Is javascript str.length calculated every time it is called or just once?

Question

Which way is more efficient? Is there a difference?

This one:

var str = 'abc';

if(str.length == 20) {
    //...
}

if(str.length == 25) {
    //...
}

// and so on

Or this one:

var str = 'abc';
var length = str.length;

if(length == 20) {
    //...
}

if(length == 25) {
    //...
}

// and so on

Answer 1

In the browsers where this might actually matter (read: IE) it will be calculated every time, so it's faster to store the value in a local variable.

http://jsperf.com/string-length

---

It used to be that

var len = someArray.length;
for (var i=0; i<len; i++) {
    // ...
}

was faster than

for (var i=0; i<someArray.length; i++) {
    // ...
}

but these days, V8's (Chrome's JS engine) optimizes the latter to run faster than the former. That's great - just remember, you don't really need to worry about performance in Chrome.

---

If you're curious to learn more about JavaScript performance, High Performance JavaScript is a solid read. Take its recommendations with a grain of salt, though, since a trick that makes code run faster in IE (6, 7, 8 or even 9) might very well make the code run slower in Chrome or Firefox 4.