sindresorhus/is · warning · TypeError

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

Error message

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

What it means

Thrown by `assert.truthy()` (assertTruthy at source/index.ts:1906) when the value is falsy: `false`, `0`, `-0`, `0n`, `''`, `null`, `undefined`, or `NaN`. Unlike the other assertions this is not a type check but a value check — its assertion signature narrows `T | Falsy` to `T`, which is why the codebase uses it to strip null/undefined/empty from union types.

Source

Thrown at source/index.ts:1908

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

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

export function assertSymbol(value: unknown, message?: string): asserts value is symbol {
	if (!isSymbol(value)) {
		throw new TypeError(message ?? typeErrorMessage('symbol', value));
	}
}

export function assertTruthy<T>(value: T | Falsy, message?: string): asserts value is T {
	if (!isTruthy(value)) {
		throw new TypeError(message ?? typeErrorMessage('truthy', value));
	}
}

export function assertTupleLike<T extends Array<TypeGuard<unknown>>>(value: unknown, guards: [...T], message?: string): asserts value is ResolveTypesOfTypeGuardsTuple<T> {
	if (!isTupleLike(value, guards)) {
		throw new TypeError(message ?? typeErrorMessage('tuple-like', value));
	}
}

export function assertTypedArray(value: unknown, message?: string): asserts value is TypedArray {
	if (!isTypedArray(value)) {
		throw new TypeError(message ?? typeErrorMessage('TypedArray', value));
	}
}

export function assertUint16Array(value: unknown, message?: string): asserts value is Uint16Array {
	if (!isUint16Array(value)) {
		throw new TypeError(message ?? typeErrorMessage('Uint16Array', value));

View on GitHub ↗ (pinned to 7821031c66)

Solutions

If `0` or `''` are legitimate values, replace with a specific check: `assert.number(value)` plus an explicit `!== undefined` guard, rather than truthiness.
Trace why the value is `null`/`undefined` — a failed lookup or missing field — and handle that case explicitly before asserting.
Provide a domain-specific `message` argument so failures identify which value was falsy.

Example fix

// before
assert.truthy(items.indexOf(target)); // fails when index is 0!
// after
const index = items.indexOf(target);
assert.truthy(index !== -1, 'target not found in items');

Defensive patterns

Strategy: validation

Validate before calling

if (!value) {
  throw new TypeError(`Expected a truthy value, got ${value}`);
}

Type guard

function isTruthy<T>(value: T): value is Exclude<T, false | 0 | 0n | '' | null | undefined> {
  return Boolean(value);
}

Try / catch

try {
  assert.truthy(value);
} catch (error) {
  if (error instanceof TypeError) {
    // value was false, 0, -0, 0n, '', null, undefined, or NaN
  }
  throw error;
}

Prevention

Know the full falsy list: false, 0, -0, 0n, '', null, undefined, NaN — legitimate values like 0 and '' fail this check
If 0 or empty string are valid inputs, use a null/undefined check (value != null) instead of a truthiness assertion
Set defaults with ?? rather than || so falsy-but-valid values survive

When it happens

Trigger: Calling `assert.truthy(value)` where value is `undefined` (missing property/return), `null` (API no-result), `0` or `''` — the last two are frequent false positives when zero or empty string are legitimate values.

Common situations: Using truthiness as a null-check on numeric fields where `0` is valid (counts, indexes, prices) or string fields where `''` is valid; lookup functions (`map.get`, `find`) returning `undefined`; NaN from failed numeric parsing sneaking through.

Related errors

AI-assisted analysis of sindresorhus/is@7821031c66 (2026-07-31). Data as JSON: /data/errors/7460784318d2ec98.json. Report an issue: GitHub ↗.