Native JS achieve replaceAll () function

Outline

js the replace () first argument can be a string, can also be a regular, when a string, can only replace one, not the overall situation, how to achieve that effect replaceAll it?

At the time of the interview, there was a problem Shredded Code, entitled: find the first string in the second string, third string and then replaced, and can not use any api, you can not use regular expressions, only with simple basic grammar . In fact, it is to let you write a native replaceAll (), and write some of their api

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// 这是我的写法，面试官仍有点不满的感觉，他说用两个for循环，一个个去检测，其实下面的做法也是类似的复杂度
// 有同学有关于js版本更好的做法，请在下方留言，大家一起学习学习
function subString (str, i, j) {
  let result = ''
  for (i; i < j; i++) {
    result += str[i]
  }
  return result
}

function replaceAll(str, before, after) {
  if (before.length > str.length) {
    return str
  }
  let searchLength = before.length
  let result = ''
  for (let i = 0; i < str.length; i++) {
    // 匹配到第一个字符
    if (str[i] === before[0]) {
      // 看before长度是否长于剩下的长度
      if (i > str.length - before.length) {
        break
      // 足够长
      } else {
        let compareStr = subString(str, i, i + searchLength)
        if (compareStr === before) {
          let front = subString(str, 0, i)
          let end = subString(str, i + searchLength, str.length - 1)
          str = front + after + end
        }
      }
    }
  }
  return str
}