IsArrayBuffer

Motivation

Given an input value, and passing it to new Uint8Array(value, byteOffset, length), the behavior is different depending on whether the input value is an ArrayBuffer or not.

• If the input value is an ArrayBuffer (or SharedArrayBuffer), the byteOffset and length are used to create a view on the buffer.
• If the input value is not an ArrayBuffer, it is treated as an iterable, and the byteOffset and length arguments are ignored.

However, checking if a value is an ArrayBuffer is not straightforward.

// ❌ Not reliable across realms
function isArrayBuffer (value) {
  return value instanceof ArrayBuffer
}

// ❌ Symbol.toStringTag can be faked
function isArrayBuffer(value) {
  return Object.prototype.toString.call(value) === '[object ArrayBuffer]';
}

// ❌ byteLength can be faked, and `Uint8Array` ignores arbitrary objects with a `byteLength` property
function isArrayBuffer(v) {
  return typeof v === 'object' && v !== null && v.byteLength !== undefined;
}

// ✅ Reliable check, but verbose
function isArrayBuffer(value) {
  try {
    Object.getOwnPropertyDescriptor(ArrayBuffer.prototype, byteLength).get.call(value);
    return true;
  } catch {
    return false;
  }
}

// ✅ Distinguish between ArrayBuffer and SharedArrayBuffer
function isArrayBuffer(value) {
  try {
    Object.getOwnPropertyDescriptor(ArrayBuffer.prototype, detached).get.call(value);
    return true;
  } catch {
    return false;
  }
}

Proposed solution

Add a built-in function ArrayBuffer.isArrayBuffer(value) that returns true if value is an ArrayBuffer, and false otherwise. And similarly, SharedArrayBuffer.isSharedBuffer(value).

ArrayBuffer.prototype.isArrayBuffer = function isArrayBuffer(value) {
  return [[ArrayBufferData]] in value && !isSharedBuffer(value);
}

SharedArrayBuffer.prototype.isSharedBuffer = function isSharedBuffer(value) {
  return [[ArrayBufferData]] in value && [[ArrayBufferData]] is shared;
}

Usages

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