In TypeScript is there any "annotation" for methods to indicate a variable won't be null after exiting it?

Question

In TypeScript with strict: true config, consider the following scenario:

export class A {

    a?: string;
    b?: number;

    init() {
        this.a = "";
        this.b = 0;
    }

    opA() {
        this.requireInit();
        const something = this.a.length; // Error here
    }

    opB() {
        this.requireInit();
        const something = this.b.toString(); // Error here
    }

    private requireInit() {
        if (this.a === undefined || this.b === undefined) {
            throw new Error("Call init before using this method");
        }
    }

}

Is there a way to mark requireInit method that after calling that method, a and b members is not undefined anymore so I do not have to use ! in every method? In C# there is a similar Attribute called MemberNotNull.

Answer 1

Making requireInit into an assertion function that asserts that its this definitely has those properties will do the trick.

class A {

    a?: string;
    b?: number;

    init() {
        this.a = "";
        this.b = 0;
    }

    opA() {
        this.requireInit();
        const something = this.a.length; // OK now
    }

    opB() {
        this.requireInit();
        const something = this.b.toString(); // OK now
    }

    private requireInit(): asserts this is { a: string, b: number } {
        if (this.a === undefined || this.b === undefined) {
            throw new Error("Call init before using this method");
        }
    }
}