Can you explain the variation between using the : syntax for declaring type
let serverMessage: UServerMessage = message;
and the as syntax?
let serverMessage = message as UServerMessage;
It appears that they yield identical outcomes in this particular scenario
One is a type annotation while the other is a type assertion.
A type annotation ensures that the compiler validates the assignment as completely valid and confirms the compatibility of message with UServerMessage.
On the other hand, a type assertion tells the compiler that despite any doubts, I am certain that message is indeed a UServerMessage. However, it is important to note that some checks are still carried out even when using type assertions. In cases where the types are very incompatible, you may encounter double assertions like message as any as UServerMessage to override this.
It is recommended to use type annotations over assertions whenever possible. Exercise caution when using type assertions and only resort to them if absolutely necessary. Think of a type assertion as a tool to force a mismatched type into place – it can be useful but remember to review your decision to ensure correctness. Confirm it's not:
Yes, they are indeed different
Firstly, consider this example
let a: string;
let b: number;
function c() {
const d = (a || b) as number; // This works
const e: number = (a || b); // This throws a typing error
}
Hence, using as number in TypeScript specifies that the value will be a number and defines the type of the result, essentially forcing TypeScript to assume it will always return a number even if that may not be the case.
On the other hand, ``: number`` defines the type of the variable, not the result. Therefore, TypeScript will ensure and verify that no other cases could occur, even though they may never happen.
I hope this explanation clears things up for you.
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