How to interview a junior engineer properly

Whiteboard puzzles select for practice, not potential. Try these three tasks instead.

Give the candidate a small, broken codebase and one hour. What they choose to fix first tells you more than any algorithm question.

Follow with a code reading exercise, then a short design conversation about a system they have actually built.

Why the broken codebase works: juniors rehearse greenfield puzzles endlessly, but real jobs are maintenance. Seed the exercise with three planted issues — a straightforward bug, a missing input validation, and a misleading comment that contradicts the code. Watch what they do first. Strong candidates read before editing, run the code to reproduce the bug, fix the highest-impact item, and say out loud what they are deprioritising. That narration is the real signal: engineers who think aloud are teachable, and teachability outranks current knowledge at this level every time.

The code reading exercise needs no special materials — hand them a genuine file from your codebase (sanitised) and ask three questions: what does this do, what would you change, where could this break? You learn whether they can navigate unfamiliar conventions, whether their critique shows taste or trivia, and how they respond when they don't understand something. Do they guess silently, ask a clarifying question, or bluff? The second behaviour predicts success; the third predicts incidents.

For the design conversation, let them choose a project from their own portfolio and go deep rather than wide. Ask why, then why again, then what broke: 'You chose Postgres — why not Mongo? What did that cost you? What failed at demo time?' Candidates discussing work they actually did reveal honest depth; candidates reciting tutorials reveal shallow layers quickly. Do not punish gaps in knowledge — probe how they'd close them. 'I'd read the docs and build a spike' is exactly what you want to hear.

Structure the whole loop around fairness to both sides. One hour of practical work maximum, paid if it exceeds an hour. Tell candidates the format in advance so preparation reflects the job. Give written feedback within two weeks either way — in a market as networked as Nigerian tech, candidates who felt respected become applicants again later and referrers always; candidates who felt used become cautionary tales in group chats. Your interview process is employer branding whether you designed it that way or not.