Proper Django CSRF validation using fetch post request

Figured this out. The issue is that fetch doesn't include cookies by default.

Simple solution is to add credentials: "same-origin" to the request and it works (with the form field but without the headers). Here's the working code:

let data = new FormData();
data.append('file', file);;
data.append('fileName', file.name);
// add form input from hidden input elsewhere on the page
data.append('csrfmiddlewaretoken', $('#csrf-helper input[name="csrfmiddlewaretoken"]').attr('value'));
fetch("/upload/", {
    method: 'POST',
    body: data,
    credentials: 'same-origin',
})

Your question is very close to success. Here is a json way if you do not want the form method. Btw, @Cory's form method is very neat.

1. Neat way with 3rd library

let data = {
    'file': file,
    'fileName': file.name,
};
// You have to download 3rd Cookies library
// https://docs.djangoproject.com/en/dev/ref/csrf/#ajax
let csrftoken = Cookies.get('csrftoken');
let response = fetch("/upload/", {
    method: 'POST',
    body: JSON.stringify(data),
    headers: { "X-CSRFToken": csrftoken },
})

2. Another cumbersome way, but without any 3rd library

let data = {
    'file': file,
    'fileName': file.name,
};
let csrftoken = getCookie('csrftoken');
let response = fetch("/upload/", {
    method: 'POST',
    body: JSON.stringify(data),
    headers: { "X-CSRFToken": csrftoken },
})

// The following function are copying from 
// https://docs.djangoproject.com/en/dev/ref/csrf/#ajax
function getCookie(name) {
    var cookieValue = null;
    if (document.cookie && document.cookie !== '') {
        var cookies = document.cookie.split(';');
        for (var i = 0; i < cookies.length; i++) {
            var cookie = cookies[i].trim();
            // Does this cookie string begin with the name we want?
            if (cookie.substring(0, name.length + 1) === (name + '=')) {
                cookieValue = decodeURIComponent(cookie.substring(name.length + 1));
                break;
            }
        }
    }
    return cookieValue;
}