ES6之数组的flat(Infinity)扁平化，实现多维数组转成一维数组

 let arr=  [1,2,[[3,4],[5,6]]]

 console.log(arr.flat(Infinity));  // [ 1, 2, 3, 4, 5, 6 ]

 console.log(arr.flat());  // [ 1, 2, [3,4],[5,6] ]

 console.log(arr.flat(1));  // [ 1, 2, [3,4],[5,6] ]

 console.log(arr.flat(2));  // [ 1, 2, 3, 4, 5, 6 ]