Building Extensions


Extension hooks are still in their infancy. If you find it impossible to add some desired behavior through an extension, file an issue.

Extensions adds to or enhances the capabilities of Buster.JS at runtime. An extension will typically use regular Buster APIs to do their bidding. However, in order to hook you into the right places, Buster.JS provides a series of extension points where you can add custom functionality.

A Buster.JS extension is an object that optionally exposes a create method, that will receive custom configuration, and one or more methods that implement an extension hook. The number of hooks is expected to increase in the near future.

Create: var instance = ext.create([options]);

If provided, the create method is called with any custom configuration for the extension. Custom configuration is any value assigned to the property named after the extension in the configuration file, e.g.:

module.exports["Some tests"] = {
    extensions: [require("my-extension")],
    "my-extension": whatever

In order for this to work, the extension must export a name property, i.e. module.exports = { name: "my-extension", ... } in the case above. Although you don’t explicitly have to, it’s good practice to name the extension after its package name.

The create method is not required. If it’s not provided, the extension will not receive its custom configuration.

Following is a description of the currently available hooks, in the order they are run.

Hook: preConfigure

The preConfigure hook is run, before the configuration groups are resolved. This allows extensions to manipulate, add and remove configuration groups.

The arguments passed to this hook are group and config.

For a usage example see buster-testbed-extension.

Hook: analyze

The analyze hook is run after the analyzer is created and can be used to flag issues about the code base.

The argument passed to this hook is analyzer.

buster-lint and buster-syntax uses the hook to warn about lint and syntax errors.

Proper documentation for the analyzer is pending. For now, refer to buster-lint for a usage example.

Hook: beforeRun

The beforeRun hook is run, after the test run configuration is fully loaded, but before the test runner has been initialized.

Currently the hook doesn’t receive arguments.

Hook: configure

The configure hook allows extensions to manipulate Resource sets assigned to a test run. Resource sets contain all files and other resources required for a test run. You can use this hook to modify only sources, only tests, framework resources, everything or any other combination. Read the documentation for buster-configuration to understand how files are loaded and how you can hook into that process.

To implement this hook, simply provide a configure method on your extension object. The following example adds a resource to the framework group:

module.exports = {
    create: function (options) {
        var instance = Object.create(this);
        instance.options = options;
        return instance;

    configure: function (group) {
        group.on("load:framework", function (resourceSet) {
                path: "/oh-yeah.js",
                content: "buster.log('Extension calling!');"

Hook: testRun

The testRun hook is executed right before tests are run. It receives one or two arguments, depending on the environment.

Browser environment: testRunner, messagingClient

Both arguments are event emitters. The testRunner is a “remoteRunner” (not yet documented). It behaves like a Test runner, i.e. it emits all the same events. However, because the run potentially includes more than one browser, the remote runner wraps all test cases in an additional top-level context which is named after the browser that ran it.

The messagingClient contains all raw messages emitted in the browsers. The messages are wrapped in an envelop that also contains information about the browser that sent it:

    data: { name: 'My context' },
    topic: 'context:start',
    clientId: 'eaebee40-ff08-4fcd-bc97-2da569e837c3',
    client: { emit: [Function] }

Node environment: testRunner

Node tests only receives a single runner argument. It is a plain Test runner.