119

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(URL, options).then(response => {
   // how to check if response has a body of type json?
   if (response.isJson()) return response.json();
});
Mark Fisher
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Sibelius Seraphini
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3 Answers3

205

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
  }
}
nils
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    Via the same strategy you could just use response.json in combination with catch; if you catch an error, it means it's not json. Wouldn't that be a more idiomatic way of handling this (instead of ditching response.json)? – Wouter Ronteltap Nov 10 '18 at 12:23
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    @WouterRonteltap : Aren't you only allowed to do one or the other. It seems like I remember that you only get one shot at response.anything(). If so, JSON is text, but text isn't necessarily JSON. Therefore, you have to do the sure-thing first, which is .text(). If you do .json() first, and it fails, I don't think you'll get the opportunity to also do .text(). If I'm wrong, please show me different. – Lonnie Best Jan 03 '19 at 09:02
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    In my opinion you can't trust the headers (even though you should, but sometimes you just can't control the server on the other side). So it's great that you also mention try-catch in your answer. – Jacob Apr 24 '19 at 08:38
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    Yes, @Lonnie Best is completely correct in this. if you call .json() and it throws an exception (because the response isn't json), you will get a "Body has already been consumed" exception if you subsequently call .text() – Andy Mar 08 '20 at 15:16
6

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.

larskarbo
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2

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;
}
Rakesh Soni
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