Code Concision - String Capitalization

I'm working on a few JavaScript functions for myself that I hope come in handy sometime. The first I wrote was a Capitalization function (yeah i know, real original. whatever)
I was just wondering if

• I'm doing anything incredibly stupid

• There's anyway to make the code below more concise (like cool JS concision features i don't know about, tightening down on program flow, etc.)

• There's anything else cool i can do

Code:

String.prototype.capitalize = function(everyWord) {
    var words = this.split(" ");
    if (everyWord == true) {
        for (i = 0; i < words.length; i++) {
            words[i] = cap(words[i]);
        }
    }
    else {
        words[0] = cap(words[0]);
    }
    return words.join(" ");

    function cap(word) {
        return word.charAt(0).toUpperCase() + word.slice(1);
    }
};
var x = "abraham lincoln yada yada";
x = x.capitalize();
alert(x);
x = x.capitalize(true);
alert(x);

Thanks!
http://jsfiddle.net/thomas4g/LpwRn/31/

Live example

String.capitalize = function(str, everyWord) {
    return _.map(_.words(str), function(val, key) {
        return key === 0 || everyWord ? _.capitalize(val) : val;
    }).join(" ");
};

Write to String not String.prototype. Extending native prototypes is not friendly.

Use underscore and underscore.string.

Basically don't re-invent the wheel ;)

StackExchange Code Review Q#2734, answer score: 4