EasyExperience level: 0-2 years · 3-5 years1 min read

JSON Syntax Rules — Interview Questions

Reviewed by Gurusankar M.

Asked inInfosysTCSCognizantAccenture
#json#syntax#valid json#rules#formatting
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Short Answer

Valid JSON: data is name/value pairs; objects use {}, arrays use []; keys and string values must use double quotes; values are string/number/boolean/null/object/array; items are comma-separated with NO trailing comma; no comments; the whole document is one value. Whitespace between tokens is ignored.

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Concept Made Simple

Coffee Chat Question

“What are the syntax rules of valid JSON?”

Remember It Faster

Mind Map Answer

Keys
double-quoted strings only
Strings
double quotes (not single)
Separators
`:` key/value, `,` between items
Not allowed
trailing comma, comments, functions

Learn by Doing

Hands-on Keyboard

json
// ✅ valid
{ "id": 1, "tags": ["a", "b"] }

// ❌ invalid: single quotes, trailing comma, comment
{ 'id': 1, "tags": ["a", "b",], /* note */ }

Think Beyond the Expected

What If?

Why does a trailing comma break JSON when JavaScript allows it?

JSON is a strict subset of JavaScript's object syntax and the spec forbids trailing commas, comments, and single quotes to keep parsers simple and unambiguous. JS object literals are more forgiving, but JSON.parse follows the strict grammar — a trailing comma throws a SyntaxError.

Real World

The classic 3am bug: someone hand-edits a config file, leaves a trailing comma or a comment, and the whole service fails to boot. Linters and JSON5/JSONC exist precisely because strict JSON is unforgiving of these.

Interviewer's Expectation

Keywords they're listening for:

  • objects {} / arrays []
  • double-quoted keys & strings
  • comma-separated, no trailing comma
  • no comments
  • one root value

Common Mistakes

  • Trailing comma after the last item
  • Single quotes or unquoted keys
  • Adding // or /* */ comments

Best Practices

  • Lint/validate JSON in CI and editors
  • Use JSONC/JSON5 only where the parser supports it
  • Never hand-edit critical JSON without validation

Follow-up Questions

  1. What is JSON5 / JSONC and why do they exist?
  2. Can the top level be an array or a primitive?
  3. How do you validate JSON syntax programmatically?

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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: Syntax (JSON)
Interview question: "What are the syntax rules of valid 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.
Open inChatGPTGeminiClaude

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