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Plugins API

What follows is the plugin API documentation: the low-level information you'll need to write a frock plugin. If you'd prefer some examples to start, please visit the examples docs, which will introduce you to the various examples provided.

Additionally, there are two special types of plugins covered in their own documents:

  • Middleware are plugins that run before your main handler; they can add functionality that your handler can use, or modify your request/response before it reaches the handler.
  • Cores are plugins that aren't tied to a handler; they are used to augment frock's core functionality, and are much less common.

Detailed Documentation

A frock plugin is just a factory function that returns a router with some specific properties. Frock plugins must take this shape:

Factory Function: createPlugin(frock, logger, options, [db]) -> handlerFn

The factory function will be called whenever the frock is run or on reload.

  • frock The frock singleton instance; all plugins that are run by frock will be passed the same instance, so you should take care to not modify it. To learn more about the properties and methods available, see the api docs.
  • logger (function) The frock logger. A bole instance that's contextualized with your plugin name and the port on which you're running. Example:'some message', someObj)
  • options the options object that was in the frock config
  • db if you requested a database, you'll find the level instance here

Additionally, the factory function must expose one method:

  • factory.validate(config) Validates a configuration object, returns a {key: 'error message'} object on invalid configuration items, and a falsey value if everything is good to go.

The factory function must return a route handler:

Route Handler: handler(req, res)

A route handler is a standard nodejs HTTP handler, with a few special methods attached:


Called when the handler is shut down. Your plugin must call the provided cb when its work is done; plugins are shut down serially, so if you fail to callback you'll hang the frock process.


If you create a db in your frockfile, you can request a db to be passed to any plugin; you can use this to create some persistence in your mocks. A database is passed per name, so it's up to you to define how you use them. This isn't meant to be safe; it's meant to be flexible. An example config:

  "db": {
    "path": "_db"
  "servers": [
      "port": 8081,
      "routes": [
          "path": "/api/whatever/*",
          "methods": ["GET"],
          "handler": "some-handler",
          "db": "some-db"

When you use this config, a level instance called "some-db" will be created in your db.path folder that was specified, and it'll be passed as the last parameter to the frock plugin's factory function.

You can also call any frock instance's dbs.register(name) to get the DB; you can use this to access other running mock's databases.

Note: Asking for a database when the level dependency isn't present will cause an error to be thrown; if you wish to use a database, make sure this package is available in the package with your frockfile.json.

Version Compatibility

When writing frock plugins that you intend to package and distribute, it's important to declare version compatibility. frock uses semver, and you can trust that breaking changes will only be introduced in major versions.

To declare compatibility, add a section to your plugin module's package.json:

  "frock": {
    "compatible-versions": "0.1.0 - 0.9.*"

The compatible-versions key follows the semver rules, semver, and is determined against frock's versions with a:

semver.satisfies(frock_version, your_compabile-versions_string)

If a plugin does not pass that test, a warning is generated when frock starts, but it will not prevent frock from running, so unexpected behavior could occur.

Note: You do not need to provide any compatibility information for plugins that are alongside your frockfile.json; this is only for plugins that are published separately.

Tips for writing Plugins

  • Use the frock.router factory if your plugin needs an internal router; frock uses commuter as its router, which has the ability to automatically deal with subroutes. This is available as a property of the frock to ensure that a frock and its plugins are using the same version of commuter.
  • If you choose to not use frock.router, ensure you're stripping the options._path from what you're passing to your router.
  • Make everything configurable; frock is very configurable but you can shoot yourself in the foot by hard-coding configuration parameters. For instance, any running mock can access any other mock's database; if you want to avoid collisions you'd best make these configurable in your frockfile.json
    • To that end, anything that is in your handler's options key is passed straight through to your plugin; put whatever you need in there.
    • Don't create your own option keys that start with underscores; these should be reserved for frock to insert options that your frock might need.
    • Don't modify the frock that you are passed; this is the global instance that is shared between all running mocks.
  • Use frock.pwd whenever you're resolving things; frock is meant to run using the directory the frockfile.json lives in as the working directory; doing something different will be unexpected for your users.
  • Socket mocks are far simpler than HTTP mocks, and haven't the full suite of options that a HTTP mock has.
  • Don't crash :) Remember that all mocks run in the same process; you bring one down and you bring down the whole thing.

Response Convenience Functions

Each http request that passes through frock has some convenience functions added that'll be available before it hits your plugin or middleware:

  • req.GET an object containing all GET parameters that were in the request's URL
  • res.json(data, [status = 200]) responds with a JSON payload, optionally setting the status:
    • data an object to be JSON serialized
    • status an optional integer status, such as 201 or 404; defaults to 200

The following error functions will take a data parameter, which can be either a string or an object; if it's an object, the data will be serialized and the content type will be set to application/json before sending:

  • res.e404([data]) send a 404
  • res.e400([data]) send a 400
  • res.e500([data]) send a 500
  • res.error(Error, [status = 500]) send a 500 with an optional error object, the difference with the other options is this one will log an error's stacktrace to the console, if present.