Unknown Type in TypeScript

TypeScript has a type called unknown.

We have a variable here of type unknown.

let input: unknown;

We give unknown type because we do not know the type of value, the user inputs.

Difference between any and unknown?

We have a any variable and string variable below:

let a: any;
let b: string;
a = 10;
b = a;

Above code does not throw error. any type can be assigned to another type like string.

Now, see the code below:

let a: unknown;
let b: string;
a = 10;
b = a;

Now b = a throws error. A variable of unknown type is not allowed to assign to another type.

So, unknown is more restrictive than any.

How to assign an unknown variable to string variable?

In order to assign a unknown type variable to string type, we need to explicitly check for string using JavaScript.

let a: unknown;
let b: string;
a = 10;

if (typeof a === "string") {
  b = a;
}

Now, TypeScript will not throw any error.