If the server doesn't respond, fetch takes it as connection failure and handles it in the catch() block. The fetch only executes then() blocks when the connection is successful. If the connection was not successful, the catch() block will be executed.

fetch('something')
 .then( response => {})
 .catch(error => {
   // handle error here
 })

Checking that fetch was successful

You looking for catch block. In the catch block you can fetch the errors. Imn the example below you have access to the error object.

fetch("https://rockosmodernserver.westus2.cloudapp.azure.com/ ", {
    method: "GET",
    }).then(response => response)
    .then(data => {
        console.log("server is up")
    })
    .catch((error) => {
      console.error('Error:', error);
      console.log("server is down!!")   
    });

If a server is not answering, he can not give you a response. Which means there is no status code.

Update: A fetch() promise will reject with a TypeError when a network error is encountered or CORS is misconfigured on the server-side, although this usually means permission issues or similar — a 404 does not constitute a network error, for example. An accurate check for a successful fetch() would include checking that the promise resolved, then checking that the Response.ok property has a value of true. The code would look something like this:

  fetch('flowers.jpg')
  .then(response => {
    if (!response.ok) {
      throw new Error('Network response was not OK');
    }
    return response.blob();
  })
  .then(myBlob => {
    myImage.src = URL.createObjectURL(myBlob);
  })
  .catch(error => {
    console.error('There has been a problem with your fetch operation:', error);
  });

https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch#checking_that_the_fetch_was_successful

First edit: This solution could be helpful, by using a timeout and track that timeout has happened. https://stackoverflow.com/a/50101022/13111978