Typescript - Update object properties dynamically with Object.entries

Question

I want to implement a function that loops over the properties of an object and applies updates to another object of the same type.

interface Car {
  tires: number;
  name: string;
}

function updateCar(car: Car, updates: Partial<Car>) {
  Object.entries(updates).forEach(([key, value]) => {
    car[key] = value;
  })
}

The issue here is that there is an error

No index signature with a parameter of type 'string' was found on type 'Car'

when casting the key to keyof Car it works, but as soon as using "noImplicitAny": true there is again an error

Type 'string | number' is not assignable to type 'never'

Is there a way to solve this issue in a type-safe matter. Casting the car to any would work but I'd like to avoid it.

Many thanks Bene

Buxtehude

Partial<Car> would allow me to pass {tires: 4, name: undefined}. Do you want that? – Andria

@AluanHaddad - the question is just targeted to the typing problem. I know that this method has some caveats and some other checks would be necessary but this is not the scope of my question :) – Buxtehude

Yes but I am trying to point out that they are related. If you just want a way to do it that won't complain, use Object.assign(car, updates) – Andria

Answer 1

This question inspired me to revisit a similar problem I ran into.

While not a direct analog of your problem (i.e. this is a pure function that returns a new object rather than modifying arguments supplied by the caller), here's an update function that retains type-safety by using object-spread instead of the Object.entries() function to do its work:

function updateFromPartial<T>(obj: T, updates: Partial<T>):T { 
    return {...obj, ...updates}; 
}

Dilapidation

Note that this returns a new object rather than updating in-place – Chivers

Great! I wasn't aware Partial exist :) – Fibroid

Thank you for this, spent days without figuring something out. – Variscite

Answer 2

That function already exists. Why not just use?

Object.assign(car, updates);

The Object.assign() method copies all enumerable own properties from one or more source objects to a target object. It returns the modified target object. MDN

Smoodge

It seems like Object.assign is not type checked, which means you can affect anything to car. For example if you Car type has a color: string but you update object comes with {color: null} this line of code will not raise any error (eventhough it should). – Allograph

Answer 3

This is about as typesafe as I could get it. The only thing left not properly typed is car[key]. This is clearly not a perfect solution.

interface Car {
  tires: number;
  name: string;
}

function updateCar(car: Car, updates: Partial<Car>) {
    for (const update in updates) {
        const key = update as keyof Car;
        (car[key] as any) = updates[key];
    }
}

Coexecutor

Answer 4

You can use Object.assign() as suggested in this answer but add typing to achieve OPs goal:

function assignFromPartial<T extends {}>(target: T, ...sources: Partial<T>[]) {
  return Object.assign(target, ...sources) as T;
}

Squire