Byte-sized Frontend Interviews: Flatten

Suppose that you're given an array that contains either a number or another array. Flatten the array such that all the values exist on a single level.

For example, if we had the array [1, [2, 3, [4, 5]]], we would expect that the result is [1, 2, 3, 4, 5].

This problem lends itself to be recursive in nature because of how an array could contain more arrays which would need to unpack so that all the values will exist on a single level.

For each item in the array, there are two possible operations:

• If the item is a number, then we add that number to our resulting array
• If the item is an array, then we should recursively call the function with the current array.

Here's a straightforward implementation for this algorithm:

function flatten(array) {
  const result = []

  for (const item of array) {
    if (Array.isArray(item)) {
      result.push(...flatten(item))
    } else {
      result.push(item)
    }
  }

  return result
}

This solution takes advantage of the spread operator in JavaScript and the variadic nature of the push method's arguments list. the result.push(...flatten(item)) line makes sure that we recursively call flatten with the item which is currently an array and then spreads the result of the recursive call into result.push which allows us to push any number of items into the array.

Another recursive solution we can offer here is by using the Array.prototype.flatMap method with recursion.

function flatten(array) {
  if (!Array.isArray(array)) {
    return array
  }

  return array.flatMap((item) => flatten(item))
}

We reduce the verbosity of the solution by provide a sleek functional solution. Array.prototype.flatMap helps us make sure that any array solution we receive gets flattened by one level.

For example, if we did something like,

;[1, 2, 3].flatMap((num) => [num, num * 2])

We would expect that the solution is [1, 2, 2, 4, 3, 6] rather than [[1, 2], [2, 4], [3, 6]] because flatMap makes sure that the values are hoisted up one level from the array.