SOURCE

function qsort(arr){
    if(arr.length<=1) return arr
    const num = arr[arr.length -1]
    const [left,right] = [[],[]]
    for(let i = 0; i<arr.length-1 ; i+=1){
        if(arr[i]<=num){left.push(arr[i])}
        else{right.push(arr[i])}
    }
    return qsort(left).concat([num],qsort(right))
}
const a = [4,2,1,0]
console.log(qsort(a))
console.log(a)