ES6语法处理数组多维展开

flat可以展开数组，里面可以接收一个参数，例如flat(3),就是展开3维数组。或者传入Infinity展开所有

 let ar = [12, 34, 3, [343, [194, [23]]]]
      let st = ar.flat(Infinity)
      console.log(st)