Typescript Generic class and Object.assign cannot see the value

One way that works with the class approach is to use a public property. For example, units will be assigned and typed properly. The only caveat is this will now be used as army.units.archers.

class Units<T> {
  public units: T;

  constructor(units: T) {
    this.units = units;
  }
}

const myUnits = {
  archers: 5,
  samurais: 10,
  berserks: 3
};

const army = new Units(myUnits);

army.units.archers;

However, if you don't need a class based approach, using a function could also work:

const Units2 = <T>(units: T): T => {
  return units;
};

const army2 = Units2(myUnits)

army2.archers;

Finally, if you only need a shallow copy, you could use spread syntax:

const army3 = { ...myUnits }

army3.archers;

All of these are typed properly. See them in the TypeScript Playground

I think using classes is overkill for what you're trying to achieve in your example. If you're trying to create a mechanism for creating new object which have the same fields as those passed into it, you can simply create a new object my spreading your initial value:

const myUnits = {
    archers: 5,
    samurais: 10,
    berserks: 3
};


const army = {...myUnits};

alert(army.archers);

And to answer your question about why Typescript restricts you, this is the beauty of Typescript, it prevents you from changing the shapes and values of your types in unexpected ways. The way your class is defined it doesn't hold any of the properties that myUnits holds.

If you really wish to go with a class based approach, (and i'd suggest sticking to a more functional approach) you could do something like the following:

class Units<T> {
    archers: number=0
    samurais: number=0
    berserks:number =0
    constructor(units: T) {
        Object.assign(this, units);
    }
}

const myUnits = {
    archers: 5,
    samurais: 10,
    berserks: 3
};


const army = new Units(myUnits);

army.archers;
army.samurais;
army.berserks;

army.a; //disallowed