1768. 交替合并字符串 #

/**
 * @param {string} word1
 * @param {string} word2
 * @return {string}
 */
var mergeAlternately = function (word1, word2) {
  let res = '',
    i = 0,
    j = 0;
  for (; i < word1.length && j < word2.length; i++, j++) {
    res += word1[i] + word2[j];
  }
  if (i === word1.length) {
    res += word2.slice(j);
  } else {
    res += word1.slice(i);
  }
  return res;
};