What is the difference between using the colon and as syntax for declaring type?

Question

What is the difference between the : syntax for declaring type

let serverMessage: UServerMessage = message;

and the as

Answer 1

Yes they are different

Firstly, let's see this example

let a: string;
let b: number;

function c() {
    const d = (a || b) as number; // Works
    const e: number = (a || b); // Throw typing error
}

so as number telling Typescript that in that case, the value will be a number (define the type of the result). It forces Typescript to think that it will always return a number (even that it could be not true).

``: number``` define the type of the variable, not the result. So Typescript will verify and ensure that there could not be another case (Even that it could never happen).

Hope that help.