Function declaration vs expression from a performance point of view?

Question

There's many javascript articles on the web comparing function declarations function foo (){} to function expressions var foo = function () {}. They usually mention things like hoisting, behaviour within conditionals, etc... But I don't recall any of them talking about performance. Is there any difference? Particularily in ECMA5's strict-mode (if that changes anything).

By performance I of course mean execution performance (including lookup, scope traversal, etc..) not declaration performance, although that would be a nice-to-know aswell.

"By performance I of course mean execution performance (including lookup, scope traversal, etc..) " There is none. Both definitions create the same kind of function object. — Felix Kling
– Felix Kling, Commented Jan 28, 2015 at 6:39
The only performance issues I could think of is from using NFE's in certain engines — kangax.github.io/nfe/#jscript-memory-management — kangax
– kangax, Commented Jan 28, 2015 at 13:10

Tushar Shahi · Accepted Answer · 2024-03-24 08:52:37Z

1

Function declarations are faster in cases where there's a potential for the function expression to be evaluated multiple times. For example, in code that is called in a loop a hoisted function is only evaluated once, but an expression would be evaluated each time. Besides that, there's no meaningful difference.

Whenever you have a question about a JavaScript performance issue, I recommend checking out JSPerf. Also, Google to see if someone already made one for your question:

edited Mar 24, 2024 at 8:52

Tushar Shahi

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answered Jan 28, 2015 at 6:18

Bjorn

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4 Comments

Wingblade Over a year ago

I literally googled for this everywhere both today and yesterday, skimming through many articles on the web. Yet somehow I completely forgot about checking for a jsperf. Also thanks for the short summary in your answer. +1, accepted

Bergi Over a year ago

Well, a function declaration in a loop would be a syntax error, so you don't really have that choice.

Bergi Over a year ago

@BjornTipling: Use strict mode, and possibly a browser that doesn't know quirky function statements

kangax Over a year ago

I was confused by the "Function declaration & expression" in that test, until I realized it's a poor, misunderstood named function expression...

deblocker · Accepted Answer · 2015-05-26 00:40:01Z

0

I executed the same tests from JSPerf in Chrome canary 45 and Firefox 37, sadly the results are opposite:

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

Chrome: fastest, FF: much slower

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

myfunc();

FF: fastest, Crome: much slower

So, the answer is: it depends from the browser/JS engine.

answered May 26, 2015 at 0:40

deblocker

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