Javascript string between

between.js

/**
* Usage  var you = 'hello you guys'.between('hello ',' guys');
* you = 'you';
*/
String.prototype.between = function (prefix, suffix) {
    s = this;
    var i = s.indexOf(prefix);
    if (i >= 0) {
        s = s.substring(i + prefix.length);
    }
    else {
        return '';
    }
    if (suffix) {
        i = s.indexOf(suffix);
        if (i >= 0) {
            s = s.substring(0, i);
        }
        else {
            return '';
        }
    }
    return s;
}