core-js/modules/es.array.join.js

'use strict';
var $ = require('../internals/export');
var IndexedObject = require('../internals/indexed-object');
var toIndexedObject = require('../internals/to-indexed-object');
var sloppyArrayMethod = require('../internals/sloppy-array-method');

var nativeJoin = [].join;

var ES3_STRINGS = IndexedObject != Object;
var SLOPPY_METHOD = sloppyArrayMethod('join', ',');

// `Array.prototype.join` method
// https://tc39.github.io/ecma262/#sec-array.prototype.join
$({ target: 'Array', proto: true, forced: ES3_STRINGS || SLOPPY_METHOD }, {
  join: function join(separator) {
    return nativeJoin.call(toIndexedObject(this), separator === undefined ? ',' : separator);
  }
});