Review a JavaScript flatten PR

## Live PR Review: JavaScript `flatten` using recursion You are reviewing a pull request that introduces a `flatten` utility. ### Intended behavior Implement `flatten(input)` such that: - `input` is an Array that may contain nested Arrays to arbitrary depth. - The function returns a **new** flat Array preserving left-to-right order. Examples: - `flatten([1, [2, 3], [[4]], 5]) -> [1, 2, 3, 4, 5]` - `flatten([]) -> []` - `flatten([1, [2, [3]]]) -> [1, 2, 3]` ### PR code under review ```js // PR version export function flatten(arr) { const out = []; function helper(x) { for (const i in x) { const v = x[i]; if (typeof v === 'object') { helper(v); } else { out.push(v); } } } helper(arr); return out; } ``` ### Tasks 1. Identify **correctness bugs** and **edge cases** this implementation fails. 2. Discuss **API/contract** questions you would clarify (e.g., how to treat non-array objects, `null`, sparse arrays, strings). 3. Suggest improvements for: - readability and maintainability - performance (time/space) - safety (e.g., stack overflow risk) 4. Propose a revised approach (pseudocode or description is fine) and a minimal set of tests you would request before approving.