bounce

Selective error catching and rewrite rules

Lead Maintainer - Eran Hammer

Introduction

Working with async/await introduces a new challange in handling errors. Unlike callbacks, which provide a dual mechanism for passing application errors via the callback err argument and developer errors via exceptions, await combines these two channels into one.

It is common practice to ignore application errors in background processing or when there is no useful fallback. In those cases, it is still imperative to allow developer errors to surface and not get swallowed.

For more information read:

• Learning to Throw Again
• Catching without Awaiting

For example:

async function email(user) {

    if (!user.address) {
        throw new Error('User has no email address');
    }

    const message = 'Welcome!';
    if (user.name) {
        message = `Welcome ${user.name}!`;
    }

    await mailer.send(user.address, message);
}

async function register(address, name) {

    const user = { address, name };
    const id = await db.user.insert(user);
    user.id = id;

    try {
        await email(user);
    }
    catch (err) { }             // Ignore errors

    return user;
}

This will fail silently every time the user has a name because it is reassigning a value to a const variable. However, because email() errors are ignored, system errors are ignored as well. The idea is that email() can be used in both critical and non-critical paths. In the critical paths, errors are checked and addressed, but in the non-critical paths, errors are simply ignored.

This can be solved by adding a rethrow() statement:

const Bounce = require('bounce');

async function register(address, name) {

    const user = { address, name };
    const id = await db.user.insert(user);
    user.id = id;

    try {
        await email(user);
    }
    catch (err) {
        Bounce.rethrow(err, 'system');  // Rethrows system errors and ignores application errors
    }

    return user;
}

Usage

rethrow(err, types, [options])

Throws the error passed if it matches any of the specified rules where:

• err - the error.
• type - a single item or an array of items of:
  • An error constructor (e.g. SyntaxError).
  • 'system' - matches any languange native error or node assertions.
  • 'boom' - matches boom errors.
  • an object where each property is compared with the error and must match the error property value. All the properties in the object must match the error but do not need to include all the error properties.
• options - optional object where:
  • decorate - an object which is assigned to the err, copying the properties onto the error.
  • override - an error used to override err when err matches. If used with decorate, the override object is modified.
  • return - if true, the error is returned instead of thrown. Defaults to false.

ignore(err, types, [options])

The opposite action of rethrow(). Ignores any errors matching the specified types. Any error not matching is thrown after applying the options.

background(operation, [action], [types], [options])

Awaits for the value to resolve in the background and then apply either the rethrow() or ignore() actions where:

• operation - a function, promise, or value that is awaited on inside a try...catch and any error thrown processed by the action rule.
• action - one of 'rethrow' or 'ignore'. Defaults to 'rethrow'.
• types - same as the types argument passed to rethrow() or ignore(). Defaults to 'system'.
• options - same as the options argument passed to rethrow() or ignore().

isBoom(err)

Returns true when err is a boom error.

isError(err)

Returns true when err is an error.

isSystem(err)

Return true when err is one of:

• EvalError
• RangeError
• ReferenceError
• SyntaxError
• TypeError
• URIError
• Node's AssertionError