 <p>The problem is that when you work in 'mode' 'no-cors', the Headers become an immutable and you will not be able to change some of its entries. One of the heads you can't change is the Content-Type. When you set 'mode' to 'no-cors' you will be able to change only these headers:</p> <ul> <li><code>Accept</code></li> <li><code>Accept-Language</code></li> <li><code>Content-Language</code></li> <li><code>Content-Type</code> and whose value, once parsed, has a MIME type (ignoring parameters) that is <code>application/x-www-form-urlencoded</code>, <code>multipart/form-data</code>, or <code>text/plain</code></li> </ul> <p>In another words, in 'mode' '-no-'cors' you can only set <code>application/x-www-form-urlencoded</code>, <code>multipart/form-data</code>, or <code>text/plain</code> to the <code>Content-Type</code>.</p> <p>So the solution is stop using fetch or changing it to 'cors' 'mode'. Of course this will only works if your server also accepts 'cors' requests.</p> <p>Here is an example of how you could enable CORS in an Apache Server</p> <pre><code>SetEnvIfNoCase Access-Control-Request-Method "(GET|POST|PUT|DELETE|OPTIONS)" IsPreflight=1 SetEnvIfNoCase Origin ".*" AccessControlAllowOrigin=$0 SetEnvIfNoCase Origin "https://(url1.com|url2.com)$" AccessControlAllowOrigin=$0 Header always set Access-Control-Allow-Origin %{AccessControlAllowOrigin}e env=AccessControlAllowOrigin Header always set Access-Control-Allow-Methods "GET, POST, PUT, DELETE, OPTIONS" env=IsPreflight Header always set Access-Control-Allow-Headers "Content-Type, Authorization, Accept, Accept-Language" env=IsPreflight Header always set Access-Control-Max-Age "7200" env=IsPreflight RewriteCond %{REQUEST_METHOD} OPTIONS RewriteCond %{ENV:IsPreflight} 1 RewriteRule ^(.*)$ $1 [R=204,L] </code></pre> <p>The above code will inject the CORS headers in the response when its necessary. With this code your server will allow CORS only from the the domains "url1.com" or "url2.com".</p> <p>Here are some references</p> <ul> <li><a href="https://developer.mozilla.org/en-US/docs/Web/API/Request/mode" rel="nofollow">https://developer.mozilla.org/en-US/docs/Web/API/Request/mode</a></li> <li><a href="https://fetch.spec.whatwg.org/#simple-header" rel="nofollow">https://fetch.spec.whatwg.org/#simple-header</a></li> </ul>