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.prototype.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.prototype.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