Convert object of a partial interface type to the "full" interface type in TypeScript

Question

Let's say I have an Interface A that looks like this:

interface A {
  prop1: string
  prop2: string
}

I initialize object obj like this:

const obj: Partial<A> = { prop1: 'xyz' }

Is there any way to cast obj to A and automatically set any properties not defined in obj but required in A to null or undefined? I would like to only use partials at the initialization of a variable if possible, and stick to the "full" type in function params.

I cannot change A to be a class.

Answer 1

As an extension to Alex Chashin's nice answer, here's a variation where the goal is to validate that a Partial<T> is a valid T and safely cast to it, otherwise return undefined.

The obj is passed as the first parameter of Object.assign (target) to maintain referential integrity. The second parameter does a (harmless) merge and satisfies that the result will be a valid T thanks to the if condition.

    interface A {
      value1: string
      value2: string
    }
    
    function validateObject(obj: Partial<A>): A | undefined {
      if (obj.value1 && obj.value2) {
        // Safe cast Partial<T> to T
        return Object.assign(obj, {
          value1: obj.value1,
          value2: obj.value2,
        });
      }
      return undefined;
    }