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

// 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.