The first one is a type annotation. The second one is a type assertion.
The type annotation tells the compiler check that the assignment is fully valid and that message is indeed compatible with UServerMessage
The type assertion tells the compiler, I know what I’m doing, message is a UServerMessage, never mind what you think you know, I have more information and I know best. Some checks are still performed even if you use type assertions so you might see double assertions message as any as UServerMessage for example if the type of message is very incompatible with UServerMessage
You should always prefer a type annotation to an assertion. Use assertion with care and only if you have to. A type assertion is a hammer to get a square peg to fit into a round hole, useful at times but you might take a second look at what you are doing to make sure its right. Make sure it’s not:
