If you are wanting to read the file without some form of user interaction, you are out of luck. JavaScript always runs in the "sandbox" of the browser and does not have access to any resources outside of the sandbox. This is done for security reasons.
However if your users are selecting files for processing you could look at the File API for JS that most modern browsers support.
https://developer.mozilla.org/en-US/docs/Web/API/File[
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