Verify a string consists strictly of a substring of itself in javascript

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Tags: javascript

I stumbled over a Stack Overflow question titled How do I check if a string is entirely made of the same substring?. Before reading through the OP’s current solution, I thought I’d give it a shot myself. I felt very clever, possibly even unique at thought, but I ended up with a solution almost identical to his.

function check (str) {
  // Length of the substring cannot be larger than half of the str length
  const maxlen = Math.floor(str.length / 2) || 1;

  for (let i = 1; i <= maxlen; i++) {
    // Skip indices that doesn't divide with the string length
    if (str.length % i !== 0) continue;

    // Remove all substr occurences and check the returned string's length
    const substr = str.substring(0, i);
    if (str.replace(new RegExp(substr, 'g'), '').length === 0)
      return true;

  return false;

Then I continued to read on in the answers section to find out that there exist theorems for this very problem. Below quotes from SO user templatetypedef

A string consists of the same pattern repeated multiple times if and only if the string is a nontrivial rotation of itself.

Then follows up with another one

If x and y are strings of the same length, then x is a rotation of y if and only if x is a substring of yy.

He then followed up with the following solution for the check function

function check(str) {
    return (str + str).indexOf(str, 1) !== str.length;

I was humbled and amazed. I urge you to go read the whole question and the different answers.

If you have any comments or feedback, please send me an e-mail. (stig at stigok dotcom).

Did you find any typos, incorrect information, or have something to add? Then please propose a change to this post.

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