Flatten An array of Integers not using Array.flat()

array-flat.js

// Write some code, that will flatten an array of arbitrarily nested arrays of integers into a flat array of integers. 
// e.g. [[1,2,[3]],4] -> [1,2,3,4]. 

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

console.log(flatten([[1,2,[3]],4]));
console.log(flatten([[1,33,22,[[[12]]],[3]],4]));