sindresorhus/is · error · TypeError

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

Error message

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

What it means

Thrown by `assertBlob()` when the value fails `isBlob` (object type check for 'Blob'). It guarantees callers can use Blob APIs (`.arrayBuffer()`, `.stream()`, `.size`) without runtime crashes.

Source

Thrown at source/index.ts:1504

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

export function assertBigInt64Array(value: unknown, message?: string): asserts value is BigInt64Array {
	if (!isBigInt64Array(value)) {
		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)

View on GitHub ↗ (pinned to 7821031c66)

Solutions

Convert binary data to a Blob before asserting: `new Blob([buffer])`.
On Node, ensure Node ≥ 18 (or import Blob from 'node:buffer') so a real Blob class exists.
If using a Blob polyfill in tests, ensure the same Blob implementation is used by producer and consumer.
Check `is(value)` in the message to see what you actually received and fix the upstream producer.

Example fix

// before
assert.blob(fileBuffer);
// after
assert.blob(new Blob([fileBuffer]));

Defensive patterns

Strategy: type-guard

Validate before calling

if (!(value instanceof Blob)) {
  value = new Blob([value]); // wrap explicitly if raw data
}

Type guard

function isBlob(value: unknown): value is Blob {
  return typeof Blob !== 'undefined' && value instanceof Blob;
}

Try / catch

try {
  assert.blob(value);
} catch (error) {
  if (error instanceof TypeError) {
    // likely a Buffer, ArrayBuffer, or string — wrap in new Blob([...]) or reject
  } else throw error;
}

Prevention

In Node, Blob exists only in Node 18+ (node:buffer) — verify runtime support before relying on it
File extends Blob, so File passes; Buffer and ArrayBuffer do not — wrap them with new Blob([data])
Check fetch/form-data payloads at the boundary where they enter your code, not deep inside

When it happens

Trigger: Calling `assert.blob(value)` with a Buffer, ArrayBuffer, Uint8Array, string, File-like plain object, or a stream instead of an actual Blob instance.

Common situations: Node.js < 18 (or older polyfill setups) where global `Blob` didn't exist and code passed Buffers instead; form-upload handlers receiving multipart data as Buffers; cross-library confusion between `File`/`Blob` polyfills (e.g., from `fetch-blob`) whose instances may not match the realm's Blob; JSDOM/test environments lacking Blob.

Related errors

AI-assisted analysis of sindresorhus/is@7821031c66 (2026-07-31). Data as JSON: /data/errors/147e15b268c36ab1.json. Report an issue: GitHub ↗.