利用数组的reduce和concat实现数组flat，并可传参
Array.prototype.flat = function(deep = 1) {
if(typeof deep !== 'number') {
throw new TypeError('深度参数必须为数字类型')
}
if(deep === 0) return this
let arr1 = this
return arr1.reduce((acc, val) => {
console.log(deep)
if(Array.isArray(val) && deep > 1) {
console.log(acc)
return acc.concat(val.flat(--deep))
} else {
return acc.concat(val)
}
}, []);// [1, 2, 3, 4]
}
