The one true way to deal with nullables in TypeScript.

nullables.ts

/**
 * The one true way to deal with nullables in TypeScript.
 * 
 * This code demonstrates a nice design pattern to deal with nullable
 * values in TypeScript. In my blog post, I describe how sane practices
 * around null values involved in both synchronous and asynchronous workflows
 * enhance the quality of the code without any downside.
 * 
 * http://blog.hmil.fr/2017/08/deal-with-nullables-like-theyre-not-even-here-good-coding-practices-in-typescript/
 * 
 * This demo was compiled with typescript 2.4.1 and tested in NodeJS v8.1.3.
 * You will need the following tsconfig.json in order to compile:
 * 
```tsconfig.json
{
    "compilerOptions": {
        "strictNullChecks": true,
        "target": "es5",
        "lib": [
            "dom", "es2015"
        ]
    }
}
```
 */


class User {
    constructor(
            public name: string) {
    }

    /** A user may want to fetch her introduction sentence from an async source so
      * we use a callback to handle the asynchronous execution flow.
      */
    public introduce(cb: (msg: string) => void): void {
        setImmediate(() => {
            cb(`Hello, my name is ${this.name}`);
        });
    }
}

class App {
    _user?: User;

    public login(user: User): void {
        this._user = user;
    }

    private get user(): User {
        if (!this._user) {
            throw new Error('No user is logged in!');
        }
        return this._user;
    }

    public renameUser(newName: string): void {
        this.user.name = newName;
    }

    public introduceUser(): Promise<string> {
        return new Promise<string>(resolve => {
            this.user.introduce((s) => resolve(s));
        })
    }
}

const app = new App();
app.introduceUser().then(console.log, console.error);
app.login(new User('John'));
app.introduceUser().then(console.log, console.error);