Array Out of Bounds: Comparison with undefined, or length check?

Question

this seems to be a common javascript idiom:

function foo (array, index) {
    if (typeof array[index] == \'undefined\')
        alert (\'out of bounds baby\'         


        

Answer 1

It's not common as far as I know, much more common is:

for (var i=0, iLen=array.length; i

You should not compare to undefined, since a member of the array may have been assigned a value of undefined, or may not have been assigned any value.

e.g.

var a = [0,,,,];

alert(a.length); // 4 (or 5 in buggy IE);

a[1] === undefined; // true but not out of bounds

The main reason for using a for loop is that array properties may not be returned in numeric order if a for..in loop is used.

A for..in loop is much more efficient in sparse arrays but the possible out-of-order access must be dealt with if it matters (as must inherited and non-numeric enumerable properties if they should be avoided).