A function to flatten array

flattener.js

function flatten(arr, result) {
    for(let i = 0; i < arr.length; i++){
        if(Array.isArray(arr[i])){
           flatten(arr[i], result) 
        }else {
            result.push(arr[i])
        }
    }
    return result
}



console.log(flatten([[1,2,[3]],4], []))
console.log(flatten([[1,2,[3, [5, 6, [9, 8, 7]]],4]], []))