Last Resort aims to help your application catch unhandled exceptions and unhandled rejected promises in the browser.

Platforms Supported

Last Resort is tested using BrowserStack. BrowserStack provides this service for free under their program for supporting open-source software.

Last Resort is supported on:

• Desktop: Chrome, Firefox, Edge, IE11, IE10, IE9, Opera, and Safari (on El Capitan, Yosemite, Mavericks).

• Mobile: iOS 9.3 down to 6. Android 4.4. Presumably later versions of Android are also fine, although we've not been able to test them.

We test against the latest versions offered by the vendors of these browsers on their respective platforms.

Borderline cases:

• The suite runs on IE9 but it does not perform a thorough check of the values obtained from the events generated by the browser. There are a few quirks there.

If you see a platform you think should be tested but isn't, then chances are that Browser Stack is not providing support for it, or is not providing stable support for it. You should inspect the karma.conf.js file to see platforms that have been commented out. If you can get these platforms to work, you are welcome to put in a pull request.

Not supported, and unlikely to be supported, ever:

• Android versions less than 4.4.

• iOS versions less than 6.

IMPORTANT IMPORTANT IMPORTANT: Last Resort depends on <global>.addEventListener("unhandledrejection", ...) being supported in your environment in order to trap unhandled rejections. If your platform does not support this, then Last Resort will not be able to trap unhandled rejections. This page suggests that only Chrome 49 and over supports it natively. On browsers that do not support it natively, you must use a promise library that provides support for unhandledrejection events. Bluebird is one such library. It is actually the library used for testing Last Resort's handling of unhandled rejections.

Note on using Last Resort in workers. Last Resort, when used with Bluebird, is able to trap unhandledrejection in workers. However, it has to use `self.onunhandledrejectionrather thanself.addEventListener. This is because a lot of platforms (including **all** Microsoft browsers) do not actually allow using self.addEventListener`` to listen for custom exceptions. See this comment for details.

Loading Last Resort

Last Resort can be loaded in the following ways:

1. In a script element. In this case it will be available as LastResort on the window object in which it has been loaded.

2. As an AMD module. It will export the same thing as what LastResort contains in the scenario above. You may name the module whatever you want, so long as you define a path for it in your paths configuration.

3. As a CommonJS module. It will export the same as the thing as the earlier options.

4. In a script element and then as an AMD or CommonJS module. This is not a common way to load scripts but Last Resort supports it. When it is loaded by script, it creates LastResort like in the first scenario above. When it is then loaded again as a module, it checks whether LastResort already exists in the global space, and returns that if it exists, rather than create a new module. If LastResort does not exist, then it will initialize itself from scratch. Note that, when loaded first through script and then as a module, LastResort will still be present in the global space if it had only been loaded through script.

Why this 4th method? This is supported in order to allow loading Last Resort as early as possible, while still allowing it to be referenced as a module, and not forcing modules that need to use it to refer to a global. Ideally, Last Resort should be loaded before any other code so that it can report errors as early as possible. In one project in which it is used, it is loaded with a script element, then we register an absolutely bare error handler that uses alert to report to the user, then more substantial scripts are loaded, including an AMD module that provides a Bootstrap-based dialog in case of error. This module loads Last Resort as an AMD module and replaces the initial barebones error handler with its more sophisticated one.

Using Last Resort

Last Resort listens to these two events:

• error, which is emitted when a thrown exception is not caught by any code.

• unhandledrejection, which is emitted when a rejected promise is not handled by any code.

In the following when we say "the events" we are referring to these two events.

Last Resort exports these items:

• install(context, options) installs Last Resort to intercept the events on context. The parameter context is often window but could be the global context of a worker too (self). It returns an instance of OnError this instance is what is now handling the events.

  The options parameter is a plain object holding possible options:

  • force when set to true will force the installation of Last Resort on the context even if it was already installed previously. Otherwise an exception is raised. If force is used and Last Resort was previously installed, the previous installation will be automatically uninstalled.

  • noUnhandledRejection when set to true will NOT install a handler to catch unhandled rejections.

• The OnError class, which has the following methods:

  • register(fn) registers the function fn to handle the events. fn will be called with the event objects generated for the error and unhandledrejection events. It should be prepared to examine the event to determine what happened exactly. Until register(fn) is called, the OnError instance does not do anything when an event is emitted.

  • uninstall() uninstalls the OnError

• isInstalled(context) returns true if install(context) was ever called. false otherwise.

• wasTriggered() returns true if any event was raised in any context.

By default, the registered function is called both when an uncaught exception happens and when an unhandled Promise rejection happens. So it has to be ready to examine its arguments and determine how to process that information depending on the types and number of the arguments passed.

When the registered function is called, Last Resort calls uninstall on the OnError` object that bears the registered function. This prevents infinite recursion if it happens that the registered function causes new errors. It is up to the developer of the registered function to ensure that this function does not screw things up while it is running. It *could* call LastResort.install`` anew if desired and register its own specialized handler. Or it could do its own error handling.

Some old browsers bypass onerror handlers attached to iframe elements and instead go straight for the one on the root window. It is up to your application to decide whether it needs to support these old browsers and use Last Resort appropriately (e.g. detect the issue and install on the root window instead of installing on an iframe).

Example

Here is a super simple example:

import * as lr from "last-resort";

const onerror: lr.OnError = lr.install(window);

function handle(ev: Event): void {
    // tslint:disable-next-line:no-console
    console.log(ev);
}

onerror.register(handle);

This example just dumps the error to the console.

Developing Last Resort

If you produce a pull request run gulp test first to make sure it is clean. If you add features, do add tests for them.