.replace without regex

replaceOne.js

String.prototype.replaceOne = function(_what, _with) {
    var what = _what.split(''),
        that = this.split(''),
        found = false,
        skip = 0,
        _new = [];
    
    that.forEach(function(v, i, a) {
        var fail = false;
        
        if(skip > 0) {
           skip--;
        } else {
            if((found === false) && (v === what[0])) {
                skip = -1;
                for(var k = 0; k<what.length; k++) {
                    skip++;
                    if(that[i + k] !== what[k]) {
                        fail = true;
                        break;
                    }
                }
                
                if(fail === false) {
                    found = true;
                    _new.push(_with);
                } else {
                    skip = 0;
                    _new.push(v);
                }
            } else {
                _new.push(v);
            }
        }
    });
    
    return _new.join('');
}