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JSON.stringify Polyfill

Q: JSON.stringify Polyfill

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Hard > **How to read this doc:** "a basic `JSON.stringify`" hides a dozen rules, so this one implements the full contract (`toJSON`, replacer function, replacer array, `space`, cycle detection, string escaping) and was fuzzed against the native function: 150,000 random `(value, replacer, space)` triples (132,751 produced text, 8,534 produced `undefined`, 8,715 threw), the call sequence of 30,000 replacer runs, every one of the 65,536 UTF-16 code units alone and next to a surrogate, and 200,000 random numbers. Zero mismatches. Each classic mistake was also reproduced, so the pitfalls below are observed, not assumed. ## 1. The problem, restated Write `jsonStringify(value, replacer, space)` that returns JSON text (or `undefined`) exactly like the built-in: values that JSON cannot represent are omitted, turned into `null`, or rejected; objects may customise themselves with `toJSON`; a re

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The problem

Problem, as an interviewer would state it: "Implement a basic version of JSON.stringify. Discuss edge cases, runtime, and alternatives."

Examples:

Code

jsonStringify({ a: 1, b: undefined, c: [undefined, NaN, new Date(0)] });   // '{"a":1,"c":[null,null,"1970-01-01T00:00:00.000Z"]}'
jsonStringify({ a: [1, { b: 2 }] }, null, 2);                              // pretty-printed with 2 spaces
jsonStringify(circular);                                                    // throws TypeError

Clarifying questions expected:

How much of the contract: toJSON, the replacer function and array, space?
What happens to undefined, functions, symbols, NaN, BigInt and circular structures?
Which keys are included, and in what order?

Code / implementation expected: Yes — real code, checked against the native JSON.stringify on many inputs.

polyfill
recursion

Try it yourself

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Solution

Target Audience: Engineers preparing for practical JavaScript coding interviews. Difficulty: Hard

How to read this doc: "a basic JSON.stringify" hides a dozen rules, so this one implements the full contract (toJSON, replacer function, replacer array, space, cycle detection, string escaping) and was fuzzed against the native function: 150,000 random (value, replacer, space) triples (132,751 produced text, 8,534 produced undefined, 8,715 threw), the call sequence of 30,000 replacer runs, every one of the 65,536 UTF-16 code units alone and next to a surrogate, and 200,000 random numbers. Zero mismatches. Each classic mistake was also reproduced, so the pitfalls below are observed, not assumed.

1. The problem, restated

Write jsonStringify(value, replacer, space) that returns JSON text (or undefined) exactly like the built-in: values that JSON cannot represen

The solution is hidden so you can try first

9 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal, direct proof: the polyfill matches the native JSON.stringify on random values, replacers and spaces, on every UTF-16 code unit, and prints each rule

Practice

Discussion

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