第一種：常規(guī)置換，借助中間變量

  let a = 1, b = 2, c;
  c = a;
  a = b;
  b = c;
  //==> a = 2, b = 1

第二種：借助Object

  let a = 1, b = 2;
  let obj = {
      a: b,
      b: a,
  }
  a = obj.a; b = obj.b;
  //==> a = 2, b = 1

第三種：ES6的賦值解構

  let a = 1, b = 2;
  [a, b] = [b, a];
  //==> a = 2, b = 1