Request header field Access-Control-Allow-Headers is not allowed by itself in preflight response

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I have come across CORS issues multiple times and can usually fix it but I want to really understand by seeing this from a MEAN stack paradigm.

Before I simply added middleware in my express server to catch these things, but it looks like there is some kind of pre-hook that is erroring out my requests.

Request header field Access-Control-Allow-Headers is not allowed by Access-Control-Allow-Headers in preflight response

I assumed that I could do this:

app.use(function(req, res, next) {
  res.header("Access-Control-Allow-Headers","*")
})

or the equivalent but this doesn’t seem to fix it. I also of course tried

app.use(function(req, res, next) {
  res.header("Access-Control-Allow-Headers","Access-Control-Allow-Headers")
})

Still no luck.

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