How to pass an instance variable into typescript decorator arguments?

What you are trying to do is not possible.

Decorators get called when the class is declared and at this time there is no instance to pass into the decorator.

For example, with this code:

class MyClass {
    startDateString: string;
    @property({ afterDate: this.startDateString })
    endDateString: string;
}
let myClass = new MyClass();

1. MyClass is declared.
2. The decorators are run on MyClass. There is no instance that exists to pass in at this point and this in the decorator argument refers to the global object—not an instance.
3. new MyClass() is called and the instance is created. Decorators aren’t called on this step. That already happened.

Take a look at the compiled JavaScript for reference:

var MyClass = (function () {
    // -- 1 --
    function MyClass() {
    }
    // -- 2 --
    __decorate([
        // see here... `this` is equal to the global object
        property({ afterDate: this.startDateString })
    ], MyClass.prototype, "endDateString", void 0);
    return MyClass;
})();
// -- 3 --
var myClass = new MyClass();

Note that using this.startDateString doesn’t throw a compile error here because this is typed as any.

So what is trying to be done here by passing in an instance property doesn’t make sense and isn’t possible.

What you could do is make startDateString static then pass it in like so: @property({ afterDate: MyClass.startDateString }).