Asynchronous string replace

stringReplaceAsync.js

function stringReplaceAsync (string, regex, replacer) {
  if (!regex.global) {
    const match = regex.exec(string)
    if (!match) return string

    const retVal = replacer(...match, match.index, string)

    if (retVal instanceof Promise) {
      return retVal.then((ret) => (
        string.slice(0, match.index) +
        ret +
        string.slice(match.index + match[0].length)
      ))
    } else {
      return (
        string.slice(0, match.index) +
        retVal +
        string.slice(match.index + match[0].length)
      )
    }
  } else {
    const accu = []
    let promise = false
    let match
    let lastIndex = 0

    while ((match = regex.exec(string)) !== null) {
      accu.push(string.slice(lastIndex, match.index))

      const retVal = replacer(...match, match.index, string)

      if (retVal instanceof Promise) {
        promise = true
      }

      accu.push(retVal)

      lastIndex = regex.lastIndex
    }

    accu.push(string.slice(lastIndex))

    return promise
    ? Promise.all(accu).then((arr) => arr.join(''))
    : accu.join('')
  }
}