How to check if the response of a fetch is a json object in javascript

You can do this cleanly with a helper function:

const parseJson = async response => {
  const text = await response.text()
  try{
    const json = JSON.parse(text)
    return json
  } catch(err) {
    throw new Error("Did not receive JSON, instead received: " + text)
  }
}

And then use it like this:

fetch(URL, options)
.then(parseJson)
.then(result => {
    console.log("My json: ", result)
})

This will throw an error so you can catch it if you want.

You could check for the content-type of the response, as shown in this MDN example:

fetch(myRequest).then(response => {
  const contentType = response.headers.get("content-type");
  if (contentType && contentType.indexOf("application/json") !== -1) {
    return response.json().then(data => {
      // The response was a JSON object
      // Process your data as a JavaScript object
    });
  } else {
    return response.text().then(text => {
      // The response wasn't a JSON object
      // Process your text as a String
    });
  }
});

If you need to be absolutely sure that the content is a valid JSON (and don't trust the headers), you could always just accept the response as text and parse it yourself:

fetch(myRequest)
  .then(response => response.text()) // Parse the response as text
  .then(text => {
    try {
      const data = JSON.parse(text); // Try to parse the response as JSON
      // The response was a JSON object
      // Do your JSON handling here
    } catch(err) {
      // The response wasn't a JSON object
      // Do your text handling here
    }
  });

Async/await

If you're using async/await, you could write it in a more linear fashion:

async function myFetch(myRequest) {
  try {
    const reponse = await fetch(myRequest);
    const text = await response.text(); // Parse it as text
    const data = JSON.parse(text); // Try to parse it as JSON
    // The response was a JSON object
    // Do your JSON handling here
  } catch(err) {
    // The response wasn't a JSON object
    // Do your text handling here
  }
}