@wodeleeway @MedinaGitHub file type is deduced by looking at the magic (first few bytes), that's what you're seeing in that code block. If the file starts with "<" the immediate guess is that it's an HTML or XML-based format.
If you are using XHR or fetch and requesting a file that isn't available, you'll get back the 404 response. Usually the body is a small HTML page, which is why you see the error in question.
Your code should defend against it by checking the status of the request. With fetch it is available as res.status in the first callback:
fetch(myRequest).then(function(res) {
if(res.status == 404) { /* file not found */ }
How to resolve the 404 errors is beyond the scope of this project.
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I found the error! in rectJS the file xlsx must be in the 'public' folder