I accustomed of using WTForms by means of Flask-WTF in my flask application. Doing server side validation is trivial. But how do I leverage this server validation to become a field level, ajax, client side validation? So, when user tab to another input fields, my application can directly goes on validating it and give validation warning/info/error.
I haven't found a resource in the internet yet
A possible solution is as follows:
On the client side you attach a handler to the blur event in all the controls in the form.
Each time the blur event occurs you run a Javascript function that collects the values of all the fields and then submits them as an ajax POST request.
On the server the view function that handles this ajax POST request instantiates the Flask-WTF form and then validates it. Any errors that resulted from validation are collected into a dictionary that is then sent in a JSON response back to the client.
For example, a successful validation could return the following JSON:
{ "errors": {} } A response that includes errors could be:
{ "errors": { "name": "This field is required", "age": "Enter a numeric value between 0 and 99" } } The client gets this JSON response and applies the required changes to the DOM to expose the errors.
If you get a new blur event before the previous one returned you will probably want to abort the pending ajax POST and start a new one with updated field values. You should have only one pending validation request at a time to avoid race conditions.
A great question. This is what we do (flask backend, jquery frontend):
