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Standard library

There is a newer version: 3.39.0

'use strict';
var $ = require('../internals/export');
var toObject = require('../internals/to-object');
var lengthOfArrayLike = require('../internals/length-of-array-like');
var setArrayLength = require('../internals/array-set-length');
var deletePropertyOrThrow = require('../internals/delete-property-or-throw');
var doesNotExceedSafeInteger = require('../internals/does-not-exceed-safe-integer');

// IE8-
var INCORRECT_RESULT = [].unshift(0) !== 1;

// V8 ~ Chrome < 71 and Safari <= 15.4, FF < 23 throws InternalError
var properErrorOnNonWritableLength = function () {
  try {
    // eslint-disable-next-line es/no-object-defineproperty -- safe
    Object.defineProperty([], 'length', { writable: false }).unshift();
  } catch (error) {
    return error instanceof TypeError;
  }
};

var FORCED = INCORRECT_RESULT || !properErrorOnNonWritableLength();

// `Array.prototype.unshift` method
// https://tc39.es/ecma262/#sec-array.prototype.unshift
$({ target: 'Array', proto: true, arity: 1, forced: FORCED }, {
  // eslint-disable-next-line no-unused-vars -- required for `.length`
  unshift: function unshift(item) {
    var O = toObject(this);
    var len = lengthOfArrayLike(O);
    var argCount = arguments.length;
    if (argCount) {
      doesNotExceedSafeInteger(len + argCount);
      var k = len;
      while (k--) {
        var to = k + argCount;
        if (k in O) O[to] = O[k];
        else deletePropertyOrThrow(O, to);
      }
      for (var j = 0; j < argCount; j++) {
        O[j] = arguments[j];
      }
    } return setArrayLength(O, len + argCount);
  }
});