Array.prototype.toLocaleString()

arr.toLocaleString([locales[, options]]);

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() (en-US), and for dates see Date.prototype.toLocaleString() (en-US).

A string representing the elements of the array.

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

• Object: Object.prototype.toLocaleString() (en-US)
• Number: Number.prototype.toLocaleString() (en-US)
• Date: Date.prototype.toLocaleString() (en-US)

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.

// 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.

• Array.prototype.toString()
• Intl
• Object.prototype.toLocaleString() (en-US)
• Number.prototype.toLocaleString() (en-US)
• Date.prototype.toLocaleString() (en-US)