Link to the video explanation:
Here is the full implementation.
/**

• @param {string} s
• @return {number}
  */
  let lengthOfLastWord = function(s) {
  let lastWordLength = 0;
  let beforeFirstNonEmptyChar = true;
  if (s.length === 0) {
  return lastWordLength;
  }
  for (let i = s.length - 1; i >= 0; i--) {
  if (s.charAt(i) !== ' ') {
  lastWordLength++;
  beforeFirstNonEmptyChar = false;
  } else {
  if (!beforeFirstNonEmptyChar) {
  break;
  }
  }
  }
  return lastWordLength;
  };