compare js object key to see if is the same typescript type

imagine this scenario of code:

type A = {
  keyOne: string;
  keyTwo: string;
};

type B = {
  keyThree: string;
  keyFour: string;
};

type C = {
  unionKey: A | B;
  keyFive: string;
};

type CA = {
  unionKey: A;
  keyFive: string;
};

type CB = {
  unionKey: B;
  keyFive: string;
};

const obj: C = {} as C;

if ('keyOne' in obj.unionKey) {
  obj.unionKey.
}

const handleType = (obj: C): CA | CB => {
  if ('keyOne' in obj.unionKey) {
    return obj as CA;
  } else return obj as CB;
};

const result = handleType({
  unionKey: {
    keyThree: 'sdfsd',
    keyFour: 'sadfasd',
  },
  keyFive: 'sdfsd',
});

result.unionKey. // ts is get confused here and could not suggesst me correct type which i'm looking for (keyThree & keyFour)

consider having obj, how can i check the type on unionKey to see which type it follows? if it is type A i want to do work X, and if it is B i want to do work Y.

the issue is the comparing js value to ts type!

Thanks in advance!

>Solution :

Change your function to use a generic type, and the type will fall pass.

eg..

function handleType<T extends C>(obj: T) {
  if ('keyOne' in obj.unionKey) {
    return obj;
  } else return obj;
};

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