Read and upload large file (> 2GB) in chunks using JavaScript, without loading entire file into RAM

I see example codes use FileReader.readAsArrayBuffer and Blob.slice. But doesn't that involve loading the entire file into memory (readAsArrayBuffer)

Yes, it will load entire file into memory as ArrayBuffer. And then slice part of it.

without loading the entire file (e.g. multiple gigabytes) into memory?

This is not possible. JS code will see ArrayBuffer as entire file.

You can send form via http post request - then browser itself will send file entirely or by chunks (multipart form data) without ability to control this process by page's code.

If you want to do custom upload via ajax request - you need to get entire file into memory (like ArrayBuffer) and then do whatever you want because of you can access file (selected by user) till file field exists and you can access its contents.

Streaming can be done only for writing files to user file system using File System API:

https://developer.mozilla.org/en-US/docs/Web/API/File_System_API

Response to question in comments

Of course, each selected file from input.files (FileList) which have File interface extends Blob which have its own stream method which can stream file into specified destination, but file will not be streamed directly from user file system to destination, as it is Blob which will be created by underlying ArrayBuffer which will be loaded fully into browser memory and streamed from that memory. That's why it is not really "stream" with low memory consumption. Real stream can be achieved for example from fetched resource.

https://developer.mozilla.org/en-US/docs/Web/API/Blob/stream

https://developer.mozilla.org/en-US/docs/Web/API/Blob/arrayBuffer

The arrayBuffer() method of the Blob interface returns a Promise that resolves with the contents of the blob as binary data contained in an ArrayBuffer.

// example of `stream` method usage
const fileInput = <HTMLInputElement>document.getElementById('fileItem');

if (fileInput && fileInput.files) {
    const file = fileInput.files[0];

    file.stream().pipeTo(destination);
}

One more example of internal details:

https://developer.mozilla.org/en-US/docs/Web/API/FileReader/readAsArrayBuffer

When the read operation is finished, the readyState property becomes DONE, and the loadend event is triggered. At that time, the result property contains an ArrayBuffer representing the file's data.

So file will be present in browser memory entirely. What to do with data data: send by http form or using ajax by parts - decides developer.

P.S. Of course browser itself can optimize file loading into memory internally but this is not noted in specification and I wouldn't rely (expect/hope) on this (this is browser dependent).