[1, 2, 3, [4, 5, 6, [7, 8], 9], 10] => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

reduce + concat

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function flattenDeep(arr) {
    return arr.reduce((acc, curr) => Array.isArray(curr) ? acc.concat(flattenDeep(curr)) : acc.concat(curr), [])
}

let a = [1, 2, 3, [4, 5, 6, [7, 8], 9], 10]
console.log(flattenDeep(a))

toString

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function flattenDeep(arr) {
    return arr.toString().split(",").map(item => Number(item))
}

let a = [1, 2, 3, [4, 5, 6, [7, 8], 9], 10]
console.log(flattenDeep(a))

这是什么高级方法 ！！！

forEach

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function flattenDeep(arr) {
    let res = []
    arr.forEach(item => {
        if (Array.isArray(item)) {
            res = res.concat(flattenDeep((item)))
        } else {
            res.push(item)
        }
    })
    return res
}

let a = [1, 2, 3, [4, 5, 6, [7, 8], 9], 10]
console.log(flattenDeep(a))

原理跟reduce的一样， 应该说 reduce 的原理跟这个一样