I agree. I would think replaceAll would behave like the following:

/* Usage:

  > 'abcdefabc'.replaceAll('abc')
  'def'

  > 'abcdefabc'.replaceAll('abc', '+')
  '+def+'

  > 'abcdefabc'.replaceAll(/abc/, '+')
  '+def+'

  > 'abcdefabc'.replaceAll(/abc/g, '+')
  '+def+'
 */

 String.prototype.replaceAll = function (re_or_str, sub) {
     return this.split(re_or_str).join(sub||'')
 }

On a side note, is it just me or does the linked polyfill for String.prototype.replaceAll [1] seem wildly complex???

[1] https://github.com/es-shims/String.prototype.replaceAll/blob...