Recursive method

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1 
function steamrollArray(arr) {
  let flat = [];
  if(arr.find((item)=>Array.isArray(item))){
    arr.forEach((item)=>{
        if(Array.isArray(item)){
          flat = [...flat,...item]
        }else{
          flat.push(item);
        }
    })
  }else{
    return arr;
  }
  return steamrollArray(flat);
}

console.log(steamrollArray([1, [2], [3, [[4]]]]));
2

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3

I’ve edited your post for readability. When you enter a code block into a forum post, please precede it with a separate line of three backticks and follow it with a separate line of three backticks to make it easier to read.

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