Portfolio Pro: Proof of Work for developer hiring

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Portfolio ProProof of work

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Passed: Run tasks with a concurrency limit | Portfolio Pro

Proof of work · Result

Graded in a sandbox

Test code not shown · 35 min suggested

Run tasks with a concurrency limit

Fifty uploads, but only three at a time.

All required tests passed

How to read this: the verdict comes from the required tests. The counts below describe how the code was written. Nothing here shows who typed it. Iterations are the times the tests were run. Versions are distinct versions of the code the tests ran on. Span is the time from first run to save.

Required tests: 6 of 6; passed
Bonus: 1 of 1; passed
Iterations: 3; graded runs
Versions: 3; distinct versions graded
Span: 2 min; first run to save
Runtime: 614 ms; in the sandbox
Saved: Oct 15, 2026

How the code arrived

First graded run: Did not pass every required test.
Largest single jump: 341 bytes added between two graded runs, counting from the starter code.

Tests

PASSreturns results in the order of the tasks, not the order they finished
PASSstarts only as many tasks as the limit allows
PASSnever runs more than the limit at once
PASSstarts the next task as soon as one finishes (not in batches)
PASSresolves an empty list to an empty array

Questions to ask in the interview

The first questions come from this Challenge. The rest come from the counts above. Each one asks the candidate to explain the code in their own words.

How do you start the next task the moment one finishes, rather than after a batch?
What happens to tasks that are already running when one of them fails?
How do you keep results in the order of the tasks array?
How would you change this to collect every failure instead of stopping at the first?
Your first graded run did not pass everything. What did you change to get it passing?

Hiring?

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Writing code?

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Submitted code

solution.js · 36 lines

1function promisePool(tasks, limit) {2  return new Promise((resolve, reject) => {3    const results = new Array(tasks.length);4    let next = 0;5    let done = 0;6    let failed = false;7 8    if (tasks.length === 0) return resolve(results);9 10    const fail = (err) => {11      failed = true;12      reject(err);13    };14 15    const launch = () => {16      if (failed || next >= tasks.length) return;17      const index = next++;18      let running;19      try {20        running = Promise.resolve(tasks[index]());21      } catch (err) {22        fail(err);23        return;24      }25      running.then((value) => {26        if (failed) return;27        results[index] = value;28        done++;29        if (done === tasks.length) resolve(results);30        else launch();31      }, fail);32    };33 34    for (let i = 0; i < Math.min(limit, tasks.length); i++) launch();35  });36}