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This problem seems simple but you need to make sure to flatten any array, regardless of the level which is what adds a bit of difficulty to the problem.

• Array.isArray()

You need to check if an element is an array or not.

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If you are dealing with an array, then you need flatten it by getting the value inside of the array. This means if you have 4 then instead of returning [4] you need to return 4. If you get [4] then the same, you want the 4. You can access it with arr[index1][index2] to go a level deeper.

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You will definitely need recursion or another way to go beyond two level arrays to make the code flexible and not hard-coded to the answers needed. Have fun!

try to solve the problem now

Solution ahead!

function steamrollArray(arr) {
  var flattenedArray = [];

  // Create function that adds an element if it is not an array.
  // If it is an array, then loops through it and uses recursion on that array.
  var flatten = function(arg) {
    if (!Array.isArray(arg)) {
      flattenedArray.push(arg);
    } else {
      for (var a in arg) {
        flatten(arg[a]);
      }
    }
  };

  // Call the function for each element in the array
  arr.forEach(flatten);
  return flattenedArray;
}

// test here
steamrollArray([1, [2], [3, [[4]]]]);

🚀 Run Code

• Create a new variable to keep flattened arrays.
• Create a function that will add non array elements to the new variable, and for the ones that are array it loops through them to get the element.
• It does that by using recursion, if the element is an array then call the function again with a layer of array deeper to check if it is an array or not. if it is not then push that non-array element to the variable that gets returned. Otherwise, keep going deeper.
• Invoke the function, the first time you will always pass it an array, so it always fall in to the isArray branch
• Return the flattened array.

• Array.push()
• Array.forEach()

function steamrollArray(arr) {
  return arr.reduce(function (flat, toFlatten) {
    return flat.concat(Array.isArray(toFlatten) ? steamrollArray(toFlatten) : toFlatten);
  }, []);
}

// test here
steamrollArray([1, [2], [3, [[4]]]]);

🚀 Run Code

• Use reduce to concatenate each element into the last element
• If the new element is an Array itself call the function recursively to flatten it before merging it with the rest of result
• Pass an empty array to reduce as initial value to make sure even the first element will be processed
• Credit goes to

• http://stackoverflow.com/a/15030117/1075499
• Array.reduce
• Array.concat

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