Guided reviewIntermediate15–25 min

Find the bugs in a queue consumer

Task: read queue.bad.ts. The program can restart, and a message can come back a second time. Say what can go wrong and what you'd fix first.

Tests: code review and understanding repeats.

This is a code-reading exercise — it isn't run. Write your notes in the editor as comments, then compare them with the checklist.

Follow-up: what about success followed by a crash before acknowledgement?

You will practise

Find bugs in asynchronous handling
Recognise problems with a repeated message
Propose an order of fixes

Related questions

Source: AI & Agents — practice tasks — An exercise written for the study programme, not a confirmed interview task.

Not run automatically — compare with the checklist

// Deliberately faulty starter for review. Do NOT run against a real service.
const seen=new Set<string>();
export async function consume(messages:{id:string;amount:number}[], api:{pay:(amount:number)=>Promise<void>}) {
  messages.forEach(async message=>{
    if(seen.has(message.id))return;
    seen.add(message.id);
    try {await api.pay(message.amount);}
    catch(error){console.log('ignored',error);}
  });
  return {success:true};
}

Tab indents · Esc, then Tab, leaves the editor · Your code is saved in this browser.

forEach(async) doesn't wait for the work to finish
The local Set disappears on restart and doesn't protect other servers
“Seen” is recorded before the payment succeeds
Errors are hidden (only console.log)
The function reports success too early

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Hints

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Reference solution

Give it a real try first — the solution is most useful after you've struggled a bit.