Mastering TypeScript 3 - Third Edition

await errors

Our promise objects generally define both a success condition as well as an error condition when calling asynchronous functions. In order to trap these error conditions when using async await syntax, we can use a try...catch block. To illustrate this, let's define a promise that returns an error, as well as an error message, as follows:

function awaitError() : Promise<string> { 
    return new Promise<string> ( 
        ( resolve: (message: string) => void, 
          reject: (error: string) => void ) =>  
          { 
             function afterWait() { 
                 console.log(`calling reject`); 
                 reject("an error occurred"); 
             }     
             setTimeout(afterWait, 1000); 
          } 
    ); 
} 

Here, we have a function named awaitError that is defining and returning a Promise<string>, and is using our standard promise ...