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const highlightPattern = (str, pattern) => {
  let onlyRepeatedStr = '';
  let lastMatchIndex = -1;
  const spaces = (n) => new Array(n).fill('-').join('');

  while ((lastMatchIndex = str.indexOf(pattern, lastMatchIndex)) >= 0) {
    onlyRepeatedStr += spaces(lastMatchIndex - onlyRepeatedStr.length) + pattern;
    // shift the beginning of the next loop .indexOf
    lastMatchIndex += pattern.length;
  }
  onlyRepeatedStr += spaces(str.length - onlyRepeatedStr.length);

  console.log(str);
  console.log(onlyRepeatedStr);
};

highlightPattern("AAAAAAbAAAAb", "b")

// Output:
// AAAAAAbAAAAb
// ------b----b