File in a document library to byte array using JavaScript

We are trying to access a file in a document library and convert it into a byte array with JavaScript CSOM.
We can get the file reference with the client object model but we cannot figure out how to convert it into a byte array.
Please help us understand how this can be done.

We need something like the File.OpenBinaryDirect() in the .NET Managed Client Object Model
We could not find the counterpart for this in the ECMA script CSOM.
The end goal is to feed this byte array to a web service that uploads the file to an external system.