js字符串拼接的问题

  var a = 123+'123';
  var b = '123'+123;
  var c = 123+'';
  var d = 123;
  console.log(typeof a); // string
  console.log(typeof b); // string
  console.log(typeof c); // string
  console.log(typeof d); // number
  console.log(typeof d.toString()); // string