sindresorhus/is · error · TypeError

Expected value which is `SharedArrayBuffer`, received value

Error message

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

What it means

Thrown by `assert.sharedArrayBuffer()` (assertSharedArrayBuffer at source/index.ts:1888) when the value is not a `SharedArrayBuffer` instance. Regular `ArrayBuffer`, typed arrays, and DataViews all fail — the two buffer types are distinct classes. This assertion also fails when `SharedArrayBuffer` itself is unavailable in the environment.

Source

Thrown at source/index.ts:1890

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

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

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

export function assertSharedArrayBuffer(value: unknown, message?: string): asserts value is SharedArrayBuffer {
	if (!isSharedArrayBuffer(value)) {
		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));

View on GitHub ↗ (pinned to 7821031c66)

Solutions

Allocate the buffer as shared: `new SharedArrayBuffer(size)` instead of `new ArrayBuffer(size)`.
In browsers, enable cross-origin isolation with COOP/COEP headers so SharedArrayBuffer exists.
If a value is a typed array, pass `view.buffer` only when the view was created over a SharedArrayBuffer; otherwise use `assert.arrayBuffer()`.

Example fix

// before
const buf = new ArrayBuffer(1024);
assert.sharedArrayBuffer(buf);
// after
const buf = new SharedArrayBuffer(1024);
assert.sharedArrayBuffer(buf);

Defensive patterns

Strategy: validation

Validate before calling

if (typeof SharedArrayBuffer === 'undefined') {
  throw new Error('SharedArrayBuffer unavailable (needs cross-origin isolation)');
}
if (!(value instanceof SharedArrayBuffer)) {
  throw new TypeError('Expected a SharedArrayBuffer');
}

Type guard

function isSharedArrayBuffer(value: unknown): value is SharedArrayBuffer {
  return typeof SharedArrayBuffer !== 'undefined' && Object.prototype.toString.call(value) === '[object SharedArrayBuffer]';
}

Try / catch

try {
  assert.sharedArrayBuffer(value);
} catch (error) {
  if (error instanceof TypeError) {
    // likely a plain ArrayBuffer, or SAB is disabled in this environment
  }
  throw error;
}

Prevention

Check environment support first — browsers require COOP/COEP headers for SharedArrayBuffer to exist
Don't confuse ArrayBuffer with SharedArrayBuffer; they are distinct types and postMessage/transfer semantics differ
Guard worker-message handlers, since buffers arriving via postMessage may be either kind

When it happens

Trigger: Calling `assert.sharedArrayBuffer(value)` with a plain `ArrayBuffer`, a `Uint8Array` (a view, not a buffer), or in browsers where SharedArrayBuffer is disabled because the page is not cross-origin isolated (missing COOP/COEP headers).

Common situations: Browsers hide SharedArrayBuffer unless the page sends `Cross-Origin-Opener-Policy: same-origin` and `Cross-Origin-Embedder-Policy: require-corp` (post-Spectre mitigation), so worker code that works in Node fails in the browser; passing a typed array's `.buffer` which is an ArrayBuffer unless explicitly allocated shared.

Related errors

AI-assisted analysis of sindresorhus/is@7821031c66 (2026-07-31). Data as JSON: /data/errors/e0ca2cf887a21fe6.json. Report an issue: GitHub ↗.