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

'use strict';
var $ = require('../internals/export');
var uncurryThis = require('../internals/function-uncurry-this');
var IndexedObject = require('../internals/indexed-object');
var toIndexedObject = require('../internals/to-indexed-object');
var arrayMethodIsStrict = require('../internals/array-method-is-strict');

var nativeJoin = uncurryThis([].join);

var ES3_STRINGS = IndexedObject !== Object;
var FORCED = ES3_STRINGS || !arrayMethodIsStrict('join', ',');

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