String.prototype.repeat()

repeat(count)

'abc'.repeat(-1)    // RangeError
'abc'.repeat(0)     // ''
'abc'.repeat(1)     // 'abc'
'abc'.repeat(2)     // 'abcabc'
'abc'.repeat(3.5)   // 'abcabcabc' (count will be converted to integer)
'abc'.repeat(1/0)   // RangeError

({ toString: () => 'abc', repeat: String.prototype.repeat }).repeat(2)
// 'abcabc' (repeat() is a generic method)

if (!String.prototype.repeat) {
  String.prototype.repeat = function(count) {
    'use strict';
    if (this == null)
      throw new TypeError('can\'t convert ' + this + ' to object');

    var str = '' + this;
    // To convert string to integer.
    count = +count;
    // Check NaN
    if (count != count)
      count = 0;

    if (count < 0)
      throw new RangeError('repeat count must be non-negative');

    if (count == Infinity)
      throw new RangeError('repeat count must be less than infinity');

    count = Math.floor(count);
    if (str.length == 0 || count == 0)
      return '';

    // Ensuring count is a 31-bit integer allows us to heavily optimize the
    // main part. But anyway, most current (August 2014) browsers can't handle
    // strings 1 << 28 chars or longer, so:
    if (str.length * count >= 1 << 28)
      throw new RangeError('repeat count must not overflow maximum string size');

    var maxCount = str.length * count;
    count = Math.floor(Math.log(count) / Math.log(2));
    while (count) {
       str += str;
       count--;
    }
    str += str.substring(0, maxCount - str.length);
    return str;
  }
}