Variable 'test' is used before being assigned - Typescript

Question

I am getting error in this implementation of typescript code. I am mapping here one type to another. But vscode shows error that variable 'test' is used before being assigned. can anyone please help?

interface A {
   name: string;
   age: string;
   sex: string;
}

interface B {
   name: any;
   age: string;
   sex: string;
 }

const modifyData = (g : B) :A => {

    let test: A;
    test.name = g.name['ru'];
    test.age = g.age;
    test.sex = g.sex;

   return test as A;
};

const g = [{
  "name": {
      "en": "George",
      "ru": "Gregor"
       },
  "age": "21",
  "sex": "Male"
},
{
  "name": {
      "en": "David",
      "ru": "Diva"
       },,
  "age": "31",
  "sex": "Male"
}];

const data = g.map(modifyData);
console.log(data);

Answer 1

To clarify a little, this hinges on the difference between "assigned" and "defined." For example:

let myDate: Date; // I've defined my variable as of `Date` type, but it still has no value.

if (!someVariable) {
   myDate = new Date();
}

console.log(`My date is ${myDate}`) // TS will throw an error, because, if the `if` statement doesn't run, `myDate` is defined, but not assigned (i.e., still has no actual value).
   

Defining simply means giving it an initial value:

let myDate: Date | undefined = undefined; // myDate is now equal to `undefined`, so whatever happens later, TS won't worry that it won't exist.

Answer 2

The error is triggered because typescript sees the variable still as undefined in some scenarios. Adding !before variable tells typescript to remove undefined or null as possibles types for variable:

let test!: A;

Definite assignment assertions doc

Playground: typescript/playground

Answer 3

It is indeed unassigned. It is defined, but it has no value.

In my humble opinion, the cleanest way would be to return a literal:

const modifyData = (g: B):A => {
    return {
        name: g.name['ru'],
        age: g.age,
        sex: g.sex
    } as A;
};