How to know if JavaScript string.replace() did anything?

Question

The replace function returns the new string with the replaces, but if there weren\'t any words to replace, then the original string is returned. Is there a way to k

Answer 1

Javascript replace is defected by design. Why? It has no compatibility with string replacement in callback.

For example:

"ab".replace(/(a)(b)/, "$1$2")
> "ab"

We want to verify that replace is done in single pass. I was imagine something like:

"ab".replace(/(a)(b)/, "$1$2", function replacing() { console.log('ok'); })
> "ab"

Real variant:

"ab".replace(/(a)(b)/, function replacing() {
  console.log('ok');
  return "$1$2";
})

> ok
> "$1$2"

But function replacing is designed to receive $0, $1, $2, offset, string and we have to fight with replacement "$1$2". The solution is:

"ab".replace(/(a)(b)/, function replacing() {
  console.log('ok');
  // arguments are $0, $1, ..., offset, string
  return Array.from(arguments).slice(1, -2)
  .reduce(function (pattern, match, index) {
    // '$1' from strings like '$11 $12' shouldn't be replaced.
    return pattern.replace(
      new RegExp("\\$" + (index + 1) + "(?=[^\\d]|$)", "g"),
      match
    );
  }, "$1$2");
});

> ok
> "ab"

This solution is not perfect. String replacement itself has its own WATs. For example:

"a".replace(/(a)/, "$01")
> "a"

"a".replace(/(a)/, "$001")
> "$001"

If you want to care about compatibility you have to read spec and implement all its craziness.