CODEWITHSUNDEEP
typescript
Eric
Eric

Reputation: 917

Typescript errors on class with generics and partials

I am having trouble using a partial on a generic type:

interface HasName {
  name: string;
}

class Tools<T extends HasName> {
  public create: (params: Partial<T>) => T;
  createWithName = (nameArg: string) => {
    this.create({ name: nameArg })
  }
}

I expect this example to work because T extends HasName should ensure that T will have a name field, and (params: Partial<T>) should match any object with any subset of the keys of T.

However, there is a typescript compilation error on the line this.create({ name: nameArg }):

Argument of type '{ name: string; }' is not assignable to parameter of type 'Partial'.

Can someone help me understand why { name: string } is not assignable to Partial<T>, when it should be based on my logic above?

Thank you in advance.

Upvotes: 2

Views: 119

Answers (1)

CRice
CRice

Reputation: 32156

In short, TypeScript is right, but as long as you're careful with what you use for T, you should be okay to just silence the error with a cast:

this.create({ name: nameArg } as Partial<T>)

As to why TypeScript is right here, it's because you can create types that extend HasName but for which {name: string} isn't a valid object of that type.

The easiest example is just to make a type where name is more specific than just string:

interface HasSpecificName extends HasName {
    name: "alice" | "bob";
}

In which case, {name: nameArg} where nameArg is any string isn't assignable to Partial<HasSpecificName> since string isn't assignable to "alice" | "bob".

Upvotes: 2

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