Type narrowing with a type assertion

The other way of performing type checking we are going to look at when using unknown is to use type assertion. Type assertion lets us tell the compiler what the type is with the as keyword.

Let's create yet another version of our logScores function as an example:

1. First, let's create a type alias for the structure we want the function parameter to be:

type Scores = { 
  name: string; 
  scores: number[] 
};

1. In our logScores function, we can now use the as keyword to tell the compiler what type to expect:

function logScores(scores: unknown) {
  console.log((scores as Scores).firstName);
  console.log((scores as Scores).scores);
}

  That's enough information for the compiler to pinpoint the problem:

The unknown type allows us to reduce our use of the any type and create more strongly-typed and robust TypeScript programs. We do have to write more code, though, when referencing unknown types. The additional code we need to write needs to check the type of the unknown variable so that the TypeScript compiler can be sure we are accessing valid members within it.