Javascript – Hidden Features of JavaScript?

hidden-featuresjavascript

What "Hidden Features" of JavaScript do you think every programmer should know?

After having seen the excellent quality of the answers to the following questions I thought it was time to ask it for JavaScript.

Even though JavaScript is arguably the most important Client Side language right now (just ask Google) it's surprising how little most web developers appreciate how powerful it really is.

Best Solution

You don't need to define any parameters for a function. You can just use the function's arguments array-like object.

function sum() {
    var retval = 0;
    for (var i = 0, len = arguments.length; i < len; ++i) {
        retval += arguments[i];
    }
    return retval;
}

sum(1, 2, 3) // returns 6