将部分接口类型的 object 转换为 TypeScript 中的“完整”接口类型

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

This answer is pretty late, but I'll leave it in case anyone comes across later.

Typescript is all about types, which are used at compile time to check if the code is correct, but removed at runtime to turn code into plain JavaScript. This means, there is no way to add any properties using typescript features.

However, you can defined a function that takes partial object and sets default properties:

function complete(obj: Partial<A>): A {
  return Object.assign({
    prop1: 'default1',
    prop2: 'default2'
  }, obj);
}

const partial: Partial<A> = { prop1: 'xyz' }
const a: A = complete(partial)
console.log(a) // { prop1: 'xyz', prop2: 'default2' }

If you want to set missing properties to undefined, the same function will work, but you won't be able to cast the returned object to A , because A requires all properties to be strings and not null or undefined

Answer 2

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;
    }