console.timeEnd(label)

console.time('100-elements');
for (var i = 0; i < 100; i++) {
  ;
}
console.timeEnd('100-elements');

Console.prototype.timeEnd = function(label) {
  var time = this._times[label];
  if (!time) {
    throw new Error('No such label: ' + label);
  }
  var duration = Date.now() - time;
  this.log('%s: %dms', label, duration);
};