sindresorhus/is · error · TypeError

Expected value which is `integer`, received value of type

Error message

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

What it means

Thrown by `assertInteger()` (source/index.ts:1693) when `Number.isInteger(value)` is false. Only actual number primitives with no fractional part pass — numeric strings, NaN, Infinity, floats, and BigInt all fail. The library asserts this before code that does indexing, counting, or bit math on the value.

Solutions

Convert string input explicitly before asserting: `Number.parseInt(raw, 10)` or `Number(raw)`, then assert.
Round intentional floats with `Math.trunc`/`Math.round` if a whole number is the real requirement.
Guard upstream computation against NaN/Infinity (e.g. division by zero) instead of letting it reach the assert.
Use `assertSafeInteger()` if the value must also be within Number.MAX_SAFE_INTEGER.

Example fix

// before
const port = process.env.PORT;
assertInteger(port); // throws — it's a string
// after
const port = Number.parseInt(process.env.PORT ?? '3000', 10);
assertInteger(port);

Defensive patterns

Strategy: validation

Validate before calling

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

Type guard

const isInteger = (v: unknown): v is number => typeof v === 'number' && Number.isInteger(v);

Prevention

Parse user/string input with Number.parseInt and verify with Number.isInteger before passing on
Beware floating-point arithmetic producing 0.30000000000000004-style values — round explicitly if an integer is intended
Remember NaN, Infinity, and numeric strings all fail the integer check; validate at the trust boundary
Per this repo's CLAUDE.md, prefer branded guard types so the false branch doesn't narrow number to never

When it happens

Trigger: Calling `assertInteger()` with a numeric string ('42'), a float (3.5), NaN or Infinity from a failed calculation, a BigInt (42n), or undefined/null from a missing field.

Common situations: Values from `process.env`, CLI args, query strings, and form inputs are strings and are never coerced; division or parse results producing floats or NaN; JSON fields typed loosely. Note `Number.isInteger(5.0)` is true but `parseInt` leftovers like '5px' become NaN via `Number()`.

Related errors

AI-assisted analysis of sindresorhus/is@7821031c66 (2026-07-31). Data as JSON: /api/errors/ad7f28f87fd3609b. Report an issue: GitHub.

Appendix: source

Thrown at source/index.ts:1695

		throw new TypeError(message ?? typeErrorMessage('Int16Array', value));
	}
}

export function assertInt32Array(value: unknown, message?: string): asserts value is Int32Array {
	if (!isInt32Array(value)) {
		throw new TypeError(message ?? typeErrorMessage('Int32Array', value));
	}
}

export function assertInt8Array(value: unknown, message?: string): asserts value is Int8Array {
	if (!isInt8Array(value)) {
		throw new TypeError(message ?? typeErrorMessage('Int8Array', value));
	}
}

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

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

export function assertMap<Key = unknown, Value = unknown>(value: unknown, message?: string): asserts value is Map<Key, Value> {
	if (!isMap(value)) {
		throw new TypeError(message ?? typeErrorMessage('Map', value));
	}
}

export function assertNan(value: unknown, message?: string): asserts value is number {
	if (!isNan(value)) {
		throw new TypeError(message ?? typeErrorMessage('NaN', value));

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