This PR makes it possible to declare optional properties and methods in classes, similar to what is already permitted in interfaces. For example:

class Bar {
    a: number;
    b?: number;
    f() {
        return 1;
    }
    g?(): number;  // Body of optional method can be omitted
    h?() {
        return 2;
    }
}

When compiled in --strictNullChecks mode, optional properties and methods automatically have undefined included in their type. Thus, the b property above is of type number | undefined and the g method above is of type (() => number) | undefined. Type guards can be used to strip away the undefined part of the type:

function test(x: Bar) {
    x.a;  // number
    x.b;  // number | undefined
    x.f;  // () => number
    x.g;  // (() => number) | undefined
    let f1 = x.f();            // number
    let g1 = x.g && x.g();     // number | undefined
    let g2 = x.g ? x.g() : 0;  // number
}

[mhegazy]

comments are out of date now.

can you add a declaration emit test.

can you also add a test with --strictNullchecks and an optional parameter property declaration, e.g. constructor(private a?: number) { }

also another one for extending classes, making optionals non-optional, and making non-optionals optional.

@ahejlsberg Nice job! Just curious, can decorators fetch information that can determine whether the target is declared optional?

can decorators fetch information that can determine whether the target is declared optional?

no, see #8126

Optional class properties don't seem to work for abstract properties in typescript 2.0.3, subclasses of the abstract class are forced to implement the property.

@Roam-Cooper that's the point of the abstract modifier, to require that an implementation is provided by a subclass. If it's optional you don't have to override it meaning it should not be abstract.

Ah, yes, wasn't thinking straight! My bad. 😃

Does this PR enable making class getters optional? I can't figure out how to mark getters as optional. I have no idea where the question mark would go.

@bradenhs I have the same question. Would be nice for my situation, where I want to make a custom derived class (which has getters) compatible with the base class definition/type-shape.

@Venryx I made an issue for this (#14417) but it was shot down. I don't think the guy reviewing the issue fully understood what I was getting at. You could create another issue and give a better explanation of the problem though. If you do let me know!