Populate an HTML Form from a FormData object

Not sure if this is the answer to your question, but if you have JQuery and JQuery View engine, you can use this:

var f = document.getElementById('test_form');
var data = FormData(f);

// Fill the form using JQuery view Engine:
$("#test_form").view(f);

See example on: https://jocapc.github.io/jquery-view-engine/docs/form

Using this and this, here is how I serialize and deserialize form data:

function formSerialize(form) {
    const data = new FormData(form);
    //https://stackoverflow.com/a/44033425/1869660
    return new URLSearchParams(data).toString();
}

function formDeserialize(form, data) {
    const entries = (new URLSearchParams(data)).entries();
    for(const [key, val] of entries) {
        //http://javascript-coder.com/javascript-form/javascript-form-value.phtml
        const input = form.elements[key];
        switch(input.type) {
            case 'checkbox': input.checked = !!val; break;
            default:         input.value = val;     break;
        }
    }
}

Warning: formDeserialize() won't clear fields that are not included in the stored data, e.g. empty radio groups or checkboxes. Also, not tested with all <input> types.

You are on the right track with creating the FormData object from the test_form ID.

You can access all the values from the FormData object as so:

var f = document.getElementById('test_form');
var data = new FormData(f);

var lastName = data.get('last_name');
var firstName = data.get('first_name');
var dob = data.get('date_of_birth');

You can also use FormData.getAll to pull all of the data from the object.

var allData = data.getAll;

Hopefully this is what you were asking, if not please let me know and we can get it figured out.