sindresorhus/is · error · TypeError

Expected value which is `empty string or whitespace`, receiv

Error message

Expected value which is `empty string or whitespace`, received value of type `${is(value)}`.

What it means

Thrown by `assertEmptyStringOrWhitespace` (source/index.ts:1584) when the value is neither the empty string nor a string consisting only of whitespace characters. On success the type narrows to `'' | Whitespace`; any string with non-whitespace content, or any non-string, throws a TypeError with the detected type.

Source

Thrown at source/index.ts:1586

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

export function assertEmptySet(value: unknown, message?: string): asserts value is Set<never> {
	if (!isEmptySet(value)) {
		throw new TypeError(message ?? typeErrorMessage('empty set', value));
	}
}

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

export function assertEmptyStringOrWhitespace(value: unknown, message?: string): asserts value is '' | Whitespace {
	if (!isEmptyStringOrWhitespace(value)) {
		throw new TypeError(message ?? typeErrorMessage('empty string or whitespace', value));
	}
}

export function assertEnumCase<T = unknown>(value: unknown, targetEnum: T, message?: string): asserts value is T[keyof T] {
	if (!isEnumCase(value, targetEnum)) {
		throw new TypeError(message ?? typeErrorMessage('EnumCase', value));
	}
}

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

export function assertEvenInteger(value: number, message?: string): asserts value is number {
	if (!isEvenInteger(value)) {
		throw new TypeError(message ?? typeErrorMessage('even integer', value));

View on GitHub ↗ (pinned to 7821031c66)

Solutions

Confirm the value is actually a string — null/undefined must be handled before the assertion
If any content is legitimate, replace the assertion with the boolean `is.emptyStringOrWhitespace(value)` and branch
Normalize the input (e.g. `value ?? ''`) upstream if blank should be represented as an empty string

Example fix

// before
assert.emptyStringOrWhitespace(null); // non-string, throws
// after
assert.emptyStringOrWhitespace(input ?? '');

Defensive patterns

Strategy: type-guard

Validate before calling

if (typeof value === 'string' && value.trim().length === 0) {
  assert.emptyStringOrWhitespace(value);
}

Type guard

function isEmptyStringOrWhitespace(value: unknown): value is string {
  return typeof value === 'string' && value.trim().length === 0;
}

Try / catch

try {
  assert.emptyStringOrWhitespace(value);
} catch (error) {
  if (error instanceof TypeError) {
    // value is non-string or contains non-whitespace characters
  } else throw error;
}

Prevention

This check requires a string — null/undefined fail it, so guard with typeof first
Use value.trim() === '' as the equivalent inline check
Be aware which whitespace definition applies (String.prototype.trim covers Unicode whitespace)

When it happens

Trigger: Calling `assert.emptyStringOrWhitespace(value)` with a string containing any non-whitespace character, or with a non-string value (number, null, undefined).

Common situations: Validating that an optional text field was left blank; a value that looks blank but contains invisible non-whitespace characters (e.g. zero-width joiners not classified as whitespace); receiving `null` instead of `''` from a form library or database column.

Related errors

AI-assisted analysis of sindresorhus/is@7821031c66 (2026-07-31). Data as JSON: /data/errors/a6b7ae6256281500.json. Report an issue: GitHub ↗.