[javascript] Uncaught (in promise) TypeError: Failed to fetch and Cors error

having a problem with getting data back from database. I am trying my best to explain the problem.

1.If I leave "mode":"no-cors" inside the code below, then I can get data back from server with Postman, but not with from my own server. Thinking it has to be my client side error

2. When I remove "mode":"no-cors" then I am getting 2 errors: -Fetch API cannot load http://localhost:3000/. Request header field access-control-allow-origin is not allowed by Access-Control-Allow-Headers in preflight response. -Uncaught (in promise) TypeError: Failed to fetch

Quick Browsing suggested to put in the "mode":"no-cors" which fixed this error, but it does not feel right thing to do.

So I thought maybe somebody has a suggestion how to approach this problem.

Really hope I was clear enough, but pretty sure I am not giving clear explanation here :S

function send(){
    var myVar = {"id" : 1};
    console.log("tuleb siia", document.getElementById('saada').value);
    fetch("http://localhost:3000", {
        method: "POST",
        headers: {
            "Access-Control-Allow-Origin": "*",
            "Content-Type": "text/plain"
        },//"mode" : "no-cors",
        body: JSON.stringify(myVar)
        //body: {"id" : document.getElementById('saada').value}
    }).then(function(muutuja){

        document.getElementById('väljund').innerHTML = JSON.stringify(muutuja);
    });
}

In my case, the problem was the protocol. I was trying to call a script url with http instead of https.

you can use solutions without adding "Access-Control-Allow-Origin": "*", if your server is already using Proxy gateway this issue will not happen because the front and backend will be route in the same IP and port in client side but for development, you need one of this three solution if you don't need extra code 1- simulate the real environment by using a proxy server and configure the front and backend in the same port

2- if you using Chrome you can use the extension called Allow-Control-Allow-Origin: * it will help you to avoid this problem

3- you can use the code but some browsers versions may not support that so try to use one of the previous solutions

the best solution is using a proxy like ngnix its easy to configure and it will simulate the real situation of the production deployment

See mozilla.org's write-up on how CORS works.

You'll need your server to send back the proper response headers, something like:

Access-Control-Allow-Origin: http://foo.example
Access-Control-Allow-Methods: POST, PUT, GET, OPTIONS
Access-Control-Allow-Headers: Origin, X-Requested-With, Content-Type, Accept, Authorization

Bear in mind you can use "*" for Access-Control-Allow-Origin that will only work if you're trying to pass Authentication data. In that case, you need to explicitly list the origin domains you want to allow. To allow multiple domains, see this post