Array flat的实现

 if (!Array.prototype.flat) {
     Array.prototype.flat = function (num = 1) {
         if (!Number(num) || Number(num) < 0) {
             return this;
         }
         var arr = []
         this.forEach((item) => {
             if (Array.isArray(item)) {
                 arr = arr.concat(item.flat(--num))
             } else {
                 arr.push(item)
             }
         })
         return arr
     }
 }

测试用例

 const arr = [1, [2, [3, 'a', [4]]]]

 console.log(arr.flat('dsdsadf'));  // [1, [2, [3, 'a', [4]]]]
 console.log(arr.flat(-32)); // [1, [2, [3, 'a', [4]]]]
 console.log(arr.flat(0));   // [1, [2, [3, 'a', [4]]]]
 console.log(arr.flat('1'));   // [1, 2, [3, 'a', [4]]]
 console.log(arr.flat('2'));    // [1, 2, 3, 'a', [4]]
 console.log(arr.flat(3));       // [1, 2, 3, 'a', 4]
 console.log(arr.flat(Infinity));     // [1, 2, 3, 'a', 4]
 console.log(arr.flat('Infinity'));   // [1, 2, 3, 'a', 4]