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

Question

I\'m using fetch polyfill to retrieve a JSON or text from a URL, I want to know how can I check if the response is a JSON object or is it only text

fetch(UR         


        

Answer 1

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.

Answer 2

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
  }
}

Answer 3

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;
}