Javascript File API Reading into buffer

Question

I am interested on reading from file to my own buffer, in order to avoid an additional copy.

There seems to be an experimental technology ReadableStreamBYOBReader . I tested in MS edge that is expected to support accordingly to Browser compatibility table , as well as caniuse .

Here is the snippet that I expected to run in a browser supporting the feature, can you spot an error or explain why it is not working?

 console.log(navigator.userAgent) document.querySelector('#input-file').addEventListener('change', async (e) => { if(e.target.files[0]){ try{ // Compare two methods of reading a file const file = e.target.files[0]; const stream = file.stream(); const slice = await file.slice(0, Math.min(file.size, 16)).arrayBuffer() const my_own_array = new Uint8Array(Math.min(file.size, 16)); // Here is where it fails in my browser const reader = stream.getReader({mode:'byob'}) console.log(await reader.read(my_own_array)); slice_array = new Int8Array(slice); // I expect both arrays to have the same data since I read the same file console.log({allEqual: slice_array.every((v, i) => v === my_wn_array[i])}) console.log(my_own_buffer); }catch(e){ console.log(e.toString()) } } })

 Select a file and the script will attempt to load it <input type=file id=input-file>

What I see here is

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/94.0.4606.71 Safari/537.36 Edg/94.0.992.38`
TypeError: Failed to execute 'getReader' on 'ReadableStream': Cannot use a BYOB reader with a non-byte stream

If it works in your browser, please, let me know in the comments.

EDIT : As noted by Xeelley I should have passed {mode:'byob'} instead of 'byob' .

Answer 1

Check MDN ReadableStream.getReader() Docs.

getReader accept 1 optional parameter with type object . So if you need BYOB reader, you should pass reader config like this:

file.stream().getReader({ mode: 'byob' })