Common JSON Parsing Errors — Interview Questions
⚡ Short Answer
The usual culprits: trailing commas, single quotes or unquoted keys, comments, unescaped control characters or quotes inside strings, wrong encoding/BOM, and truncated payloads ('Unexpected end of input'). A frequent one is calling JSON.parse on something that isn't JSON at all — like an HTML error page ('Unexpected token < in JSON').
☕Coffee Chat Question
Concept Made Simple
“What are common JSON parsing errors and their causes?”
🧠Mind Map Answer
Remember It Faster
⌨️Hands-on Keyboard
Learn by Doing
JSON.parse("{'a':1}"); // ❌ single quotes -> SyntaxError
JSON.parse('{"a":1,}'); // ❌ trailing comma
JSON.parse(''); // ❌ Unexpected end of input
JSON.parse('<html>...'); // ❌ Unexpected token < (got HTML)
JSON.parse('{"a":1}'); // ✅ ok🔥What If?
Think Beyond the Expected
Your API call fails with 'Unexpected token < in JSON at position 0' — what happened?
You tried to JSON.parse an HTML page, not JSON. Usually the request hit a proxy/error page or a 404/500 that returned HTML, and the client parsed the body blindly. Fix it by checking the HTTP status and Content-Type before parsing, and by logging the raw response body.
😂Real World
'Unexpected token < in JSON' is one of the most-Googled front-end errors — it almost always means the endpoint returned an HTML error page. Checking status/content-type before parsing prevents it.
🎯Interviewer's Expectation
Keywords they're listening for:
⚠️Common Mistakes
- ✗Parsing before checking HTTP status/content-type
- ✗Assuming a 200 always means valid JSON
- ✗Not logging the raw body on failure
✅Best Practices
- ✓Verify status + Content-Type before parsing
- ✓Wrap parsing in try/catch
- ✓Log the raw response when parsing fails
🔁Follow-up Questions
- 1How do you handle these errors gracefully?
- 2Why check Content-Type before parsing?
- 3How does encoding/BOM cause parse failures?
🧩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: "What are common JSON parsing errors and their causes?" 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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