ObjectManipulator.ts

/*
https://typescript-exercises.github.io/#exercise=15&file=%2Findex.ts

export class ObjectManipulator {

    constructor(protected obj) {}

    public set(key, value) {
        return new ObjectManipulator({...this.obj, [key]: value});
    }

    public get(key) {
        return this.obj[key];
    }

    public delete(key) {
        const newObj = {...this.obj};
        delete newObj[key];
        return new ObjectManipulator(newObj);
    }

    public getObject() {
        return this.obj;
    }
}

*/



type ObjectWithNewProp<T, K extends string, V> = T & {[NK in K]: V};

export class ObjectManipulator<T> {
    constructor(protected obj: T) {}

    public set<K extends string, V>(key: K, value: V): ObjectManipulator<ObjectWithNewProp<T, K, V>> {
        return new ObjectManipulator({...this.obj, [key]: value} as ObjectWithNewProp<T, K, V>);
    }

    public get<K extends keyof T>(key: K): T[K] {
        return this.obj[key];
    }

    public delete<K extends keyof T>(key: K): ObjectManipulator<Omit<T, K>> {
        const newObj: T = {...this.obj};
        delete newObj[key];
        return new ObjectManipulator(newObj);
    }

    public getObject(): T {
        return this.obj;
    }
}

const obj1 = new ObjectManipulator({ a: 1, b: 2})
const obj2 = obj1.set('c',  'tres')
const obj3 = obj2.delete('a')

obj1.get('c') // Error, obj1 does not contain key 'c'
obj2.get('c') // Correct
obj3.get('a') // Error, obj3 does not contain key 'a'