sindresorhus/is · error · TypeError

Expected value which is `WeakSet`, received value of type `$

Error message

Expected value which is `WeakSet`, received value of type `${is(value)}`.

What it means

Thrown by `assertWeakSet()` when the value is not a WeakSet instance. `isWeakSet` compares the toString tag (`getObjectType(value) === 'WeakSet'`), so a regular Set, an Array, or an API-compatible mock will all be rejected. Like the other assert functions in this library, it throws a TypeError naming the expected type and the detected type of what was actually received.

Source

Thrown at source/index.ts:2002

// eslint-disable-next-line @typescript-eslint/no-restricted-types
export function assertWeakMap<Key extends object = object, Value = unknown>(value: unknown, message?: string): asserts value is WeakMap<Key, Value> {
	if (!isWeakMap(value)) {
		throw new TypeError(message ?? typeErrorMessage('WeakMap', value));
	}
}

// eslint-disable-next-line @typescript-eslint/no-restricted-types
export function assertWeakRef<T extends object = object>(value: unknown, message?: string): asserts value is WeakRef<T> {
	if (!isWeakRef(value)) {
		throw new TypeError(message ?? typeErrorMessage('WeakRef', value));
	}
}

// eslint-disable-next-line @typescript-eslint/no-restricted-types
export function assertWeakSet<T extends object = object>(value: unknown, message?: string): asserts value is WeakSet<T> {
	if (!isWeakSet(value)) {
		throw new TypeError(message ?? typeErrorMessage('WeakSet', value));
	}
}

export function assertWhitespaceString(value: unknown, message?: string): asserts value is string {
	if (!isWhitespaceString(value)) {
		throw new TypeError(message ?? typeErrorMessage('whitespace string', value));
	}
}

export default is;

export type {
	ArrayLike,
	Class,
	EvenInteger,
	FiniteNumber,
	Integer,
	NaN,

View on GitHub ↗ (pinned to 7821031c66)

Solutions

Construct the value with `new WeakSet()` instead of `new Set()` at the failing call site.
If members can be primitives, keep a `Set` and assert with `assertSet` — WeakSet cannot hold primitives.
If both collection types are valid inputs, use non-throwing guards (`is.weakSet(value) || is.set(value)`) and branch instead of asserting.

Example fix

// before
const visited = new Set();
assertWeakSet(visited); // throws TypeError

// after
const visited = new WeakSet();
assertWeakSet(visited); // passes

Defensive patterns

Strategy: type-guard

Validate before calling

if (is.weakSet(value)) {
  assert.weakSet(value); // safe
}

Type guard

import is from '@sindresorhus/is';

function ensureWeakSet(value: unknown): WeakSet<object> | undefined {
  return is.weakSet(value) ? value : undefined;
}

Try / catch

try {
  assert.weakSet(value);
} catch (error) {
  if (error instanceof TypeError) {
    handleInvalidInput(error.message);
  } else {
    throw error;
  }
}

Prevention

Use the boolean guard is.weakSet(value) as a branch condition rather than catching the assert's TypeError for expected non-WeakSet inputs
A Set is not a WeakSet — make sure the producing code calls new WeakSet(), not new Set()
WeakSets can only hold objects and are not serializable; anything deserialized from JSON or IPC will fail this assert
Annotate parameters as WeakSet<object> in TypeScript so the compiler flags wrong collection types before runtime

When it happens

Trigger: Calling `assertWeakSet(value)` (or `is.assert.weakSet(value)`) with a `Set`, an array of objects, null/undefined, or a duck-typed object exposing add/has/delete. Any value whose Symbol.toStringTag-derived type is not exactly 'WeakSet' triggers it.

Common situations: Most often hit when converting a `Set` used for object-membership tracking (e.g. visited-node sets in graph/serialization code) to a `WeakSet` and one code path still constructs a Set; or when trying to use a WeakSet with primitive members — since WeakSet only holds objects, developers fall back to Set and the assertion then fails.

Related errors

AI-assisted analysis of sindresorhus/is@7821031c66 (2026-07-31). Data as JSON: /data/errors/4e26a4d727cbf6b6.json. Report an issue: GitHub ↗.