How to Best Handle Errors in Async/Await Try...Catch Blocks in TypeScript?

Correct Syntax for Try...Catch Blocks with Async/Await

When using the convenient Async/Await feature in TypeScript, an issue arises regarding the declaration of variables when they await a response. Traditionally, they must be declared outside of the try...catch block to be used later, as shown below:

let createdUser;
try {
    createdUser = await this.User.create(userInfo);
} catch (error) {
    console.error(error);
}

console.log(createdUser);

It has been suggested that it's best practice to avoid placing business logic within the try block. However, using the recommended approach requires declaring the awaited variable outside the block, which can be cumbersome.

Best Practice for Error Handling

It is indeed considered best practice to avoid placing multiple lines of business logic inside the try body to ensure that all exceptions arising from the value are caught. A more comprehensive approach is as follows:

try {
    const createdUser = await this.User.create(userInfo);

    console.log(createdUser);
    // Business logic goes here
} catch (error) {
    console.error(error); // Exception from creation or business logic
}

Alternatives for Selective Error Handling

There are three alternatives if you wish to only catch and handle errors originating from the promise:

1. Declare the Variable Outside and Branch: This involves branching depending on whether an exception occurred, as demonstrated below:

let createdUser; // Or use 'var' inside the block
try {
    createdUser = await this.User.create(userInfo);
} catch (error) {
    console.error(error); // Exception from creation
}

if (createdUser) { // User was created successfully
    console.log(createdUser);
    // Business logic goes here
}

2. Test the Exception Type: Test the caught exception for its specific type and handle or rethrow it accordingly:

try {
    const createdUser = await this.User.create(userInfo);

    console.log(createdUser);
    // Business logic goes here
} catch (error) {
    if (error instanceof CreationError) {
        console.error(error); // Exception from creation
    } else {
        throw error;
    }
}

3. Use then with Callbacks: This approach uses two callbacks instead of try/catch, providing the simplest and most recommended solution:

await this.User.create(userInfo).then(createdUser => {
    // User was created successfully
    console.log(createdUser);
    // Business logic goes here
}, error => {
    console.error(error); // Exception from creation
});

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