Array.prototype.toLocaleString()

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

// 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');
      }

      const a = Object(this);

      // 2. Let len be ? ToLength(? Get(A, "length")).
      const 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
      const separator = ',';

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

      // 5. Let firstElement be ? Get(A, "0").
      const 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 »
      //       )
      //     )
      let r = firstElement === null ?
        '' : firstElement.toLocaleString(locales, options);

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

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

        // b. Let nextElement be ? Get(A, ToString(k)).
        const 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;
    }
  });
}

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

// "￥7,￥500,￥8,123,￥12"