Array.prototype.toLocaleString()

The toLocaleString() method returns a string representing the elements of the array. The elements are converted to Strings using their toLocaleString methods and these Strings are separated by a locale-specific String (such as a comma “,”).

Syntax

toLocaleString();
toLocaleString(locales);
toLocaleString(locales, options);

Parameters

locales Optional: A string with a BCP 47 language tag, or an array of such strings. For the general form and interpretation of the locales argument, see the Intl page.
options Optional: An object with configuration properties, for numbers see Number.prototype.toLocaleString(), and for dates see Date.prototype.toLocaleString().

Return value

A string representing the elements of the array.

Polyfill

// https://tc39.github.io/ecma402/#sup-array.prototype.tolocalestring
if (!Array.prototype.toLocaleString) {
  Object.defineProperty(Array.prototype, 'toLocaleString', {
    value: function(locales, options) {
      // 1. Let O be ? ToObject(this value).
      if (this == null) {
        throw new TypeError('"this" is null or not defined');
      }

      var a = Object(this);

      // 2. Let len be ? ToLength(? Get(A, "length")).
      var len = a.length >>> 0;

      // 3. Let separator be the String value for the
      //    list-separator String appropriate for the
      //    host environment's current locale (this is
      //    derived in an implementation-defined way).
      // NOTE: In this case, we will use a comma
      var separator = ',';

      // 4. If len is zero, return the empty String.
      if (len === 0) {
        return '';
      }

      // 5. Let firstElement be ? Get(A, "0").
      var firstElement = a[0];
      // 6. If firstElement is undefined or null, then
      //  a.Let R be the empty String.
      // 7. Else,
      //  a. Let R be ?
      //     ToString(?
      //       Invoke(
      //        firstElement,
      //        "toLocaleString",
      //        « locales, options »
      //       )
      //     )
      var r = firstElement == null ?
        '' : firstElement.toLocaleString(locales, options);

      // 8. Let k be 1.
      var k = 1;

      // 9. Repeat, while k < len
      while (k < len) {
        // a. Let S be a String value produced by
        //   concatenating R and separator.
        var s = r + separator;

        // b. Let nextElement be ? Get(A, ToString(k)).
        var nextElement = a[k];

        // c. If nextElement is undefined or null, then
        //   i. Let R be the empty String.
        // d. Else,
        //   i. Let R be ?
        //     ToString(?
        //       Invoke(
        //        nextElement,
        //        "toLocaleString",
        //        « locales, options »
        //       )
        //     )
        r = nextElement == null ?
          '' : nextElement.toLocaleString(locales, options);

        // e. Let R be a String value produced by
        //   concatenating S and R.
        r = s + r;

        // f. Increase k by 1.
        k++;
      }

      // 10. Return R.
      return r;
    }
  });
}

If you need to support truly obsolete JavaScript engines that don't support Object.defineProperty, it's best not to polyfill Array.prototype methods at all, as you can't make them non-enumerable.

Examples

Using locales and options

The elements of the array are converted to strings using their toLocaleString methods.

Object: Object.prototype.toLocaleString()
Number: Number.prototype.toLocaleString()
Date: Date.prototype.toLocaleString()

Always display the currency for the strings and numbers in the prices array:

var prices = ['￥7', 500, 8123, 12];
prices.toLocaleString('ja-JP', { style: 'currency', currency: 'JPY' });

// "￥7,￥500,￥8,123,￥12"

For more examples, see also the Intl, NumberFormat, and DateTimeFormat pages.

Specifications

Specification
ECMAScript Language Specification (ECMAScript) # sec-array.prototype.tolocalestring
ECMAScript Internationalization API Specification (ECMAScript Internationalization API) # sup-array.prototype.tolocalestring

Browser compatibility

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet
`toLocaleString`	1	12	1	5.5	4	1	≤37	18	4	10.1	1	1.0
`locales_parameter`	24	79	52	No	15	6.1	≤37	25	56	14	6.1	2.0
`options_parameter`	24	79	52	No	15	6.1	≤37	25	56	14	6.1	2.0