Design a JSON parser
PremiumMachine codingHard
- Recursive descent
- Tokenizer
- RFC 8259
Quick answer
This machine coding interview question asks you to design a JSON parser. Aim for a design that meets its requirements at the stated scale, then explain its main components, data flow and trade-offs. Interviewers want to see how you scope the problem, size it and defend each choice.
The problem
Build a JSON parser, the piece every language ships to turn text such as {"id": 7, "tags": ["a"]} into values and back. It must accept exactly valid JSON, reject everything else with a clear position, and print values as text again. The hard parts are string escapes, the number grammar and deep nesting.
What must a JSON parser do?
- Parse: parse(text) returns a value tree (null, boolean, number, string, array or object) or throws a ParseError with message, line and column.
- Grammar: Follow RFC 8259: any value at the top level, no trailing commas, no comments, no single quotes, string keys only, and whitespace limited to space, tab, line feed and carriage return.
How well must it do it?
- Exact errors: Every error names the line, the column and what was expected, for example line 3, column 14: expected ',' or '}' but found ']'.
- Linear time: Parsing is O(n) in the input size, with no backtracking and no copying of the rest of the input per token.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Recursive descent
- Tokenizer
- RFC 8259
- Unicode escapes
- Error reporting
- Serialization
How you’ll practise
- Requirements
- Entities and interfaces
- Core implementation
- Edge cases and tests
- Extensions
- Wrap-up
Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.
What do interviewers look for?
In any machine coding design round.
- Clarifies the requirements before drawing anything
- Uses a few numbers that actually change the design
- Draws a clear path from the client to storage
- Picks storage and caching with reasons, not by habit
- Explains what breaks first under load and how to fix it
- States trade-offs openly instead of claiming a perfect design
Questions people ask about this interview question
- Which design type is this question?
- It is a machine coding question. You design a JSON parser and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Hard. It suits engineers preparing for senior interviews, where you are expected to drive the discussion and go deep on trade-offs.
- What should I cover first?
- Start with the requirements: what the system must do, how well it must do it and which numbers change the design. Agree on scope with the interviewer before you design anything.
- Can I practise it for free?
- It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.
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Updated · Written by the SevyDevy team