SOURCE

function flat(arr, depth=1){
    return depth>0 && Array.isArray(arr)
    ? arr.reduce((prev,curr)=>{
        if(Array.isArray(curr))return [...prev, ...flat(curr, depth-1)] 
        return [...prev,curr]
    },[])
    :arr
}

const res = flat([1,2,[3,4,[5,6,[7,8],11]],9,10],3);

console.log(res)

function flatStack(arr, depth=1){
    for(let i = 0;l<arr.length;i++){
        let depthNum = depth
        while(depthNum >0){
            if(Array.isArray(arr[i]) {
                arr.splice(i,1,...arr[i])
                depthNum --
            }
        }
        
    }
}