sindresorhus/is · error · TypeError

Expected value which is `PropertyKey`, received value of typ

Error message

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

What it means

Thrown by `assert.propertyKey()` (assertPropertyKey at source/index.ts:1864) in the @sindresorhus/is library when the value fails `isPropertyKey`, meaning it is not a valid object property key. A PropertyKey in JavaScript is a `string`, `number`, or `symbol` — the only types usable to index an object. The message interpolates `is(value)` to report the actual detected type of the rejected value.

Source

Thrown at source/index.ts:1866

		throw new TypeError(message ?? typeErrorMessage('positive number', value));
	}
}

export function assertPrimitive(value: unknown, message?: string): asserts value is Primitive {
	if (!isPrimitive(value)) {
		throw new TypeError(message ?? typeErrorMessage('primitive', value));
	}
}

export function assertPromise<T = unknown>(value: unknown, message?: string): asserts value is Promise<T> {
	if (!isPromise(value)) {
		throw new TypeError(message ?? typeErrorMessage('Promise', value));
	}
}

export function assertPropertyKey(value: unknown, message?: string): asserts value is PropertyKey {
	if (!isPropertyKey(value)) {
		throw new TypeError(message ?? typeErrorMessage('PropertyKey', value));
	}
}

export function assertRegExp(value: unknown, message?: string): asserts value is RegExp {
	if (!isRegExp(value)) {
		throw new TypeError(message ?? typeErrorMessage('RegExp', value));
	}
}

export function assertSafeInteger(value: unknown, message?: string): asserts value is number {
	if (!isSafeInteger(value)) {
		throw new TypeError(message ?? typeErrorMessage('safe integer', value));
	}
}

export function assertSet<T = unknown>(value: unknown, message?: string): asserts value is Set<T> {
	if (!isSet(value)) {
		throw new TypeError(message ?? typeErrorMessage('Set', value));

View on GitHub ↗ (pinned to 7821031c66)

Solutions

Inspect the failing value — ensure the argument is a string, number, or symbol before asserting.
If the key may be missing, guard with `if (key !== undefined)` or use `is.propertyKey(key)` for a non-throwing check.
Convert non-key values explicitly, e.g. `String(key)`, if coercion is the intended behavior.

Example fix

// before
const key = obj.id; // may be undefined or an object
assert.propertyKey(key);
// after
const key = String(obj.id ?? '');
assert.propertyKey(key);

Defensive patterns

Strategy: type-guard

Validate before calling

const t = typeof value;
if (t !== 'string' && t !== 'number' && t !== 'symbol') {
  throw new TypeError(`Expected a PropertyKey, got ${t}`);
}

Type guard

function isPropertyKey(value: unknown): value is PropertyKey {
  const t = typeof value;
  return t === 'string' || t === 'number' || t === 'symbol';
}

Try / catch

try {
  assert.propertyKey(value);
} catch (error) {
  if (error instanceof TypeError) {
    // value was not string | number | symbol — reject the input
  }
  throw error;
}

Prevention

Use is.propertyKey(value) before calling assert.propertyKey so failure is a branch, not a throw
Type dynamic key parameters as PropertyKey instead of any so the compiler catches most misuse
Watch for objects used as keys — they get coerced to '[object Object]' elsewhere but fail this check

When it happens

Trigger: Calling `assert.propertyKey(value)` with anything other than a string, number, or symbol — e.g. an object, array, boolean, null, or undefined that was intended to be used as a dynamic object key.

Common situations: Building dynamic property access where the key comes from user input, JSON, or a Map key that turned out to be an object; passing `undefined` because an optional key argument was omitted; accidentally passing the whole object `{key: 'x'}` instead of `'x'`.

Related errors

AI-assisted analysis of sindresorhus/is@7821031c66 (2026-07-31). Data as JSON: /data/errors/5141fffce33acb93.json. Report an issue: GitHub ↗.