Typescript interface default values

Question

I have the following interface in TypeScript:

interface IX {
    a: string,
    b: any,
    c: AnotherType
}

I declare a variable of that t

Answer 1

While @Timar's answer works perfectly for null default values (what was asked for), here another easy solution which allows other default values: Define an option interface as well as an according constant containing the defaults; in the constructor use the spread operator to set the options member variable

interface IXOptions {
    a?: string,
    b?: any,
    c?: number
}

const XDefaults: IXOptions = {
    a: "default",
    b: null,
    c: 1
}

export class ClassX {
    private options: IXOptions;

    constructor(XOptions: IXOptions) {
        this.options = { ...XDefaults, ...XOptions };
    }

    public printOptions(): void {
        console.log(this.options.a);
        console.log(this.options.b);
        console.log(this.options.c);
    }
}

Now you can use the class like this:

const x = new ClassX({ a: "set" });
x.printOptions();

Output:

set
null
1