https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Array/flat#reduce_concat_isarray_recursivity

let ary = [1, [2, [3, [4, 5]]], 6];
let str = JSON.stringify(ary);

用 reduce 實現陣列的 flat 方法
function flatten(ary){
    return ary.reduce((pre,cur)=>{
        return pre.concat(Array.isArray(cur)?flatten(cur):cur)         
    },[])
}
flatten( [1, 2, [3, 4], [5, [6, 7]]])

遞迴
let result = [];
let fn = function(ary) {
for(let i = 0; i < ary.length; i++) }{
let item = ary[i];
if (Array.isArray(ary[i])){
fn(item);
} else {
result.push(item);
}
}
}

擴充套件運算子
while (ary.some(Array.isArray)) {
ary = [].concat(...ary);
}
 

• flat([1, 2, 3, [4, 5]]) // [1,2,3,4,5]
• flat([1, 2, 3, [[4, 5]]]) // [1,2,3,[4,5]]
• flat([1, 2, 3, [[4, 5]]], 2) // [1,2,3,4,5]
• flat([1,2,3,[1,2,[1,2,3]]]) [1,2,3,1,2,[1,2,3]]

function flat(array, depth=1) {
 let result=[]
    for (let i = 0; i < array.length; i++) {
        let item = array[i]
        if (depth > 0 && Array.isArray(item)) {
            result = result.concat(flat(item, depth - 1))
        } else {
            result.push(item)
        }
    }
    return result
}
 

function flat(array, depth = 1) {
    let count = 0
    let result = array
    while (count < depth) {
        for (let i = 0; i < result.length; i++) {
            // 如果是陣列，則將此項拍平
            if (Array.isArray( result[i])) {
                let item=result.splice(i, 1)
                result = [...result, ...item]                
            }
        }
        count++
    }
    return result
}
 

function flat(array, depth = 1) {
    return array.reduce((result, item) => {
        if (depth > 0 && Array.isArray(item)) {
            result = [
                ...result,
                ...flat(item, depth - 1)
            ]
        } else {
            result.push(item)
        }
        return result
    }, [])
}

function flat(array,depth=1){
    return depth>0?array.reduce((pre,cur)=>{
      return pre.concat(Array.isArray(cur)?flat(cur,depth-1):cur)
    },[]):array.slice()
}
flat([1,2,3,[1,2,[1,2,3]]])