Implement JSON.stringify

The question

Deceptively simple, and that’s the point. Anyone can write the recursion. The interviewer is watching for whether you know the type rules, and in particular whether you notice that undefined behaves differently depending on where it appears.

The solution

Returning JavaScript's undefined (rather than the string "undefined") from the recursive call is what lets the object and array branches apply their different rules.

function jsonStringify(value) {
  if (value === null) return "null";

  const type = typeof value;
  if (type === "number") return Number.isFinite(value) ? String(value) : "null";
  if (type === "boolean") return String(value);
  if (type === "string") return quote(value);

  if (Array.isArray(value)) {
    // undefined and functions become null inside an array, not omitted.
    const items = value.map((item) => {
      const text = jsonStringify(item);
      return text === undefined ? "null" : text;
    });
    return "[" + items.join(",") + "]";
  }

  if (type === "object") {
    const pairs = [];
    for (const key of Object.keys(value)) {
      const text = jsonStringify(value[key]);
      // Keys whose value is undefined are dropped entirely.
      if (text !== undefined) pairs.push(quote(key) + ":" + text);
    }
    return "{" + pairs.join(",") + "}";
  }

  // undefined, functions and symbols have no JSON representation.
  return undefined;
}

function quote(text) {
  let out = '"';
  for (const char of text) {
    if (char === '"') out += '\\"';
    else if (char === "\\") out += "\\\\";
    else if (char === "\n") out += "\\n";
    else if (char === "\r") out += "\\r";
    else if (char === "\t") out += "\\t";
    else if (char < " ") out += "\\u" + char.charCodeAt(0).toString(16).padStart(4, "0");
    else out += char;
  }
  return out + '"';
}

How it works

One branch per type, with null checked first, because typeof null is "object" and would otherwise fall through to the object branch.

undefined is the interesting one. The recursive call returns JavaScript’s undefined, not the string, and each container decides what that means: an object drops the key, an array writes null. Strings need escaping rather than just quoting, since a quote or a newline inside one produces JSON that won’t parse back. And NaN and Infinity become null, which surprises people.

The tests it has to pass

This is the part most explanations leave out. "It works" is not a claim you can check by reading. These are the 2 cases Practice Pad runs against your solution, and the implementation above passes all of them.

expectEqual(jsonStringify({ name: "Ada", scores: [3, 5], active: true }), '{"name":"Ada","scores":[3,5],"active":true}');

expectEqual(
  [jsonStringify("text"), jsonStringify(0), jsonStringify(false), jsonStringify(null)],
  ['"text"', "0", "false", "null"],
);

Edge cases to watch out for

Treating undefined the same everywhere

JSON.stringify({ a: undefined }) is "{}", but JSON.stringify([undefined]) is "[null]". Returning the string "undefined" from the recursion makes both wrong at once.

Checking typeof before checking null

typeof null === "object", so a null value handled by the object branch produces "{}" instead of "null". Test for null first.

Quoting strings without escaping them

Wrapping a string in quotes isn’t enough. A string containing a quote character or a newline produces invalid JSON that won’t parse back.

Using Object.entries and forgetting insertion order

Not usually a bug, but worth knowing: integer-like keys are enumerated in ascending numeric order before string keys, so { 2: "a", b: "c", 1: "d" } doesn’t stringify in the order it was written.

Follow up questions

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