Sending A Json object through Fetch API to a Flask Server leads to a 400 Bad Request

I was trying to send some JSON data to the sever following some online articles and the official flask documentation, but I keep getting a 400 error. What I’ve tried hasn’t worked so far.

I have read that if flask doesn’t get properly formated JSON data it pushes this error and also that the read of the request must specify Content-Type header as application/json or else the same happens.

I copied some of my code off the official Documentation and this is what i have so far:

a view function inside my flask application:

@main.route('/test', methods=['GET','POST'])
def test():

    if request.method == 'POST':
#this gets me the 400 when run

    return render_template('test.html')

the following scrip tag inside test.html:

    let data = {"data": "Hello World!"}

    document.querySelector('#main').addEventListener('click', function () {
        fetch('/test', {
            method: "POST",
            body: JSON.stringify(data),
            headers: {"Content-Type": "application/json"},
            credentials: "same-origin"
        .then(response => console.log(response.json))

Every time I hit the button to POST the data I get the following showing in my terminal


So I assume what is causing all of this is that the Content-Type header of the HTTP request is not setting properly.

Any ideas on how I could fix this would be apreciated

>Solution :

Based on your code and the error message you are receiving, the issue might be related to the request header not being set correctly.

You are setting the Content-Type header in your JavaScript code to "application/json", which is correct. However, your Flask view function does not check the Content-Type header correctly. You can check it like this:

if request.is_json:
    data = request.get_json()
    print("Request is not JSON")

This checks whether the request is in JSON format using the is_json property of the request object. If it is, then you can use the get_json() method to extract the data. If it’s not, you can print a message or return an error.

Additionally, you might want to add a try-except block to catch any errors that might occur when parsing the JSON data:

if request.is_json:
        data = request.get_json()
    except Exception as e:
        print("Error parsing JSON data:", e)
        return "Bad Request", 400
    print("Request is not JSON")
    return "Bad Request", 400

This will catch any exceptions when parsing the JSON data and return a "Bad Request" error with a 400 status code.

I hope this helps!

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