Is there a way to use a typescript interface with optional keys, but then concretize that the keys are there?

Issue

This code:

interface Dog {
  name?: string;
  size?: number;
}

const entity: Dog = {
  name: 'max',
}

const name: string = entity.name;

Causes this error:

Type 'string | undefined' is not assignable to type 'string'.

I can avoid the error by removing the entity type:

interface Dog {
  name?: string;
  size?: number;
}

const entity = {
  name: 'max',
}

const name: string = entity.name;

But then I lose the auto-complete feature.

Is there a way to win both? To have autocomplete, and let the code know which keys are there based on the initializer?

E.g. using Required<Dog> isn't a good solution because I don't want to initialize size. The real use-case I have actually has a much bigger interface.

Solution

What I do is to define a generic identity check function that can be used for any interfaces:

function identityCheck<T = never>() {
  return <I>(input: I & T) => input as I;
}

then create a concrete check-function for the Dog interface:

const dogIdentity = identityCheck<Dog>();

finally use it to create the constant:

const myDog = dogIdentity({
    name: 'bello',
})
// name is of type string
myDog.name.slice();

Typescript Playground Example

Answered By - TmTron