withastro/astro · error · Error

Cannot convert undefined to an object.

Error message

Cannot convert undefined to an object.

What it means

`AstroCookie.json()` parses the cookie's string value with `JSON.parse`, but first guards against an `undefined` value because `JSON.parse(undefined)` yields `Unexpected token u` — a confusing message. By throwing the explicit 'Cannot convert undefined to an object.' the API communicates that no cookie value exists to deserialize. Note the guard compares against `undefined` strictly, which is only possible when the cookie was constructed without a value.

Source

Thrown at packages/astro/src/core/cookies/cookies.ts:55

		options?: AstroCookieSetOptions,
	): void;
	delete(key: string, options?: AstroCookieDeleteOptions): void;
}

const DELETED_EXPIRATION = new Date(0);
const DELETED_VALUE = 'deleted';
const responseSentSymbol = Symbol.for('astro.responseSent');

const identity = (value: string) => value;

class AstroCookie implements AstroCookieInterface {
	public value: string;
	constructor(value: string) {
		this.value = value;
	}
	json() {
		if (this.value === undefined) {
			throw new Error(`Cannot convert undefined to an object.`);
		}
		return JSON.parse(this.value);
	}
	number() {
		return Number(this.value);
	}
	boolean() {
		if (this.value === 'false') return false;
		if (this.value === '0') return false;
		return Boolean(this.value);
	}
}

class AstroCookies implements AstroCookiesInterface {
	#request: Request;
	#requestValues: Record<string, string | undefined> | null;
	#outgoing: Map<string, [string, string, boolean]> | null;
	#consumed: boolean;

View on GitHub (pinned to d081033d5f)

Solutions

1. Check the cookie exists and has a value before calling `.json()`: `const c = Astro.cookies.get('data'); if (c) { const v = c.json(); }`.
2. Ensure the code constructing `AstroCookie` passes a real string (default to `''`).
3. If using `.get()` returns undefined, treat absence explicitly rather than coercing.

Example fix

// before
const data = Astro.cookies.get('cart').json();

// after
const cart = Astro.cookies.get('cart');
const data = cart ? cart.json() : {};

const c = Astro.cookies.get('key'); if (!c) return; const v = c.json();

function hasCookieValue(c) { return c != null && c.value !== undefined && c.value !== ''; }

try { data = cookie.json(); } catch (e) { if (/undefined/.test(e.message)) data = {}; else throw e; }

When it happens

Trigger: Calling `.json()` on an `AstroCookie` whose `value` is `undefined` — typically when a cookie accessor returned a placeholder/empty cookie. In practice the `AstroCookie` wrapper is constructed with a string, so this fires when a code path constructs `new AstroCookie(undefined as any)` or a similar coercion.

Common situations: A library/integration building an `AstroCookie` from an unguarded lookup; deserializing a cookie whose value was deleted/emptied; TypeScript bypass (`as any`) hiding an undefined.

Related errors

• ResponseSentError
• AstroResponseHeadersReassigned
• UnknownContentCollectionError
• RenderUndefinedEntryError
• CacheNotEnabled

AI-assisted analysis of withastro/astro@d081033d5f (2026-08-12). Data as JSON: /api/errors/6da1e17dff301aee. Report an issue: GitHub.