Flatten Nested Array

1. An array that contains arrays is a recursive structure - each inner array is the same problem, only smaller. That is the signal to reach for recursion.
2. Walk the input left to right, keeping one result array.
3. If the current item is an array, flatten it with the same function and append everything it returns.
4. Otherwise it is a plain value - append it as it is.
5. Return the result once every item has been visited.
6. Why O(n) despite a recursive call inside a loop: each value is visited exactly once, however deep it sits. The recursion divides the work; it does not repeat it.
7. Space is O(n) for the result, plus O(d) of call stack, where d is the deepest level of nesting. The explicit-stack follow-up below trades that call stack for an ordinary array.

Common mistakes

• Flattening only one level. result.concat(item) opens an array once, so [1, [2, [3, 4]]] becomes [1, 2, [3, 4]] - the nested [3, 4] survives.
• Reaching for arr.flat(Infinity). It gives the right answer, but the question is asking you to implement exactly that.
• Losing the order with a stack. A stack is last in, first out, so popping [1, [2, 3], 4] naively yields 4, 3, 2, 1. Push children right to left, or reverse the result at the end.
• Very deep nesting. Each level costs one call-stack frame, and around twenty thousand levels throws a RangeError in JavaScript. The explicit stack has no such limit.
• result.push(...flattenArray(item)) on a huge inner array. Spreading passes every element as a separate argument, and around a million of them throws a RangeError. Pushing in a loop avoids it.
• Using typeof item === "array" to detect arrays. typeof reports "object" for arrays; Array.isArray(item) is the correct check.