text: /********************************************************
  默认值可以引用解构赋值的其他变量，但该变量必须已经声明
  ********************************************************/
  
  let [x = 1, y = x] = [];     
  console.log(x,y);// 1 1
  
  let [a = 1, b = a] = [2];   
  console.log(a,b);//2 2
  
  let [u = 1, v = u] = [1, 2]; 
  console.log(1,2);//1 2
  
  // l 取值 r 但是 r 没有定义
  let [l = r, r = 1] = []; //ReferenceError: r is not defined
  console.log(l,r);