buf.copy(target[, targetStart[, sourceStart[, sourceEnd]]])

• target <Buffer> | <Uint8Array> 要复制到的 Buffer 或 Uint8Array。
• targetStart <integer> target 内开始写入的偏移量。 默认值: 0。
• sourceStart <integer> buf 内开始复制的偏移量。 默认值: 0。
• sourceEnd <integer> buf 内停止复制的偏移量（不包括）。 默认值: buf.length.
• 返回: <integer> 复制的字节数。

将数据从 buf 的区域复制到 target 的区域，即使 target 内存区域与 buf 重叠。

TypedArray#set() 执行相同的操作，可用于所有 TypedArray，包括 Node.js Buffer，尽管它采用不同的函数参数。

// 创建两个 `Buffer` 实例。
const buf1 = Buffer.allocUnsafe(26);
const buf2 = Buffer.allocUnsafe(26).fill('!');

for (let i = 0; i < 26; i++) {
  // 97 是 'a' 的十进制 ASCII 值。
  buf1[i] = i + 97;
}

// 将 `buf1` 字节 16 到 19 复制到 `buf2` 中，从 `buf2` 的字节 8 开始。
buf1.copy(buf2, 8, 16, 20);
// 这相当于：
// buf2.set(buf1.subarray(16, 20), 8);

console.log(buf2.toString('ascii', 0, 25));
// 打印: !!!!!!!!qrst!!!!!!!!!!!!!

// 创建 `Buffer` 并将数据从一个区域复制到同一 `Buffer` 内的重叠区域。

const buf = Buffer.allocUnsafe(26);

for (let i = 0; i < 26; i++) {
  // 97 是 'a' 的十进制 ASCII 值。
  buf[i] = i + 97;
}

buf.copy(buf, 0, 4, 10);

console.log(buf.toString());
// 打印: efghijghijklmnopqrstuvwxyz

• target <Buffer> | <Uint8Array> A Buffer or Uint8Array to copy into.
• targetStart <integer> The offset within target at which to begin writing. Default: 0.
• sourceStart <integer> The offset within buf from which to begin copying. Default: 0.
• sourceEnd <integer> The offset within buf at which to stop copying (not inclusive). Default: buf.length.
• Returns: <integer> The number of bytes copied.

Copies data from a region of buf to a region in target, even if the target memory region overlaps with buf.

TypedArray#set() performs the same operation, and is available for all TypedArrays, including Node.js Buffers, although it takes different function arguments.

// Create two `Buffer` instances.
const buf1 = Buffer.allocUnsafe(26);
const buf2 = Buffer.allocUnsafe(26).fill('!');

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf1[i] = i + 97;
}

// Copy `buf1` bytes 16 through 19 into `buf2` starting at byte 8 of `buf2`.
buf1.copy(buf2, 8, 16, 20);
// This is equivalent to:
// buf2.set(buf1.subarray(16, 20), 8);

console.log(buf2.toString('ascii', 0, 25));
// Prints: !!!!!!!!qrst!!!!!!!!!!!!!

// Create a `Buffer` and copy data from one region to an overlapping region
// within the same `Buffer`.

const buf = Buffer.allocUnsafe(26);

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf[i] = i + 97;
}

buf.copy(buf, 0, 4, 10);

console.log(buf.toString());
// Prints: efghijghijklmnopqrstuvwxyz