缓冲区与 TypedArray

版本历史

版本	变更
v3.0.0	类 `Buffer` 现在继承自 `Uint8Array`。

Buffer 实例也是 JavaScript Uint8Array 和 TypedArray 实例。所有 TypedArray 方法都可在 Buffer 上使用。但是，Buffer API 和 TypedArray API 之间存在细微的不兼容。

特别是：

TypedArray.prototype.slice() 创建 TypedArray 部分的副本，而 Buffer.prototype.slice() 在现有 Buffer 上创建视图而不进行复制。这种行为可能会有意外，并且仅存在于旧版兼容性中。 TypedArray.prototype.subarray() 可用于在 Buffer 和其他 TypedArray 上实现 Buffer.prototype.slice() 的行为。
buf.toString() 与其对应的 TypedArray 不兼容。
多种方法，例如 buf.indexOf()，支持额外的参数。

有两种方式可以从 Buffer 创建新的 TypedArray 实例：

将 Buffer 传给 TypedArray 构造函数将复制 Buffer 的内容，解释为整数数组，而不是目标类型的字节序列。

const buf = Buffer.from([1, 2, 3, 4]);
const uint32array = new Uint32Array(buf);

console.log(uint32array);

// 打印: Uint32Array(4) [ 1, 2, 3, 4 ]

传入 ArrayBuffer 底层的 Buffer 将创建与 Buffer 共享其内存的 TypedArray。

const buf = Buffer.from('hello', 'utf16le');
const uint16array = new Uint16Array(
  buf.buffer,
  buf.byteOffset,
  buf.length / Uint16Array.BYTES_PER_ELEMENT);

console.log(uint16array);

// 打印: Uint16Array(5) [ 104, 101, 108, 108, 111 ]

通过以相同的方式使用 TypedArray 对象的 .buffer 属性，可以创建新的 Buffer，它与 TypedArray 实例共享相同的分配内存。 Buffer.from() 在这种情况下表现得像 new Uint8Array()。

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// 复制 `arr` 的内容。
const buf1 = Buffer.from(arr);

// 与 `arr` 共享内存。
const buf2 = Buffer.from(arr.buffer);

console.log(buf1);
// 打印: <Buffer 88 a0>
console.log(buf2);
// 打印: <Buffer 88 13 a0 0f>

arr[1] = 6000;

console.log(buf1);
// 打印: <Buffer 88 a0>
console.log(buf2);
// 打印: <Buffer 88 13 70 17>

使用 TypedArray 的 .buffer 创建 Buffer 时，可以通过传入 byteOffset 和 length 参数仅使用底层 ArrayBuffer 的一部分。

const arr = new Uint16Array(20);
const buf = Buffer.from(arr.buffer, 0, 16);

console.log(buf.length);
// 打印: 16

Buffer.from() 和 TypedArray.from() 具有不同的签名和实现。具体来说，TypedArray 变体接受第二个参数，该参数是在类型化数组的每个元素上调用的映射函数：

TypedArray.from(source[, mapFn[, thisArg]])

但是，Buffer.from() 方法不支持使用映射函数：

History

Version	Changes
v3.0.0	The `Buffer`s class now inherits from `Uint8Array`.

Buffer instances are also JavaScript Uint8Array and TypedArray instances. All TypedArray methods are available on Buffers. There are, however, subtle incompatibilities between the Buffer API and the TypedArray API.

In particular:

While TypedArray.prototype.slice() creates a copy of part of the TypedArray, Buffer.prototype.slice() creates a view over the existing Buffer without copying. This behavior can be surprising, and only exists for legacy compatibility. TypedArray.prototype.subarray() can be used to achieve the behavior of Buffer.prototype.slice() on both Buffers and other TypedArrays.
buf.toString() is incompatible with its TypedArray equivalent.
A number of methods, e.g. buf.indexOf(), support additional arguments.

There are two ways to create new TypedArray instances from a Buffer:

Passing a Buffer to a TypedArray constructor will copy the Buffers contents, interpreted as an array of integers, and not as a byte sequence of the target type.

const buf = Buffer.from([1, 2, 3, 4]);
const uint32array = new Uint32Array(buf);

console.log(uint32array);

// Prints: Uint32Array(4) [ 1, 2, 3, 4 ]

Passing the Buffers underlying ArrayBuffer will create a TypedArray that shares its memory with the Buffer.

const buf = Buffer.from('hello', 'utf16le');
const uint16array = new Uint16Array(
  buf.buffer,
  buf.byteOffset,
  buf.length / Uint16Array.BYTES_PER_ELEMENT);

console.log(uint16array);

// Prints: Uint16Array(5) [ 104, 101, 108, 108, 111 ]

It is possible to create a new Buffer that shares the same allocated memory as a TypedArray instance by using the TypedArray object’s .buffer property in the same way. Buffer.from() behaves like new Uint8Array() in this context.

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// Copies the contents of `arr`.
const buf1 = Buffer.from(arr);

// Shares memory with `arr`.
const buf2 = Buffer.from(arr.buffer);

console.log(buf1);
// Prints: <Buffer 88 a0>
console.log(buf2);
// Prints: <Buffer 88 13 a0 0f>

arr[1] = 6000;

console.log(buf1);
// Prints: <Buffer 88 a0>
console.log(buf2);
// Prints: <Buffer 88 13 70 17>

When creating a Buffer using a TypedArray's .buffer, it is possible to use only a portion of the underlying ArrayBuffer by passing in byteOffset and length parameters.

const arr = new Uint16Array(20);
const buf = Buffer.from(arr.buffer, 0, 16);

console.log(buf.length);
// Prints: 16

The Buffer.from() and TypedArray.from() have different signatures and implementations. Specifically, the TypedArray variants accept a second argument that is a mapping function that is invoked on every element of the typed array:

TypedArray.from(source[, mapFn[, thisArg]])

The Buffer.from() method, however, does not support the use of a mapping function: