Buffer.from offsets

[friesendrywall]

Version

v20.15.0

Platform

Microsoft Windows NT 10.0.19045.0 x64

Subsystem

No response

What steps will reproduce the bug?

Use following example, the complete buffer is copied, byteOffset and length are ignored as near as I can prove.

const buff = Buffer.alloc(32);
for (let i = 0; i < 32; i++) {
    buff[i] = i;
}
const buff2 = Buffer.from(buff, 5, 5);
console.log(buff2);

How often does it reproduce? Is there a required condition?

Always.

What is the expected behavior? Why is that the expected behavior?

Expect a partial buffer copy like
<Buffer 05 06 07 08 09>

What do you see instead?

<Buffer 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f>

Additional information

If this is supposed to work, it should be fixed, but if not, improve the docs.

[aduh95]

Looking at the docs (https://nodejs.org/api/buffer.html#static-method-bufferfrombuffer), it shows the version that takes a Buffer only take one argument, so I think it's expected the additional ones you provide are ignored.

node/doc/api/buffer.md

Lines 1249 to 1272 in ee8e841

	### Static method: `Buffer.from(buffer)`
	<!-- YAML
	added: v5.10.0
	-->
	* `buffer` {Buffer|Uint8Array} An existing `Buffer` or [`Uint8Array`][] from
	which to copy data.
	Copies the passed `buffer` data onto a new `Buffer` instance.
	```mjs
	import { Buffer } from 'node:buffer';
	const buf1 = Buffer.from('buffer');
	const buf2 = Buffer.from(buf1);
	buf1[0] = 0x61;
	console.log(buf1.toString());
	// Prints: auffer
	console.log(buf2.toString());
	// Prints: buffer
	```

What do you think should be improved in the docs?

---

I think buff.slice(5, 5) is what you're looking for.

[friesendrywall]

Directly underneath that shows Static method: Buffer.from(object[, offsetOrEncoding[, length]])
which doesn't do anything. I guess remove that if it doesn't do anything?

[climba03003]

I guess remove that if it doesn't do anything?

I think you misunderstanding the document, the static method you mention requires the object to provide either Symbol.toPrimitive or valueOf().
Unfortunately, Buffer doesn't provide any of them. So, it fallback to the basic one which take only the first argument.

[aduh95]

Buffer does have a valueOf() method which returns itself:

$ node -p 'Buffer.from("Hello World!").valueOf()'
<Buffer 48 65 6c 6c 6f 20 57 6f 72 6c 64 21>

I think the docs are not precise enough; in fact, if an object does not fit into the other variations, we fallback to valueOf or Symbol.toPrimitive and call the variation that corresponds to the new value.

[jazelly]

Since buffer.valueOf() returns itself, then it is strictly equal with original buffer. It falls back to Buffer.from(buffer).

It comes from the code here and here.

I think what you want is not implemented but you can achieve that by Buffer.from(buff.buffer, 5, 5). However, that will return an Uint8Array which might be pointless to your use case. You probably will get [00, 00, 00, 00, 00]

An example

const b1 = Buffer.alloc(8, [1,2,3,4,5,6,7,8]);
const b2 = Buffer.from(b1, 2, 5); // [00, 00, 00, 00, 00];
const b3 = Buffer.alloc(8, new Uint8Array([1,2,3,4,5,6,7,8]));
const b4 = Buffer.from(b3, 2, 5); // same as b3
const b5 = Buffer.from(b3.buffer, 2, 5); // truncated b3

It might be worth it to document this behaviour? since Buffer is also an 'object'.