访问打字稿属性的获取器和设置器

Question

I have a question about typescript properties: Is it possible to get the setter and getter of a typescript property or to declare a function argument to be of a property of X type?

The reason is to get some sort of "reference" to a variable which is not possible in plain JS without writing getter/setter wrappers or access the variable via parent object itself (obj["varname"]).

For example (with some working code and other parts speculative):

//A sample class with a property
class DataClass<T> {
    private T val;

    public get value(): T {
        return this.val;
    }

    public set value(value: T) {
        this.val = value;
    }
}

//Different ways of modifing a member "by reference"
class ModifyRef {
    public static void DoSomethingByGetterAndSetter(getter: () => string, setter: (val: string) => void) {
        var oldValue = getter();
        setter("new value by DoSomethingByGetterAndSetter");
    }

    public static void DoSomethingByObject(obj: Object, name: string) {
        var oldValue = obj[name];
        obj[name] = "new value by DoSomethingByObject";
    }

    //Is something like this possible?
    public static void DoSomethingByProperty(somePropery: property<string>) {
        var oldVlaue = someProperty;
        someProperty = "new value by DoSomethingByProperty";
    }
}

var inst = new DataClass<string>();

//Calling the DoSomethingByProperty if possible
ModifyRef.DoSomethingByProperty(inst.value);

//Or if not is something like this possible
ModifyRef.DoSomethingByGetterAndSetter(inst.value.get, inst.value.set);

Answer 1

The simplest way to do this would be to provide methods, rather than a property:

//A sample class with a property
class DataClass<T> {
    private val: T;

    public getValue(): T {
        return this.val;
    }

    public setValue(value: T) {
        this.val = value;
    }
}

class ModifyRef {
    public static DoSomethingByGetterAndSetter(getter: () => string, setter: (val: string) => void) {
        var oldValue = getter();
        setter("new value by DoSomethingByGetterAndSetter");
    }
}

var inst = new DataClass<string>();

//Or if not is something like this possible
ModifyRef.DoSomethingByGetterAndSetter(inst.getValue, inst.setValue);

Answer 2

I've long found it very surprising that languages with properties don't include a convenient way to make a reference to a property, and have daydreamed about having this feature in C# . It ought to work on local variables as well.

A popular pattern for this kind of first-class or reified property is a single function that can be called in two ways:

1. no arguments: returns current value.
2. one argument: sets value, returns undefined .

Or in TypeScript terms:

interface Property<T> {
    (): T;
    (newVal: T): void;
}

The methods of jQuery objects often work like this. An example of this pattern in modelling pure data is in Knockout , in which such properties also support change subscriptions, and there's a rather elegant pattern for defining computed properties that automatically recompute when their dependencies change.