Replace random substring in JavaScript

Question

I am trying to replace random substring with another within a string. This is my code

function replaceRandomSubstring(str, substr, repl) {
  var amount = str.match(substr)
  var newstr = str;
  if (amount.length != -1) {
    var index = Math.floor(Math.random() * amount.length)
    var i = 0;
    do {
      if (i == index) newstr = newstr.replace(substr, repl)
      else newstr = newstr.replace(substr, "placeholder")
      i++;
    }
    while (i < index)
    newstr = newstr.split("placeholder").join(substr)
  }
  return newstr;
}

What's happening is that it replaces the very first substring not random one

Can you show an example of the values you're passing to the function. — Teemu
– Teemu, Commented Aug 9, 2020 at 8:11
@Teemu replaceRandomSubstring(teststring, "test", "replacement") and teststring contains: "test, test, test, test, test" — Dominicentek Gaming
– Dominicentek Gaming, Commented Aug 9, 2020 at 8:16
OK, and you want to replace a single substr only, not a random amount of the occurrences? — Teemu
– Teemu, Commented Aug 9, 2020 at 8:23

Alexander B. · Accepted Answer · 2020-08-09 08:37:41Z

1

This is because amount doesn't return all matches of the substrings. It returns the first match.

Use String.prototype.matchAll() instead:

function replaceRandomSubstring(str, substr, repl) {
  const amount = [...str.matchAll(substr)]; // collect all substring matches into one array

  if (amount.length !== -1) {
    const index = Math.floor(Math.random() * amount.length);

    let i = 0;

    do {
      if (i === index) str = str.replace(substr, repl);
      else str = str.replace(substr, 'placeholder');
      i++;
    } while (i <= index);

    str = str.split('placeholder').join(substr);
  }
  return str;
}

edited Aug 9, 2020 at 8:37

answered Aug 9, 2020 at 8:24

Alexander B.

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