sindresorhus/is · error · TypeError

Expected value which is `valid length`, received value of ty

Error message

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

What it means

Thrown by `assertValidLength()` in @sindresorhus/is when the value fails `isValidLength`, i.e. it is not a safe integer >= 0 (`Number.isSafeInteger(value) && value >= 0`). It validates values meant to be used as array/string lengths or counts.

Source

Thrown at source/index.ts:1981

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

export function assertUrlString(value: unknown, message?: string): asserts value is UrlString {
	if (!isUrlString(value)) {
		throw new TypeError(message ?? typeErrorMessage('string with a URL', value));
	}
}

export function assertValidDate(value: unknown, message?: string): asserts value is Date {
	if (!isValidDate(value)) {
		throw new TypeError(message ?? typeErrorMessage('valid Date', value));
	}
}

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

// 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

View on GitHub ↗ (pinned to 7821031c66)

Solutions

Convert and round appropriately: `Math.floor(value)` or `Math.trunc(value)` for computed lengths, and clamp with `Math.max(0, value)` if underflow is expected.
Parse strings to numbers first: `Number.parseInt(str, 10)` and validate the result isn't NaN.
Find the arithmetic that produced the negative/fractional value — the assertion usually flags an upstream logic bug (off-by-one, division), not a formatting problem.

Example fix

// before
const pages = total / pageSize;
assert.validLength(pages);
// after
const pages = Math.ceil(total / pageSize);
assert.validLength(pages);

Defensive patterns

Strategy: validation

Validate before calling

function isValidLength(value) {
  return Number.isSafeInteger(value) && value >= 0;
}
if (!isValidLength(len)) {
  throw new RangeError(`Invalid length: ${len}`);
}

Type guard

if (is.validLength(value)) {
  // value is a non-negative safe integer
}

Try / catch

try {
  assert.validLength(value);
} catch (error) {
  if (error instanceof TypeError) { /* negative, fractional, NaN, or unsafe integer */ }
  throw error;
}

Prevention

A valid length is a non-negative safe integer — guard against NaN, negatives, fractions, and Infinity from arithmetic
Validate lengths derived from user input or division/subtraction before using them to size arrays or slices
Use Number.isSafeInteger(n) && n >= 0 as the equivalent pre-check

When it happens

Trigger: `assert.validLength(value)` with a negative number, a float (e.g. 2.5), NaN, Infinity, a number beyond Number.MAX_SAFE_INTEGER, a numeric string like '5', or a BigInt.

Common situations: Lengths computed by division producing floats (`total / pageSize`); parsing lengths from headers or query strings without Number conversion; offset arithmetic underflowing to negative values; Content-Length beyond safe-integer range.

Related errors

AI-assisted analysis of sindresorhus/is@7821031c66 (2026-07-31). Data as JSON: /data/errors/db73ec21b3de0ae8.json. Report an issue: GitHub ↗.