Chrome & Opera, it's actually faster in FF. I'd say that it doesn't matter too much though, because if you are declaring enough functions that this becomes an issue, you are doing something wrong. — idbentley
– idbentley, Commented Mar 25, 2011 at 14:40
@idbentley "if you are declaring enough functions that this becomes an issue, you are doing something wrong". A case like jsperf.com/function-declaration-vs-function-expression-2 is very common in codes nowadays right? Yes, there are many other important stuffs to be considered while optimizing. But shouldn't this also be considered? — Anish
– Anish, Commented Mar 25, 2011 at 15:33
Sure, doing some like that is common, but doing it enough times that it will have a noticeable effect on performance is unlikely. It is for example nothing compared to doing an AJAX request, or a DOM manipulation. My statement before intended to say that doing a large number of function declarations (enough to cause performance effects) is a code smell, and you should consider refactoring. — idbentley
– idbentley, Commented Mar 25, 2011 at 17:42

Marcel Korpel · Accepted Answer · 2011-03-25 15:18:19Z

2

Firefox also has non-standard Function Statements, which makes it possible to conditionally choose between function declarations (per spec, you can't). Just using an example of Juriy "kangax" Zaytsev:

if (true) {
    function foo(){ return 1; }
} else {
    function foo(){ return 2; }
}
foo(); // 1
// Note that other clients interpet `foo` as function declaration here, 
// overwriting first `foo` with the second one, and producing "2", not "1" as a result

So those are compiled at execution time, instead of in the parse-phase:

Function statements are NOT declared during variable instantiation. They are declared at run time, just like function expressions.

Other browsers probably will pre-compile function declarations, making them perform faster at run time, but Firefox has to interpret function declarations at run time, causing not much difference between the speed of function declarations and function expressions in Gecko-based browsers.

answered Mar 25, 2011 at 15:18

Marcel Korpel

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4 Comments

Anish Over a year ago

"Firefox has to interpret function declarations at run time" That's bad. is it fixed in the new versions?

Marcel Korpel Over a year ago

@Anish: Fixed? It's a feature: "JavaScript 1.5, NES 6.0: Added conditional function declarations (Netscape extension)". That said, the team is working to make the JS engine as fast as possible.

Anish Over a year ago

From kangax's article I understand that Firefox interprets function declarations at run time only if they're a part of function statement. When function declarations are defined normally, I guess they're defined during parse time?

Marcel Korpel Over a year ago

@Anish: You must be right, as function statements are only declared at run time, but I can still use function declarations at the bottom of my script, which means there must be a functional difference between the two. I must have been trapped by the syntax. Thanks for pointing this out!

Mike Park · Accepted Answer · 2011-03-25 14:40:36Z

1

This has to be a browser dependent issue. Some browsers may compile the javascript differently if it's an expression rather than a declaration and some might treat them exactly the same.

answered Mar 25, 2011 at 14:40

Mike Park

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