Checkbox input undefined when unchecked - AngularJS

Question

I have an issue with my AngularJS checkbox where they become undefined when they are unchecked. Can't figure out why this is happening.

<input type="checkbox" name="available" ng-model="formData.available" ng-true-value="1" ng-false-value="0" required>

With the above code; default on page load is 0. when Its checked it becomes 1. When I uncheck it it becomes undefined

Any help appreciated.

Answer 1

Thanks to the comments I've realised that the required option is not necessary and was the cause of the issue. Thanks guys!

Answer 2

If you need the required attribute set ng-model-options="{ allowInvalid: true }" so undefined becomes false on the model.

https://docs.angularjs.org/api/ng/directive/ngModelOptions