First things first: a word of caution – do this only if you absolutely have to. TypeScript has these checks for a reason, and probably in 99% of the cases you should respect and abide these restrictions. But if your scenario falls within the 1% where you must in a subclass override a private method of a superclass – this post shows a way to do this.

Let’s say we have a class that defines a generic starship, and what happens to it when it’s hit by a weapon:

type Hit = 'phaser' | 'antimatter spread' | 'photon torpedo'

class Starship {
    private totalDamage: number;

    constructor(private shipName: string) {
        this.totalDamage = 0;
    }

    private addDamage(damage: number) {
        this.totalDamage += damage;
    }

    public isHitBy(hit: Hit) {
        console.log(`${this.shipName} is hit by ${hit}!`)

        switch(hit) {
            case 'phaser':
                this.addDamage(100);
                break;
            case 'antimatter spread':
                this.addDamage(300);
                break;
            case 'photon torpedo':
                this.addDamage(500);
                break
        }
    }
}

You can create your own class of starship based on this generic class, then build a ship of the new class, and simulate it being hit by a weapon:

class Galaxy extends Starship {
    constructor(name: string) {
        super(name);
    }
}

const enterprise = new Galaxy('Enterprise');
enterprise.isHitBy('photon torpedo');

Output:

"Enterprise is hit by photon torpedo!"

This is fine, but as you’ve seen the base class has a secret feature to calculate damage inflicted by the weapon. It is secret, because it’s defined by a private method, and is not exposed to derived classes. But, like Kirk in Kobayashi Maru, you want to break the rules, and have this forbidden knowledge. Continue reading →