Steamroller Help me

Hey guys i need help to solve the Steamroller…here is my code.It’s suppose to flatten arrays which it does however it doesnt pass any test

Your code so far

function steamrollArray(arr) {
  // I'm a steamroller, baby
  
  for(var i =0,len = arr.length;i<len;i++){
    
    if(!Array.isArray(arr[i])){
        arrFlatten.push(arr[i]);
    }
    else{
      steamrollArray(arr[i]);
    }
  }
  return arrFlatten;
}

steamrollArray([1, [2], [3, [[4]]]]);



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Link to the challenge:
https://www.freecodecamp.org/challenges/steamroller