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

Do not test for undefined. You should use the length of the array. There are cases where it simply does not work to test for undefined because undefined is a legal value for legitimate array entry. Here's a legal JS array:

var legalArray = [4, undefined, "foo"];

And you can access it like this:

var legalArray = [4, undefined, "foo"];

var result = "";
for (var i = 0; i < legalArray.length; i++) {
    result += legalArray[i] + "
";
}

$("#result").html(result);

Generates this output:

4
undefined
foo

As seen in this jsFiddle: http://jsfiddle.net/jfriend00/J5PPe/