Fix: JSON.stringify() Drops undefined

The Three Behaviors

1. Object Key With undefined Value — Dropped

const user = { name: "Alice", age: undefined, role: "admin" };
JSON.stringify(user);
// => '{"name":"Alice","role":"admin"}'
//    ↑ "age" key is completely gone

2. Array Element undefined — Becomes null

const scores = [10, undefined, 30];
JSON.stringify(scores);
// => '[10,null,30]'
//      ↑ undefined became null, not dropped

3. Function Values — Also Dropped

const obj = { name: "Alice", greet: () => "hi" };
JSON.stringify(obj);
// => '{"name":"Alice"}'
//    ↑ function key "greet" is gone

NaN and Infinity Also Lose Their Values

Unlike undefined, NaN and Infinity keep their keys but become null:

JSON.stringify({ score: NaN, limit: Infinity, neg: -Infinity });
// => '{"score":null,"limit":null,"neg":null}'

Solutions

Replacer Function — Convert undefined to null

Pass a replacer as the second argument to preserve all keys:

const user = { name: "Alice", age: undefined };

JSON.stringify(user, (key, val) => (val === undefined ? null : val));
// => '{"name":"Alice","age":null}'
//    ↑ key preserved, value is null

safeStringify Helper

A reusable utility that also handles NaN and Infinity:

function safeStringify(value: unknown, indent?: number): string {
  return JSON.stringify(
    value,
    (_, val) => {
      if (val === undefined) return null;
      if (typeof val === "number" && !isFinite(val)) return null;
      return val;
    },
    indent
  );
}

safeStringify({ name: "Alice", age: undefined, score: NaN });
// => '{"name":"Alice","age":null,"score":null}'

Quick Reference

Validate Your JSON Below

Paste the output of JSON.stringify() here to inspect what survived serialization.