JavaScript Array.prototype.flat() 方法详解与手动实现

flat()
flat(depth)

const arr = [0, 1, 2, [3, 4]]
arr.flat()
// [0, 1, 2, 3, 4]

Array.prototype.myFlat = function (depth = 1) { // 需要一个参数，默认值为 1
    const result = []

    const flatten = (arr, currentDepth) => {
        for (let i = 0; i < arr.length; i++) {
            if (Object.hasOwn(arr, i)) { // 忽略稀疏数组的空槽
                if (Array.isArray(arr[i]) && currentDepth < depth) { // 展开的元素必须是数组
                    flatten(arr[i], ++currentDepth)
                } else {
                    result.push(arr[i])
                }
            }
        }
    }

    flatten(this, 0)
    return result
}

Array.prototype.myFlat = function (depth = 1) {
    const result = []

    const flatten = (arr, currentDepth) => {
        for (let i = 0; i < arr.length; i++) {
            if (Object.hasOwn(arr, i)) {
                if (Array.isArray(arr[i]) && currentDepth < depth) {
                    flatten(arr[i], ++currentDepth)
                } else {
                    result.push(arr[i])
                }
            }
        }
    }

    flatten(this, 0)
    return result
}

console.log('扁平普通数组')
const arr1 = [1, 2, [3, 4]]
console.log(arr1.myFlat())
console.log(arr1.flat())
// [1, 2, 3, 4]

const arr2 = [1, 2, [3, 4, [5, 6]]]
console.log(arr2.myFlat())
console.log(arr2.flat())
// [1, 2, 3, 4, [5, 6]]

const arr3 = [1, 2, [3, 4, [5, 6]]]
console.log(arr3.myFlat(2))
console.log(arr3.flat(2))
// [1, 2, 3, 4, 5, 6]

const arr4 = [1, 2, [3, 4, [5, 6, [7, 8, [9, 10]]]]]
console.log(arr4.myFlat(Infinity))
console.log(arr4.flat(Infinity))
// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]


console.log('稀疏数组中使用 flat')
const arr5 = [1, 2, , 4, 5]
console.log(arr5.myFlat()) 
console.log(arr5.flat())
// [1, 2, 4, 5]

const arr6 = [1, , 3, ["a", , "c"]]
console.log(arr6.myFlat()) 
console.log(arr6.flat())
// [ 1, 3, "a", "c" ]

const arr7 = [1, , 3, ["a", , ["d", , "e"]]]
console.log(arr7.myFlat())
console.log(arr7.flat()) 
// [ 1, 3, "a", ["d", empty, "e"] ]

console.log(arr7.myFlat(2))
console.log(arr7.flat(2)) 
// [ 1, 3, "a", "d", "e"]


console.log('在非数组对象上使用 flat')
const arrayLike = {
    length: 3,
    0: [1, 2],
    1: { length: 2, 0: 3, 1: 4 },
    2: 5,
}
console.log(Array.prototype.myFlat.call(arrayLike))
console.log(Array.prototype.flat.call(arrayLike))
// [1, 2, { "0": 3, "1": 4, "length": 2 }, 5]