Flatten Nested Array
Write a function that takes a deeply nested array and returns a flat (one-dimensional) array containing all the values. The function should optionally accept a depth parameter — if given, flatten only up to that many levels. This tests recursion, array manipulation, and knowledge of modern JS APIs.
Examples
Input: [1, [2, [3, [4]], 5]]
Output: [1, 2, 3, 4, 5]
// All levels flattened (default: Infinity depth).
Input: [1, [2, [3, [4]]]] with depth = 1
Output: [1, 2, [3, [4]]]
// Only the first level is flattened.
Input: [] (empty)
Output: []
Constraints
- Elements can be numbers, strings, or nested arrays.
- depth parameter (optional) limits how many levels to flatten; Infinity flattens all.
- Should not mutate the original array.
arrayrecursionstackes6reduce
Important
Interview Tip
Start by mentioning Array.prototype.flat(Infinity) to show you know the built-in. Then implement manually — the recursive approach is clearest and most interview-friendly. If asked about deeply nested arrays (stack overflow risk), present the iterative stack solution.
Approach: Brute Force
Repeatedly flatten one level at a time using concat spread, looping with Array.some(Array.isArray) until no nested arrays remain. Simple to explain but creates many intermediate arrays.
Complexity
Time: O(n × d)Space: O(n × d)
Pros
- Very simple to explain
- No recursion needed
Cons
- Creates O(d) intermediate arrays — slow for deep nesting
- Does not support depth parameter easily