[monolithed]

String.prototype.at = function(str) {
    var tl = this.length, sl = str.length;
    for(var i=j=0; i<tl; i++) {
        if(this[i] == str[j] && sl == ++j && sl != i) return ++i-j;
    }
    return -1;
}

var s1 = 'ababc', l1 = 'abac', l2 = 'bc', l3 = 'aa', l4 = 'O_o';
 
alert([
    [s1.at(l1), s1.indexOf(l1)],
    [s1.at(l2), s1.indexOf(l2)],
    [s1.at(l3), s1.indexOf(l3)],
    [s1.at(l4), s1.indexOf(l4)]
].join('\n'));