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HardDSAAsked in 20246 min read

Flatten a Deeply Nested Array Iteratively (No Recursion, No Call-Stack Limit)

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The problem

Problem, as an interviewer would state it: "Implement a function to flatten a nested array completely, iteratively (no recursion) using a stack. It must correctly handle sparse-array holes and circular references."

Examples:

Code
flatten([1, [2, [3, [4, [5]]]], 6]); // [1, 2, 3, 4, 5, 6]
flatten([1, , [2, , [, 3]], , 4]);   // [1, 2, 3, 4]   (holes are skipped)

Clarifying questions expected:

  • What should a circular reference do — throw, skip, or stop early?
  • Should an array that is merely shared (referenced twice, but not inside itself) be flattened each time?
  • Should there be a depth limit, matching Array.prototype.flat(depth)?

Code / implementation expected: Yes — real, direct proof against native flat, at depths where recursion (and native flat) fail.

  • array
  • stack
  • algorithm

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6 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal, direct proof: an explicit-stack flatten matches native flat, skips holes, distinguishes cycles from shared arrays, and survives depths where native flat throws
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