[JavaScript] Format currency: from cents<number> to <string> or viceversa (e.g. 120030040 --> "1.200.300,40" --> 120030040).

format-currency.js

/**
 * @license
 * Copyright (c) 2017-present, Luca Bertolasi. All Rights Reserved.
 *
 * This software is licensed under the terms of the MIT license,
 * a copy of which can be found at https://opensource.org/licenses/MIT
 */

function formatCurrency(amount, strToNum) {
      // default behaviour:
      // input 'amount' should be expressed in cents: 120030040
      // output will be in european format: 1.200.300,40

      // 'strToNum' (bool) behaviour:
      // input 'amount' should be expressed in european format: 1.200.300,40
      // output will be in cents: 120030040

      // input validation
      const MATCH_NON_DIGITS_REGEXP = /[^\d]/gi
      let amountStr

      if (typeof amount === 'number' && !isNaN(amount)) {
        amountStr = Math.abs(amount).toString().replace(MATCH_NON_DIGITS_REGEXP, '')

      } else if (typeof amount === 'string') {
        let _amount = amount.replace(MATCH_NON_DIGITS_REGEXP, '')
        // no need to 'Math.abs(...)' here becase the live above stripped any non-digit characters, including minus sign
        // 'parseInt(...)' will remove leading zeroes
        amountStr = _amount.length === 0 ? '0' : parseInt(_amount).toString()

      } else {
        amountStr = '0'
      }

      if (strToNum) { return parseInt(amountStr) }

      // not enought digits
      if (amountStr.length === 1) { amountStr = `00${amountStr}`; }
      if (amountStr.length === 2) { amountStr = `0${amountStr}`; }

      // at this point there are at least 3 digits
      let amountArr = amountStr.split('');

      // set local vars
      let cents = '';
      let ones = '';
      let tens = '';
      let hundreds = '';
      let thousands = '';
      let thousandsCount = 0;
      let formattedAmount = '';

      // cents
      cents = amountArr.pop();
      cents = `${amountArr.pop()}${cents}`;

      // ones
      ones = amountArr.pop();

      // minimum digits amount
      formattedAmount = `${ones},${cents}`;

      // extra digit: tens
      if (amountArr.length > 0) {
        tens = amountArr.pop();
        formattedAmount = `${tens}${formattedAmount}`;

        // extra digit: hundreds
        if (amountArr.length > 0) {
          hundreds = amountArr.pop();
          formattedAmount = `${hundreds}${formattedAmount}`;

          // extra digits: thousands
          while (amountArr.length > 0) {
            // group thousands triplets and add separator
            thousands = '';
            thousandsCount = 0;
            do {
              thousands = `${amountArr.pop()}${thousands}`;
              thousandsCount += 1;
            } while (amountArr.length > 0 && thousandsCount < 3);

            formattedAmount = `${thousands}.${formattedAmount}`;

          } // thousands
        } // hundreds
      } // tens

      return formattedAmount;
    } // formatCurrency