Buffer.from what does it do?

[ubershmekel]

• Version: v7.9.0
• Platform: Darwin yuvmac.dd-wrt 16.6.0 Darwin Kernel Version 16.6.0: Fri Apr 14 16:21:16 PDT 2017; root:xnu-3789.60.24~6/RELEASE_X86_64 x86_64
• Subsystem: Buffer

> Buffer.from(Buffer.from('1234567890'), 1, 3);
<Buffer 31 32 33 34 35 36 37 38 39 30>
> Buffer.from(Buffer.from('1234567890').buffer, 0, 3);
<Buffer 78 dc bf>
> Buffer.from('1234567890').buffer
ArrayBuffer { byteLength: 8192 }

• According to https://nodejs.org/api/buffer.html#buffer_class_method_buffer_from_arraybuffer_byteoffset_length we should be getting a truncated buffer when supplying an offset and length. This is not the case in the first invocation - Buffer.from just copies the original buffer.
• What does .buffer mean? It seems to allow the truncation to occur but has the wrong bytes in it.

These are questions, but I believe they correspond to either bugs in the docs or implementation.

See also #7134 where .slice is the recommended solution, but the underlying problem isn't resolved.

[vkurchatkin]

There is no bug. Truncation only works if first argument is ArrayBuffer, not Buffer.

What does .buffer mean? It seems to allow the truncation to occur but has the wrong bytes in it.

See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray/buffer

[ubershmekel]

@vkurchatkin it's returning the wrong bytes when I use the .buffer in the second invocation.

[vsemozhetbyt]

@ubershmekel I think it is possibly connected with Buffer.poolSize and internal preallocation:
https://nodejs.org/api/buffer.html#buffer_class_property_buffer_poolsize