indexOf.js

String.prototype.indexOf = function(query, start) {
  var streak = 0;

  start = Math.min(start || 0, this.length - 1);

  // TODO stop at this.length - query.length
  for (var i = start; i < this.length; i++) {
    if (this[i] === query[streak])
      streak++;
    else
      streak = 0;

    if (streak === query.length)
      return i - streak + 1;

    if (i === this.length - 1)
      return -1;
  }
}