repeatify.js

String.prototype.repeatify = function(x) {
  // Array.apply(null, {length: x}) creates an array of undefined values with length x
  return Array.apply(null, {length: x}).map(() => this).join('');
};
// or a better but less fun solution
String.prototype.repeatify = function(x) {
  return Array(x + 1).join(this);
};
console.log('hi'.repeatify(10));