Efficiency of calling array.length in javascript

Question

Most looping examples I see in javascript use the array.length property in the for loop itself, like so:

var numbers = [1,2,3,4,5];
for (var i = 0; i < numbers.length; i++) {
  // do something
}

However, sometimes I see the array.length property being written to a variable, and then the variable value is used in the loop, instead:

var numbers = [1,2,3,4,5];
var len = numbers.length;
for (var i = 0; i < len; i++) {
  // do something
}

Coming from a C# background, I've never had to worry about this. However, in Javascript is the second method more efficient and why?

you might want to take a look at this: jsperf.com/fastest-array-loops-in-javascript/30 — Ben Jackson
– Ben Jackson, Commented Nov 3, 2013 at 12:30
Note that aside from efficiency, these two differ in terms of what happens if the array changes length during the loop. — Stuart
– Stuart, Commented Nov 3, 2013 at 12:39

Isaiah Taylor · Accepted Answer · 2013-11-03 12:30:59Z

1

The second is more efficient because it does not have to make an operation on anything. In the first, it just has to reference a data cell each time.

answered Nov 3, 2013 at 12:30

Isaiah Taylor

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