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@cucumber/screenplay

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Cucumber Screenplay is a small library for Cucumber.js that enables better acceptance tests (Gherkin Scenarios):

See the credits section for details about prior work that inspired this library.

When you use Cucumber Screenplay, your step definitions are typically one-liners:

When('{actor} logs in successfully', async function (actor: Actor) {
  await actor.attemptsTo(logIn(`${actor.name}@test.com`, 'valid-password'))
})

You can provide several implementations of logIn - one that interacts with the user interface, but also one that interacts with the API layer underneath the user interface via direct function calls or HTTP requests.

This forces you to avoid UI language in your scenarios like "fill in field" and "click button", because it doesn't make sense to do that in a logIn implementation that isn't using the UI. Likewise, it forces you to avoid using HTTP language like "execute HTTP POST /login", because it doesn't make sense to do this in the logIn implementation that uses the UI.

These constraints encourage you to write readable scenarios that describe what users can do rahter than how your system is implemented. Your scenarios become living documentation that can be understood by everyone on the team.

Assemblies

With Cucumber Screenplay you can evolve an acceptance test suite that you can run with multiple configurations, or assemblies. The assembly diagrams below illustrate how:

DOM-HTTP-DomainDOM-DomainHTTP-DomainDomain
DOM-HTTP-DomainDOM-DomainHTTP-DomainDomain

Watch Cucumber creator Aslak Hellesøy explain how assemblies can be used to build acceptance tests that run in milliseconds:

Watch the video

Installation

First, add the library to your project:

npm install @cucumber/screenplay --save-dev

Usage

This guide will walk you through the usage of the @cucumber/screenplay step by step. For a full example, please refer to this repo.

Actors

The central concept in @cucumber/screenplay is the Actor. An actor object represents a user interacting with the system.

In order to access actor objects from your step definitions, you first need to define an {actor} parameter type.

Create a file called features/support/World.ts (if you haven't already got one) and add the following code:

import { defineParameterType, setWorldConstructor } from '@cucumber/cucumber'
import { ActorWorld, ActorParameterType } from '@cucumber/screenplay'

// Define an {actor} parameter type that creates Actor objects
defineParameterType(ActorParameterType)

// Define your own World class that extends from ActorWorld
export default class World extends ActorWorld {
}
setWorldConstructor(World)

Your step definitions will now be passed Actor objects for {actor} parameters, for example:

When Martha logs in

When('{actor} logs in', async function (actor: Actor) {
  // tell the actor to perform a task
})

Keep reading to learn how to define tasks.

Perfoming tasks

Now that your step definitions can be passed Actor objects, we need to define tasks that the actor can perform to achieve a particular goal.

A task is a function that returns another function that expects an Actor parameter.

Add the following to features/support/tasks/logIn.ts:

type LogIn = (email: string, password: string) => Action<string>

export const logIn: LogIn = (email, password) => {
  return (actor: Actor) => {
    // Just a dummy implementation for now - we'll come back and flesh this out later
    return '42'
  }
}

Back in the step definition we can now import this task:

import { logIn } from '../support/tasks/logIn'

When('{actor} logs in', async function (actor: Actor) {
  const userId = await actor.attemptsTo(logIn(`${actor.name}@test.com`, 'valid-password'))
})

Tasks and Interactions

The screenplay pattern encourages you to decompose complex tasks into multiple interaction:

                         +--------+
                         | Action |
                         +--------+
                              ^
                              |
                    +---------+---------+
                    |                   |
+-------+       +---+--+          +-----+-------+
| actor |------>| task |--------->| interaction |
+-------+       +------+    0..N  +-------------+

In @cucumber/screenplay, both tasks and interactions are of type Action. The library does not make a distinction between them, it is up to you how you decompose tasks into interactions.

See shout.ts for an example of a task that delegates to two interactions.

Tasks and Questions

In addition to Actor#attemptsTo there is also an Actor#ask method. It has exactly the same signature and behaviour as Actor#attemptsTo. It often makes your code more readable if you use Actor#attemptsTo in your When step definitions that modify system state, and Actor#ask in Then step definitions that query system state.

For example:

export type InboxMessages = () => Action<readonly string[]>

export const inboxMessages: InboxMessages = (userId) => {
  return (actor: Actor) => {
    return ['hello', 'world']
  }
}

And in the step definition:

import { inboxMessages } from '../support/tasks/inboxMessages'

Then('{actor} should have received the following messages:', function (actor: Actor, expectedMessages: DataTable) {
  const receivedMessages = actor.ask(inboxMessages())
  assert.deepStrictEqual(receivedMessages, expectedMessages.rows.map(row => row[0]))
})

Using different task implementations

It can often be useful to have multiple implementations of the same task. This allows you to build new functionality incrementally with fast feedback.

For example, you might be working on a new requirement that allows users to log in. You can start by building just the server side domain logic before you implement any of the HTTP layer or UI around it and get quick feedback as you progress.

Later, you can run the same scenarios again, but this time swapping out your tasks with implementations that make HTTP requests or interact with a DOM - without changing any code.

If you look at the shouty example included in this repo, you will see that we organized our tasks in two directories:

features
├── hear_shout.feature
└── support
    └── tasks
        ├── dom
        │   ├── inboxMessages.ts
        │   ├── moveTo.ts
        │   └── shout.ts
        └── session
            ├── inboxMessages.ts
            ├── moveTo.ts
            └── shout.ts

If your World class extends from ActorWorld, it will automatically load tasks from the directory specified by the tasks world parameter:

# Use the dom tasks
cucumber-js --world-parameters '{ "tasks": "support/tasks/dom" }'

# Use the session tasks
cucumber-js --world-parameters '{ "tasks": "support/tasks/session" }'

Here is what the World looks like (if you are using CommonJS modules):

import { setWorldConstructor } from '@cucumber/cucumber'
import { ActorWorld, defineActorParameterType, Action } from '@cucumber/screenplay'
import { InboxMessages, Shout, StartSession } from './tasks/types'

export default class World extends ActorWorld {
  // These tasks will be loaded automatically
  public startSession: StartSession
  public shout: Shout
  public inboxMessages: InboxMessages
}
setWorldConstructor(World)

If you are using ES Modules (by setting "type": "module" in package.json), you need to change ypur World constructor, and add a Before step:

import { setWorldConstructor } from '@cucumber/cucumber'
import { ActorWorld, defineActorParameterType, Action } from '@cucumber/screenplay'
import { InboxMessages, Shout, StartSession } from './tasks/types'
export default class World extends ActorWorld {
  // These tasks will be loaded automatically
  public startSession: StartSession
  public shout: Shout
  public inboxMessages: InboxMessages
  constructor(props: IWorldOptions) {
    super({ ...props, packageType: 'module' })
  }
}
setWorldConstructor(World)

Before(async () => {
  if (this.promise) {
    await this.promise
  }
})

If you're using this technique, you also need to adapt your step definitions to reference tasks from the world (this):

When('{actor} shouts {string}', async function (this: World, actor: Actor, message: string) {
  await actor.attemptsTo(this.shout(message))
})

Sharing data between steps

Your actors have the abililty to remember and recall data between steps. For example:

When('{actor} logs in', async function (actor: Actor<World>) {
  const userId = await actor.attemptsTo(logIn(`${actor.name}@test.com`, 'valid-password'))
  actor.remember('userId', userId)
})

Then('{actor} should be logged in', function (actor: Actor) {
  assert.ok(actor.recall('userId'))
})

You can also pass a function to remember to memoize a value:

function getSomething(actor: Actor) {
  actor.recall('something', () => ({ foo: 'bar' }))
}

// The same object is returned every time
assert.strictEqual(getSomething(actor), getSomething(actor))

Note: the data remembered is scoped by Actor, so you cannot access data remembered by one actor from another one. You can have multiple actors storing different data with the same key. Every Actor is discarded at the end of each scenario, so you won't be able to recall anything from previous scenarios.

Accessing the world from actors

If your tasks need to access data in the world, they can do so via the Actor#world property. If you're doing this you should also declare the generic type of the actor in the task implementation:

export const moveTo: MoveTo = (coordinate) => {
  // We're declaring the World type of the actor so that we can access its members
  return async (actor: Actor<World>) => {
    actor.world.shouty.moveTo(actor.name, coordinate)
  }
}

Asynchronous behaviour and eventual consistency

In a distributed system it may take some time before the outcome of an action propagates around the whole system.

For example, in a chat application, when one user sends a message, it may take a few milliseconds before the other users receive the message, because it travels through a network, even when it's all on your machine.

In cases like this you can use the eventually function to periodically check for a specific condition:

Then('{actor} hears {actor}’s message', async function (this: World, listener: Actor<World>, shouter: Actor) {
  const shouterLastMessage = shouter.recall('lastMessage')

  await eventually(() => {
    const listenerMessages = listener.ask(this.inboxMessages())
    assert.deepStrictEqual(listenerMessages, [shouterLastMessage])
  })
})

The eventually function accepts a single argument - a zero argument condition function. If the condition function throws an error, it will be called again at a regular interval until it passes without throwing an exception. If it doesn't pass or finish within a timeout period, a timeout error is thrown.

The default interval is 50ms and the default timeout is 1000ms. This can be overridden with a second { interval: number, timeout: number } argument after the condition.

Advanced Configuration

Below are some guidelines for more advanced configuration.

Using Promises

The default type of an Action is void. If your system is asynchronous (i.e. uses async functions that return a Promise), you can use the PromiseAction type instead of Action.

Using an explicit ActorLookup

If you cannot extend ActorWorld, you can add an ActorLookup field to your existing world class like so:

import { ActorLookup } from '@cucumber/screenplay'

class World {
  private readonly actorLookUp = new ActorLookup()

  public findOrCreateActor(actorName: string): Actor<World> {
    return this.actorLookUp.findOrCreateActor(this, actorName)
  }
}

Overriding ActorParameterType options

The defineParameterType(ActorParameterType) function call defines a parameter type named {actor} by default, and it uses the RegExp /[A-Z][a-z]+/ (a capitailsed string).

If you want to use a different name or regexp, you can override these defaults:

defineParameterType({ ...ActorParameterType, name: 'acteur' })
defineParameterType({ ...ActorParameterType, regexp: /Marcel|Bernadette|Hubert/ })
defineParameterType({ ...ActorParameterType, name: 'acteur', regexp: /Marcel|Bernadette|Hubert/ })

Design recommendations

When you're working with @cucumber/screenplay and testing against multiple layers, we recommend you use only two task implementations:

A Session represents a user (actor) having an interactive session with your system. A Session will typically be used in two places of your code:

Session is an interface that is specific to your implementation that you should implement yourself. Your UI code will use it to interact with the server. This separation of concerns prevents network implementation details to bleed into the UI code.

You'll typically have two implementations of your Session interface - HttpSession and DomainSession.

The HttpSession is where you encapsulate all of the fetch, WebSocket and EventSource logic. This is the class your UI will use in production. You will also use it in tests.

The DomainSession is an implementation that talks directly to the server side domain layer with direct function calls (without any networking). This implementation will only be used in tests.

By organising your code this way, you have four ways you can assemble your system when you run your Cucumber Scenarios.

In the example we use world parameters to control how to interact with the system and how the system is assembled.

Credits

This library is inspired by the Screenplay Pattern. This list is a chronological order of events, implementations and writings related to the evolution of the pattern.