Handling Invalid JSON — Interview Questions
⚡ Short Answer
Never assume input is valid: wrap parsing in try/catch (or a safeParse helper that returns a result/null), verify HTTP status and Content-Type first, and after parsing validate the shape (required fields/types) before use. On failure, log the raw payload and return a clear error (e.g. 400) rather than letting the parser throw uncaught.
☕Coffee Chat Question
Concept Made Simple
“How do you handle invalid or malformed JSON?”
🧠Mind Map Answer
Remember It Faster
⌨️Hands-on Keyboard
Learn by Doing
function safeParse(text) {
try {
return { ok: true, data: JSON.parse(text) };
} catch (e) {
return { ok: false, error: e.message };
}
}
const r = safeParse('{bad json');
console.log(r.ok ? r.data : "Invalid JSON: " + r.error);Invalid JSON: Unexpected token b ...
🔥What If?
Think Beyond the Expected
Should a public API crash or return a 500 when it receives malformed JSON?
Neither — return 400 Bad Request with a clear message. Malformed input is a client error, not a server fault, so a 500 is misleading and an uncaught throw can take down the request handler. Parse defensively, and respond with 400 plus enough detail for the caller to fix their payload.
😂Real World
Robust services treat every inbound body as hostile: guard, safe-parse, validate, and respond with a precise 400. This is also a security posture — malformed or oversized JSON is a common probing technique.
🎯Interviewer's Expectation
Keywords they're listening for:
⚠️Common Mistakes
- ✗Letting JSON.parse throw uncaught
- ✗Returning 500 for a client's bad JSON
- ✗Skipping shape validation after a successful parse
✅Best Practices
- ✓Safe-parse + validate at every boundary
- ✓Return 400 with actionable messages
- ✓Cap request body size
🔁Follow-up Questions
- 1Why is malformed input a 400 and not a 500?
- 2How does JSON Schema fit into validation?
- 3How do you guard against oversized payloads?
🧩Related Technologies
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Plain-language foundations
I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner. Topic: Parsing (JSON) Interview question: "How do you handle invalid or malformed JSON?" Please: 1. Explain the core idea in simple, plain language, using an everyday analogy. 2. Define any technical terms you use. 3. Walk through one small, concrete example. 4. Finish with a single sentence I can easily remember. Keep the tone friendly and assume I'm new to this topic.
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