How to do try catch and finally statements in TypeScript?

Solution 1:

Edit

Typescript 4.0 added the ability to specify unknown and any on catch variables (Issue) And typescript 4.4 added the ability to make unknown the default on catch variables (PR) uning the useUnknownInCatchVariables flag.

With this flag the following is now possible:

catch(e){
    result = e.message; // error under useUnknownInCatchVariables 
    if (typeof e === "string") {
        e.toUpperCase() // works, `e` narrowed to string
    } else if (e instanceof Error) {
        e.message // works, `e` narrowed to Error
    }
}

Specifying arbitrary types on catch variables is still not supported.

Original answer

Typescript does not support annotations on the catch variable. There is a proposal to allow this but it is still being discussed (see here)

Your only solution is to use a type assertion or an extra variable

catch(_e){
    let e:Error= _e;
    result = e.message;
}

catch(e){
    result = (e as Error).message;
}

Unfortunately this will work as well and is completely unchecked:

catch(e){
    result = e.MessageUps;
}

Note

As you can read in the discussion on the proposal, in JS not everything that is thrown has to be an Error instance, so beware of this assumption

Maybe tslint with no-unsafe-any would help catch this.

Solution 2:

With TypeScript 4.0, you can set unknown as catch clause variable type:

unknown is safer than any because it reminds us that we need to perform some sorts of type-checks before operating on our values. (docs)

try {  /* ... */ }
catch (e: unknown) { // <-- note `e` has explicit `unknown` type
    e.message // errors
    if (typeof e === "string") {
        e.toUpperCase() // works, `e` narrowed to string
    } else if (e instanceof Error) {
        e.message // works, `e` narrowed to Error
    }
    // ... handle other error types 
}

Playground

Update: TypeScript 4.4 provides a config flag --useUnknownInCatchVariables to let catch-variables default to type unknown. This is also automatically enabled with the --strict flag.

Solution 3:

Firstly, you need to define the result variable

let result;

Secondly, you can't define the type of e - as the message said, so in case you want to force the type of e, use

catch(e){
    result = (e as Exception).Message;
}

catch(e){
    result = (<Exception>e).Message;
}

Otherwise, it should still work because e will have the type as any

catch (e) {
    result = e.Message;
}