How to Check if the Response of a Fetch is a JSON Object in JavaScript?

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In JavaScript, when making HTTP requests using the Fetch API, it’s common to expect JSON data as a response from the server. However, before working with the response data, it’s essential to verify whether it is indeed a JSON object or not.

The below approaches can help you to check if the response is in JSON format or not:

Table of Content

By using Content-Type Header
By Parsing Response Body

By using Content-Type Header

This approach utilizes the Content-Type header of the response to check if it contains JSON data. If the header includes application/json, the response is parsed as JSON. Otherwise, it will be considered non-JSON.

We fetch data from the API endpoint using the Fetch API.
In the then block, we first check if the response status is OK using response.ok. If not, we throw an error.
We then examine the content-type header of the response to determine if it contains JSON data. If it does, we parse the JSON data using response.json().
If the response is successfully parsed as JSON, we assign it to the responseData variable and log it to the console.
If any errors occur during the fetch operation or parsing, we catch and handle them in the catch block.

Example: The below code will explain the use of the Content-Type header to check the response format.

JavaScript

let responseData;
fetch("https://dummyjson.com/products")
    .then(response => {
        if (!response.ok) {
            throw new Error(
                'Network response was not ok');
        }
        const contentType = response.headers.
            get('content-type');
        if (contentType && contentType.
                includes('application/json')) {
            return response.json();
        } else {
            throw new Error(
                'Response is not JSON');
        }
    })
    .then(json => {
        responseData = json;
        console.log(
            'Response is a JSON object:', 
            responseData);
    })
    .catch(error => {
        console.error(
            'There was a problem with the fetch operation:', 
            error);
    });

Output:

By Parsing Response Body

In this approach, we will attempt to parse the response body directly in JSON using theresponse.json() method. The errors will be handled using the catch callback to identify non-JSON responses.

Example: The below code will explain how you can parse fetched response to JSON.