findSubstr.js

const findSubstr = (needle, haystack) => {
  let atIdx = -1;
  let found = false;
  const needleArr = needle.split('');
  const haystackArr = haystack.split('');
  
  for(let i = 0; i <= haystackArr.length; i++) {
    found = true;
    if (needle[0] === haystackArr[i]) {
      for(let j = 0; j <= needleArr.length; j++) {
        if (needle[j] !== haystackArr[i+j]) {
          found = false;
          break;
        }
      }
      atIdx = i;
      break;
    }
  }
  
  return (found ? atIdx : -1);
}

console.log('find sseiro in travesseiro', findSubstr('sseiro', 'travesseiro'));