Question

In JS are the lengths of an array cached or does it depend on the different engines/browsers?

Normally I’d assume that browsers’ JS engines are pretty dumb and cache the length of arrays, for example:

var a   = [ ];
var l   = l;

function arrayPush(i)
{
    l   = a.push( i );
}

function arrayPop()
{
    var r   = a.pop();

    l   = a.length;

    return r;
}

(as a brief example, of course it’ll be silly to replicate every array function but if it speeds stuff up it then it’s worth it)

Answer 1

The array length is cached. It is updated each time the array is manipulated.

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When you invoke the .push() method on the array, the array length is updated in step 6 of the algorithm:

Call the [[Put]] internal method of O with arguments “length”, n, and true.

Source: http://es5.github.com/x15.4.html#x15.4.4.7

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When you invoke the .pop() method of an array, the array length is updated in step 5.d of the algorithm:

Call the [[Put]] internal method of O with arguments “length”, indx, and true.

Source: http://es5.github.com/x15.4.html#x15.4.4.6

---

When you assign a value to the array at an given index, the [[DefineOwnProperty]] internal method is invoked. The array length is updated in step 4.e.ii of the algorithm:

Call the default [[DefineOwnProperty]] internal method (8.12.9) on A passing “length”, oldLenDesc, and false as arguments. This call will always return true.

Source: http://es5.github.com/x15.4.html#x15.4.5.1