sindresorhus/is · error · TypeError

Expected value which is `boolean`, received value of type `$

Error message

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

What it means

Thrown by `assertBoolean()` when the value is not a primitive `true`/`false` per `isBoolean` (typeof check; Boolean objects don't count in strict primitive terms). Ensures code branching on the value gets a genuine boolean rather than a truthy/falsy stand-in.

Source

Thrown at source/index.ts:1510

		throw new TypeError(message ?? typeErrorMessage('BigInt64Array', value));
	}
}

export function assertBigUint64Array(value: unknown, message?: string): asserts value is BigUint64Array {
	if (!isBigUint64Array(value)) {
		throw new TypeError(message ?? typeErrorMessage('BigUint64Array', value));
	}
}

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

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

// eslint-disable-next-line @typescript-eslint/no-unsafe-function-type
export function assertBoundFunction(value: unknown, message?: string): asserts value is Function {
	if (!isBoundFunction(value)) {
		throw new TypeError(message ?? typeErrorMessage('bound Function', value));
	}
}

/**
Note: [Prefer using `Uint8Array` instead of `Buffer`.](https://sindresorhus.com/blog/goodbye-nodejs-buffer)
*/
export function assertBuffer(value: unknown, message?: string): asserts value is NodeBuffer {
	if (!isBuffer(value)) {
		throw new TypeError(message ?? typeErrorMessage('Buffer', value));
	}
}

View on GitHub ↗ (pinned to 7821031c66)

Solutions

Coerce string flags explicitly before asserting: `value === 'true'` for env vars/query params.
Default optional flags: `assert.boolean(options.flag ?? false)` or validate presence separately.
Fix the config source to use unquoted JSON booleans.
For DB values, map 0/1 to Boolean at the data-access layer.

Example fix

// before
assert.boolean(process.env.DEBUG);
// after
assert.boolean(process.env.DEBUG === 'true');

Defensive patterns

Strategy: type-guard

Validate before calling

if (typeof value !== 'boolean') {
  // coerce deliberately: value = value === 'true' — never rely on truthiness
}

Type guard

function isBoolean(value: unknown): value is boolean {
  return typeof value === 'boolean';
}

Try / catch

try {
  assert.boolean(value);
} catch (error) {
  if (error instanceof TypeError) {
    // strings 'true'/'false', 0/1, or undefined slipped through — coerce explicitly at the boundary
  } else throw error;
}

Prevention

Environment variables and query params are always strings — parse 'true'/'false' explicitly before passing them on
Avoid Boolean object wrappers (new Boolean(true)); they fail typeof checks
Give optional boolean parameters explicit defaults (flag = false) so undefined never reaches the assertion

When it happens

Trigger: Calling `assert.boolean(value)` with the strings 'true'/'false', 0/1, undefined, null, or a `new Boolean(...)` wrapper object.

Common situations: Environment variables and query-string parameters are always strings ('true' !== true); JSON configs hand-edited with quoted booleans; database drivers returning 0/1 tinyint for boolean columns; optional flags left undefined instead of defaulted.

Related errors

AI-assisted analysis of sindresorhus/is@7821031c66 (2026-07-31). Data as JSON: /data/errors/638d76e7de499b99.json. Report an issue: GitHub ↗.