ngModel.NgModelController.$isEmpty()

This is called when we need to determine if the value of an input is empty.

For instance, the required directive does this to work out if the input has data or not.

The default $isEmpty function checks whether the value is undefined, '', null or NaN.

You can override this for input directives whose concept of being empty is different from the default. The checkboxInputType directive does this because in its case a value of false implies empty.