Validating 'X-CSRFToken': csrftoken

{% csrf_token %}

<script>
const csrftoken = document.querySelector('[name=csrfmiddlewaretoken]').value;

let submitFormData = () =>
{
    const request = new Request(
        URL,
        {headers: {'X-CSRFToken': csrftoken}}
    );

    return fetch(request, {
        method: 'POST',
        mode: 'same-origin',
        body: JSON.stringify({ test: "a" })
    })
    .then(response => response.json())
    .then(response =>
    {
        data = response['time'];
        return data;
    });
}
</script>

How do I validate the 'X-CSRFToken': csrftoken header on the server-side in Django ?

Django does it in the CsrfViewMiddleware. You can review that module to see how it’s done.