How to check if the response of a fetch is a json object in javascript
JavascriptJsonFetch ApiJavascript Problem Overview
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();
});
Javascript Solutions
Solution 1 - 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 => {
// 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
}
}
Solution 2 - 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.
Solution 3 - Javascript
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;
}
Solution 4 - Javascript
I recently published an npm
package that includes common utility functions.
one of these functions that I implemented there is just like the nis's async/await
answer that you can use as bellow:
import {fetchJsonRes, combineURLs} from "onstage-js-utilities";
fetch(combineURLs(HOST, "users"))
.then(fetchJsonRes)
.then(json => {
// json data
})
.catch(err => {
// when the data is not json
})
you can find the source on Github