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

I created a basic test case for this:

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

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?

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

I created a basic test case for this: http://jsperf./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./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?

• javascript
• function
• performance

Share Improve this question Follow edited May 23, 2017 at 12:33 CommunityBot 111 silver badge asked Mar 25, 2011 at 14:35 AnishAnish 1,16444 gold badges1515 silver badges2828 bronze badges 6

• 3 What browser or javascript engine are you using? – Daniel A. White Commented Mar 25, 2011 at 14:37
• Its actually faster in firefox. – Daniel A. White Commented Mar 25, 2011 at 14:38
• 3 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 bees an issue, you are doing something wrong. – idbentley Commented Mar 25, 2011 at 14:40
• @idbentley "if you are declaring enough functions that this bees an issue, you are doing something wrong". A case like jsperf./function-declaration-vs-function-expression-2 is very mon in codes nowadays right? Yes, there are many other important stuffs to be considered while optimizing. But shouldn't this also be considered? – Anish Commented Mar 25, 2011 at 15:33
• Sure, doing some like that is mon, but doing it enough times that it will have a noticeable effect on performance is unlikely. It is for example nothing pared 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 Commented Mar 25, 2011 at 17:42

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2 Answers 2

Sorted by: Reset to default 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 piled 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-pile 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.

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

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