A safer String.prototype.replace

sub.js

// "substitute" is too hard to type
function sub (str, candidate, replacer) {
  /*
    NOTE: Using String.prototype.replace is bad because:
    1) casting an arbitrary string to a RegExp is unsafe
    2) replacement has obscure substitution rules ($ is special)
  */

  let match = str.indexOf(candidate)
  if (match === -1) {
    return str
  }

  let matches = 0
  let newStr = ''
  let cursor = 0
  while (match !== -1) {
    newStr += str.slice(cursor, match) + replacer(matches)

    cursor = match + candidate.length
    matches++
    match = str.indexOf(candidate, cursor)
  }

  newStr += str.slice(cursor)
  return newStr
}

const test = require('ava')
test('sub replace all matches with the replacement', t => {
  t.is(
    sub('a b a', 'a', () => 'b'),
    'b b b'
  )
})

test('sub should call replacer with the match index', t => {
  t.is(
    sub('a a a a', 'a', i => i.toString()),
    '0 1 2 3'
  )
})

test('sub should not call replacer if there are no matches', t => {
  sub('no match', 'zzz', () => t.fail())
  t.pass()
})