Replaces the last occurence of a pattern in a string.

  • Use typeof to determine if pattern is a string or a regular expression.
  • If the pattern is a string, use it as the match.
  • Otherwise, use the RegeExp constructor to create a new regular expression using the RegExp.source of the pattern and adding the 'g' flag to it. Use String.prototype.match() and Array.prototype.slice() to get the last match, if any.
  • Use String.prototype.lastIndexOf() to find the last occurence of the match in the string.
  • If a match is found, use String.prototype.slice() and a template literal to replace the matching substring with the given replacement.
  • If no match is found, return the original string.
const replaceLast = (str, pattern, replacement) => {
  const match =
    typeof pattern === 'string'
      ? pattern
      : (str.match(new RegExp(pattern.source, 'g')) || []).slice(-1)[0];
  if (!match) return str;
  const last = str.lastIndexOf(match);
  return last !== -1
    ? `${str.slice(0, last)}${replacement}${str.slice(last + match.length)}`
    : str;
replaceLast('abcabdef', 'ab', 'gg'); // 'abcggdef'
replaceLast('abcabdef', /ab/, 'gg'); // 'abcggdef'
replaceLast('abcabdef', 'ad', 'gg'); // 'abcabdef'
replaceLast('abcabdef', /ad/, 'gg'); // 'abcabdef'