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Array Iteration and Properties

Although arrays in JavaScript are objects, there are no good reasons to use the for in loop in for iteration on them. In fact, there are a number of good reasons against the use of for in on arrays.

Note: JavaScript arrays are not associative arrays. JavaScript only has objects for mapping keys to values. And while associative arrays preserve order, objects do not.

Because the for in loop enumerates all the properties that are on the prototype chain and because the only way to exclude those properties is to use hasOwnProperty, it is already up to twenty times slower than a normal for loop.


In order to achieve the best performance when iterating over arrays, it is best to use the classic for loop.

var list = [1, 2, 3, 4, 5, ...... 100000000];
for(var i = 0, l = list.length; i < l; i++) {

There is one extra catch in the above example, which is the caching of the length of the array via l = list.length.

Although the length property is defined on the array itself, there is still an overhead for doing the lookup on each iteration of the loop. And while recent JavaScript engines may apply optimization in this case, there is no way of telling whether the code will run on one of these newer engines or not.

In fact, leaving out the caching may result in the loop being only half as fast as with the cached length.

The length Property

While the getter of the length property simply returns the number of elements that are contained in the array, the setter can be used to truncate the array.

var foo = [1, 2, 3, 4, 5, 6];
foo.length = 3;
foo; // [1, 2, 3]

foo.length = 6;
foo; // [1, 2, 3, undefined, undefined, undefined, 4]

Assigning a smaller length truncates the array. Increasing it creates a sparse array.

In Conclusion

For the best performance, it is recommended to always use the plain for loop and cache the length property. The use of for in on an array is a sign of badly written code that is prone to bugs and bad performance.

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