sindresorhus/is · error · TypeError

Expected value which is `number`, received value of type `${

Error message

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

What it means

Thrown by `assert.number()` in the `@sindresorhus/is` library when the given value is not of primitive type `number`. The assert family wraps the `is.*` predicates and throws a TypeError so the value can be safely narrowed via TypeScript's `asserts value is number`. The message interpolates `is(value)` to report the actual detected type of what was received.

Source

Thrown at source/index.ts:1805

}

// eslint-disable-next-line @typescript-eslint/no-restricted-types
export function assertNull(value: unknown, message?: string): asserts value is null {
	if (!isNull(value)) {
		throw new TypeError(message ?? typeErrorMessage('null', value));
	}
}

// eslint-disable-next-line @typescript-eslint/no-restricted-types
export function assertNullOrUndefined(value: unknown, message?: string): asserts value is null | undefined {
	if (!isNullOrUndefined(value)) {
		throw new TypeError(message ?? typeErrorMessage('null or undefined', value));
	}
}

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

export function assertNumericString(value: unknown, message?: string): asserts value is `${number}` {
	if (!isNumericString(value)) {
		throw new TypeError(message ?? typeErrorMessage('string with a number', value));
	}
}

// eslint-disable-next-line @typescript-eslint/no-restricted-types
export function assertObject(value: unknown, message?: string): asserts value is object {
	if (!isObject(value)) {
		throw new TypeError(message ?? typeErrorMessage('Object', value));
	}
}

export function assertObservable(value: unknown, message?: string): asserts value is ObservableLike {
	if (!isObservable(value)) {

View on GitHub ↗ (pinned to 7821031c66)

Solutions

1. Convert the value first, e.g. `const n = Number(raw)` before `assert.number(n)`.
2. If the input is legitimately a numeric string, use `assert.numericString()` instead.
3. Log `is(value)` from the error message to identify what the caller actually passed and fix the producer.
4. If undefined is allowed, guard with `if (value !== undefined) assert.number(value)`.

Example fix

// before
const port = process.env.PORT;
assert.number(port); // throws: received 'string'
// after
const port = Number(process.env.PORT);
assert.number(port);

if (typeof value !== 'number' || Number.isNaN(value)) {
  throw new TypeError(`Expected number, got ${typeof value}`);
}

function isNumber(value: unknown): value is number {
  return typeof value === 'number' && !Number.isNaN(value);
}

try {
  ow(value, ow.number);
} catch (error) {
  if (error instanceof ArgumentError) {
    // report invalid input to caller; do not substitute a default
    throw new TypeError(`Invalid numeric input: ${error.message}`);
  }
  throw error;
}

When it happens

Trigger: Calling `assert.number(value)` (or `assertNumber`) with anything whose typeof is not 'number' — strings like '42', BigInt, null, undefined, NaN is accepted since typeof NaN === 'number'. Common at API boundaries where JSON/query params arrive as strings.

Common situations: Parsing query strings, env vars, or form inputs where numbers arrive as strings; forgetting to call Number()/parseFloat before validation; a config file storing a number as a quoted string; upstream API changing a field from number to string.

Related errors

• Expected value which is `non-empty array`, received value of
• Expected value which is `non-empty map`, received value of t
• Expected value which is `non-empty object`, received value o
• Expected value which is `non-empty set`, received value of t
• Expected value which is `non-empty string`, received value o

AI-assisted analysis of sindresorhus/is@7821031c66 (2026-07-31). Data as JSON: /data/errors/1baf61f3d84eca0b.json. Report an issue: GitHub ↗.