How Misusing JSON.stringify Almost Cost My Friend His Year‑End Bonus

My friend "胖头" (nickname) took over a module after a colleague left, and a bug appeared when the page submitted a form because the JSON payload omitted the value field.

The issue stemmed from JSON.stringify silently dropping properties whose values are undefined, turning them into missing fields that the server could not parse, leading to the failure.

JSON.stringify follows several rules: objects with a toJSON() method define their own serialization, primitive wrappers are converted to primitives, undefined, functions and symbols are omitted in objects (or become null in arrays), Symbol‑keyed properties are ignored, Dates are serialized as ISO strings, and circular references throw a TypeError.

To keep the value field, we can preprocess the data, replacing undefined with an empty string before calling JSON.stringify:

let newSignInfo = signInfo.map(it => {
  const value = typeof it.value === 'undefined' ? '' : it.value;
  return { ...it, value };
});
console.log(JSON.stringify(newSignInfo)); // '[{"fieldId":539,"value":""},...]'

Beyond the fix, I also wrote a simple custom jsonstringify function that mimics the native behavior, handling cyclic references, BigInt errors, and the various type conversions.

const jsonstringify = (data) => {
  // detect cycles
  const isCyclic = (obj) => { /* ... */ };
  if (isCyclic(data)) throw new TypeError('Converting circular structure to JSON');
  if (typeof data === 'bigint') throw new TypeError('Do not know how to serialize a BigInt');
  // handle primitives, arrays, objects, toJSON, etc.
  // (implementation omitted for brevity)
  return result;
};

The story ends with a reminder: understand JSON.stringify’s edge cases, test small changes thoroughly, and you can avoid costly bugs that threaten bonuses or even jobs.