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// reduce实现 Array.prototype.flat，数组扁平化

const selfFlat = function (depth = 1) {

let arr = Array.prototype.slice.call(this)

if(depth === 0 ) return arr

return arr.reduce((pre, cur) => {

if (Array.isArray(cur)) {

// 需要用 call 绑定 this 值，否则会指向 window

return [...pre, ...selfFlat.call(cur,depth-1)]

} else {

return [...pre, cur]

}

}, [])

}

Array.prototype.selfFlat || (Object.defineProperty(Array.prototype, 'selfFlat', {

value: selfFlat,

enumerable: false,

configurable: true,

writable: true

}))

let arr = [1, 2, [3, 4, [5, 6, 7, 8], 9], 10, 11, 12, [13, 14]]

console.log(arr.selfFlat())