react-telephone-input/src/format_number.ts

React component for entering and validating international telephone numbers

import R from 'cramda';

const { first, tail } = R;

function formatNumber(
  text: string,
  pattern: string | null,
  autoFormat = false,
) {
  if (!text || text.length === 0) {
    return '+';
  }

  // for all strings with length less than 3, just return it (1, 2 etc.)
  // also return the same text if the selected country has no fixed format
  if ((text && text.length < 2) || !pattern || !autoFormat) {
    return `+${text}`;
  }

  const formattedObject = pattern.split('').reduce(
    (acc, character) => {
      if (acc.remainingText.length === 0) {
        return acc;
      }

      if (character !== '.') {
        return {
          formattedText: acc.formattedText + character,
          remainingText: acc.remainingText,
        };
      }

      return {
        formattedText: acc.formattedText + first(acc.remainingText),
        remainingText: tail(acc.remainingText),
      };
    },
    { formattedText: '', remainingText: text.split('') },
  );
  return formattedObject.formattedText + formattedObject.remainingText.join('');
}

export default formatNumber;