When we use a programming language everyday, now and then we find ourselves stumbling upon intricacies of that language that need some clarification. Today I encountered this as I was using JavaScript to convert an object to an XML string. While working through the object, there was a particular piece of the code that needed to evaluate the first character of a string. For those of you versed in JavaScript, you understand that there are actually a number of ways to accomplish this, most effectively by treating the string like an array [because JavaScript doesn't mind] or by using the handy charAt() function. Both seem to work fine… but what's the difference you might ask?

var test = 'string';
    test[0];
    // ---> returns "s"

var test = 'string';
    test.charAt(0);
    // ---> returns "s"

Doesn't seem to be a difference here. I was pretty stumped until my colleague, Nathan, suggested I try an invalid index.

var test = 'string';
    test[5];
    // ---> returns ""

var test = 'string';
    test.charAt(5);
    // ---> returns null

A-ha!

As you can see, charAt() returns a string regardless of the argument actually being a valid index or not. This is extremely useful in the case that you need the returned value to be a string for later usage.

Grabbing the value directly from the string as you would do with an array is also a useful technique if you need to know that it is indeed an invalid index.

Also, could one assume that comparing against null performs better than performing a string comparison?

Update: As it turns out, IE does not support the test[] method of accessing the value. It is advised that you always use test.CharAt().

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This entry was posted on Thursday, April 24th, 2008 at 12:25 pm and is filed under JavaScript.