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

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 => {      // process your JSON data further    });  } else {    return response.text().then(text => {      // this is text, do something with it    });  }});

If you need to be absolutely sure that the content is 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())  .then(text => {    try {        const data = JSON.parse(text);        // Do your JSON handling here    } catch(err) {       // It is text, do you 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); // Fetch the resource    const text = await response.text(); // Parse it as text    const data = JSON.parse(text); // Try to parse it as json    // Do your JSON handling here  } catch(err) {    // This probably means your response is text, do you text handling here  }}

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.

Use a JSON parser like JSON.parse:

function IsJsonString(str) {    try {        var obj = JSON.parse(str);         // More strict checking              // if (obj && typeof obj === "object") {         //    return true;         // }    } catch (e) {        return false;    }    return true;}