MediumExperience level: 3-5 years · 8-15 years1 min read

Common JSON Parsing Errors — Interview Questions

Reviewed by Gurusankar M.

Asked inAmazonMicrosoftCognizantDeloitte
#json#parsing errors#unexpected token#syntax error#debugging
Report issue

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').

On this page

Concept Made Simple

Coffee Chat Question

“What are common JSON parsing errors and their causes?”

Remember It Faster

Mind Map Answer

Unexpected token <
parsing an HTML/error page as JSON
Trailing comma
comma after last item
Unexpected end
truncated / empty body
Bad quotes
single quotes / unescaped chars

Learn by Doing

Hands-on Keyboard

javascript
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

Think Beyond the Expected

What If?

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:

  • trailing comma / single quotes
  • unescaped chars
  • truncated body
  • wrong content (HTML)
  • check status/content-type first

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

  1. How do you handle these errors gracefully?
  2. Why check Content-Type before parsing?
  3. How does encoding/BOM cause parse failures?

Continue Learning with AI

Take this question deeper with your favourite AI assistant. Pick a depth, copy the prompt, or open it directly — AI is your learning companion, not a shortcut.

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.
Open inChatGPTGeminiClaude

Was this answer helpful?

Affiliate links

Tools for Your Developer Workspace

Equipment for a comfortable, productive setup.

As an Amazon Associate I earn from qualifying purchases.