Typescript - Kotlin-like copy function

Question

In Kotlin a data class can be copied using its auto-generated copy function.

val jack = User(name = "Jack", age = 1)
val olderJack = jack.copy(age = 2)

The copy function signature, as per documentation (Docs):

fun copy(name: String = this.name, age: Int = this.age) = User(name, age)

Can something like this be done in TypeScript?
Is this an acceptable solution, or can it be improved somehow?

export class User {
    constructor(
            public name: string,
            public age: number) {}

    copy(parameters: {
        name?: string,
        age?: number
    }): User {
        const { name, age } = parameters
        return new User(name || this.name, age || this.age)
    }
}

This could be solved even with a Partial<User>:

copy(partialUser: Partial<User>): User {
   return new User(
            partialUser.name || this.name,
            partialUser.age || this.age)
}

Fenton · Accepted Answer · 2018-11-26 11:37:58Z

4

You can deconstruct the item as shown below:

class User {
    constructor(
            public name: string,
            public age: number) {}
}

const jack = new User("Jack", 1);

const newJack = { ...jack, age: 2 };

This results in the copy, except the age, which is now 2.

The ordering is important (i.e. age must follow the object deconstruction).

Extended example below:

class User {
    constructor(
        public name: string,
        public age: number) { }

    copy(user: Pick<User, 'name' | 'age'>) {
        return new User(user.name, user.age);
    }
}

const jack = new User("Jack", 1);

const newJack = jack.copy({ ...jack, age: 2 });

console.log(newJack);

edited Nov 26, 2018 at 11:37

answered Nov 26, 2018 at 11:33

Fenton

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7 Comments

LppEdd Over a year ago

Wouldn't I lose eventual instance / static functions?

Fenton Over a year ago

I have added a wider example where newJack is the full User.

LppEdd Over a year ago

Thanks! Do you find Pick more appropriate then Partial?

Fenton Over a year ago

With Pick you say "it must have the members I state", wheras Partial says "I don't know what it's going to have". In our case, we can be quite strict that we expect name and age to be supplied and we don't care about copy. So the following will be caught as an error in our Pick example: const newJack = jack.copy({ age: 2 });

LppEdd Over a year ago

Interesting! I could even just use (user: Pick<User>): User { ... } then. Do you think that my Partial example was not appropriate? I ask so I'm able to understand if I'm doing right or wrong in general terms

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