JSON.stringify returns undefined — what to do

JSON.stringify(undefined) returns undefined (the JavaScript primitive), not the string "undefined". This surprises many developers and causes subtle bugs when the result is used as a response body, stored, or passed to JSON.parse.

When JSON.stringify returns undefined

• Top-level undefined: JSON.stringify(undefined) → undefined
• Top-level function: JSON.stringify(() => {}) → undefined
• Top-level symbol: JSON.stringify(Symbol('x')) → undefined

JSON.stringify(undefined)   // undefined
JSON.stringify(() => {})    // undefined
JSON.stringify(Symbol('x')) // undefined

// Passing these results to JSON.parse:
JSON.parse(JSON.stringify(undefined))
// SyntaxError: Unexpected token u in JSON at position 0

Properties silently dropped inside objects

Values that can't be serialized are dropped silently when they appear as object property values:

JSON.stringify({
  name: "Ada",
  fn: () => {},        // dropped
  sym: Symbol('x'),   // dropped
  x: undefined,       // dropped
})
// '{"name":"Ada"}'

undefined in arrays becomes null

JSON.stringify([1, undefined, 3])
// '[1,null,3]'  — undefined becomes null, not dropped

Fixes

Replace undefined with null before serializing

const value = someFunction() ?? null;  // undefined → null
JSON.stringify(value);  // ✓ produces "null"

Use a replacer to control what's omitted

JSON.stringify(obj, (key, val) => val === undefined ? null : val);
// converts all undefined values to null instead of dropping them

Use JSON.stringify with a fallback

const result = JSON.stringify(value) ?? '{"error":"not serializable"}';

Checking before you parse

const raw = JSON.stringify(data);
if (raw === undefined) throw new Error('Data is not JSON-serializable');
sendToServer(raw);