_array-copy-within.js

// 22.1.3.3 Array.prototype.copyWithin(target, start, end = this.length)
'use strict';
var toObject = require('./_to-object');
var toAbsoluteIndex = require('./_to-absolute-index');
var toLength = require('./_to-length');

module.exports = [].copyWithin || function copyWithin(target /* = 0 */, start /* = 0, end = @length */) {
  var O = toObject(this);
  var len = toLength(O.length);
  var to = toAbsoluteIndex(target, len);
  var from = toAbsoluteIndex(start, len);
  var end = arguments.length > 2 ? arguments[2] : undefined;
  var count = Math.min((end === undefined ? len : toAbsoluteIndex(end, len)) - from, len - to);
  var inc = 1;
  if (from < to && to < from + count) {
    inc = -1;
    from += count - 1;
    to += count - 1;
  }
  while (count-- > 0) {
    if (from in O) O[to] = O[from];
    else delete O[to];
    to += inc;
    from += inc;
  } return O;
};