Code working in Chrome with native fetch, but not Safari with the polyfill.

I have the following code:

fetch('//api.mysite/url', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json'
  }
});

My Access-Control-Request-Headers contains content-type (note case), which works using native fetch in Chrome, but fails in Safari using this polyfill:

Failed to load resource: Request header field Content-Type is not allowed by Access-Control-Allow-Headers.

Initially I thought it was something to do with case, but lowercasing Content-Type didn’t seem to fix it.

Per RFC 7230, headers should be case insensitive anyway: "Each header field consists of a case-insensitive field name followed by a colon (“😊, optional leading whitespace, the field value, and optional trailing whitespace.”

Any ideas what could be causing this? If it’s a bug, happy to contribute a fix.

[cxoltero]

try:

{
  method: 'POST',
  headers: {
    'Content-Type': 'application/json'
  },
  mode: 'cors'
}

[bradwoods]

I had an issue using the fetch API to get an image from an AWS S3 bucket. I was receiving a CORS issue on some of the images. Adding

headers: {
    'Content-Type': 'application/json'
},

fixed the issue.