Singularly

Singularly is an implementation of a the interactor design pattern. An interactor is a simple, single-purpose javascript class. It's functional in nature, prescriptive, clean, and easy to test.

Interactors are meant to encapsulate your application's business logic. Each interactor represents one thing that your application does.

Getting Started

yarn add singularly

or

npm install singularly --save

Context

An interactor is given a context. The context contains everything the interactor needs to do its work.

When an interactor does its single purpose, it affects it's given context.

Adding to the Context

As an interactor runs it can add information to the context.

this.context.user = user

Failing the Context

When something goes wrong in your interactor, you can flag the context as failed, which halts all subsequent interactors or organizers.

These are ideal ways to fail an interactor because you can always expect that context.error.message is set

this.context.fail(someInstanceOfError)
this.context.fail(new Error("You can't triple stamp a double stamp"))
this.context.fail({ message: "You can't triple stamp a double stamp" })

These are less ideal ways to fail an interactor.

// This isn't really useful, and has no structure
this.context.fail()
// This can be confusing because errors should have structure
this.context.fail("Refusing to continue")

When given an argument, the fail method will also update context.error The argument should be an object, or an error object as mentioned above.

const context = await FailedOrganizer.call()
context.failure // true
context.success // false
context.error // the argument to context.fail

When you don't call this.context.fail then this will automatically mark the context as a success unless an exception has occurred

const context = await SuccessfulOrganizer.call()
context.success // true
context.failure // false
context.error // undefined

Dealing with Failure

this.context.fail always throws an error of type FailedContextError.

Normally, however, these exceptions are not seen because the call method swallows exceptions.

In the recommended usage, the invoking code (maybe a controller) invokes the interactor using the class method call, then checks the success value of the context.

Types of errors that you might encounter:

try {
  await MyOrganizer.call()
} catch (err) {
  8565// When you call context.fail, the err.name will be "FailedContextError"
  err.name === "FailedContextError"
  
  // When an exception occurs, the err.name will be that of the exception
  err.name === "Error"

  // when an error occurs in a rollback
  err.message === "RollbackError"
  err.name === "AggregateError" 
  err.errors === [ TheOriginalError, TheErrorThatOccuredDuringRollback ]
}

Hooks

Hooks apply to both interactors and organizers

Interactors access the context via this.context. Organizers access the context via the method argument. You should only extend from Organizer to define hooks.

class MyInteractor extends Interactor {
  before () {
    this.context.emails_sent = 0
  }

  call () {}
}

class MyOrganizer extends Organizer {
  before (context) {
    context.emails_sent = 0;
  }
}

Before Hook

Sometimes an interactor or an organizer needs to prepare its context before it is even run. This can be done with the before hook.

before () {
  this.context.emails_sent = 0
}

After Hook

Interactors and organizers can also perform teardown operations after the instance is run.

after () {
  this.context.user = this.context.user.reload()
}

NB: After hooks are only run on success. If the fail! method is called, the after hook is not run.

Skip Hook

Sometimes you may want to skip calling an interactor or an organizer. The skip hook allows you to bypass the call invocation

skip () {
  return this.context.user.isAdmin
}

Method Sequence

1: skip 2: before 3: call (Do not define call when creating a new Organizer) 4: after

An Example Interactor

Your application could use an interactor to authenticate a user.

class AuthenticateUser extends Interactor {
  async call () {
    if (await this.authenticate()) {
      this.context.token = this.context.user.secret_token
    } else {
      this.context.fail({ message: "AuthenticateUser.failure" })
    }
  }

  async authenticate () {
    const { email, password } = this.context
    this.context.user = User.authenticate({ email, password })
    return this.context.user;
  }
}

To define an interactor, simply create a class that extends Interactor and a call instance method. The interactor can access its context from within call.

Interactors in a Controller

Most of the time, your application might use its interactors from controllers. The following controller:

class SessionsController {
  constructor(req, res) {
    this.req = req
    this.res = res
  }

  create () {
    const context = AuthenticateUser.call(this.sessionContext)
    if (context.success) {
      this.session.user_token = context.token
      this.res.redirect(/* wherever */)
    }
  }

  get sessionContext () {
    return {
      email: this.req.body.email,
      password: this.req.body.password
    }
  }
}

The call class method is the proper way to invoke an interactor. The object argument is converted to the interactor instance's context. The call instance method is invoked along with any hooks that the interactor might define. Finally, the context (along with any changes made to it) is returned.

TIP: Name your interactors after your business logic, not your implementation. CancelAccount will serve you better than DestroyUser as the account cancellation interaction takes on more responsibility in the future.

The Future™

SPOILER ALERT: Your use case won't stay so simple.

In my experience, a simple task like authenticating a user will eventually take on multiple responsibilities:

• Welcoming back a user who hadn't logged in for a while
• Prompting a user to update his or her password
• Locking out a user in the case of too many failed attempts
• Sending the lock-out email notification

The list goes on, and as that list grows, so does your controller. This is how fat controllers are born.

If instead you use an interactor right away, as responsibilities are added, your controller (and its tests) change very little or not at all. Choosing the right kind of interactor can also prevent simply shifting those added responsibilities to the interactor.

Kinds of Interactors

There are two kinds of interactors built into the Interactor library: basic interactors and organizers.

Interactors

A basic interactor is a class that extends from Interactor and defines call.

class UploadFile extends Interactor {
  async call () {
    this.context.uploadedFile = await new FileUploader(this.context.file).upload()
  }
}

Basic interactors are the building blocks. They are your application's single-purpose units of work.

Interactors can be organized or called directly

// Call upload file directly
const uploadFile = new UploadFile(initialContext)
await uploadFile.call();

// Organize Upload File
const organizer = new Organizer(UploadFile, /* maybe some other interactors too */)
await organizer.call(initialContext);

Organizers

An organizer's single purpose is to run other interactors or organizers.

import { Organizer } from "singularly"

export default new Organizer(CreateOrder, ChargeCard, SendThankYou)

The organizer passes its context to the interactors that it organizes, one at a time and in order. Each interactor may change that context before it's passed along to the next interactor.

Invoking organizers, initial context, and options

You invoke an organizer by invoking the call method. The call method arguments are call(initialContext)

The optional initialContext argument is an object that will become the initial context for the organizer and made available to all subsequent organizers and interactors.

Rollback

If any one of the organized interactors fails its context, the organizer stops. If the ChargeCard interactor fails, SendThankYou is never called.

In addition, any interactors that had already run are given the chance to undo themselves, in reverse order. Simply define the rollback method on your interactors:

class CreateOrder extends Interactor {
   call () {
    context.order = Order.create(params)
   }

   rollback () {
    context.order.destroy()
   }
}

NOTE: The interactor that fails is not rolled back. Because every interactor should have a single purpose, there should be no need to clean up after any failed interactor.

Testing Interactors

When written correctly, an interactor is easy to test because it only does one thing. Take the following interactor:

class AuthenticateUser extends Interactor {
  call () {
    if (user = User.authenticate(username, password)) {
      context.user = user
      context.token = user.secret_token
    } else {
      context.fail!(message: "AuthenticateUser.failure")
    }
  }
}

You can test just this interactor's single purpose and how it affects the context.

describe("AuthenticateUser" () => {
  describe("call", () => {
    describe("when given valid credentials", () => {
      /*
      * NOTE:
      * for brevity, assume that you stubbed User.authenticate to return a valid user
      */
      test("it succeeds", async () => {
        const context = await (new AuthenticateUser(validCredentialsContext)).call()
        expect(context.success).toBe(true)
      })

      test("provides the user", async () => {
        const context = await (new AuthenticateUser(validCredentialsContext)).call()
        expect(context.user).toBeTruthy()
      })

      test("provides the user's secret token", async () => {
        const context = await (new AuthenticateUser(validCredentialsContext)).call()
        expect(context.token).toBe("token")
      })
    })

    describe("when given invalid credentials", () => {
      /*
      * NOTE:
      * for brevity, assume that you stubbed User.authenticate to return null
      */
      test("it fails", async () => {
        const context = await (new AuthenticateUser(invalidCredentialsContext)).call()
        expect(context.failure).toBe(true)
      })

      test("provides a failure message", async () => {
        const context = await (new AuthenticateUser(invalidCredentialsContext)).call()
        expect(context.error.message).toBe("AuthenticateUser.failure")
      })
    })
  })
})

This is written using Jest, but the same principle applies to any testing framework.

Isolation

It's a good idea to stub User.authenticate in our test rather than creating users in the database. That's because our purpose in authenticate-user.test.js is to test just the AuthenticateUser interactor. The User.authenticate method should be put through its own paces in it's own test.

It's a good idea to define your own interfaces to your models. Doing so makes it easy to draw a line between which responsibilities belong to the interactor and which to the model. The User.authenticate method is a good, clear line. Imagine the interactor otherwise:

class AuthenticateUser extends interactor {
  call () {
    const user = User.findBy({ email: this.context.email })
    // Yuck! 🤢
    if (user) {
      // Ewww! Gross 🤮
      if (await bcrypt.compare(this.context.password, user.encrypted_password) === this.context.password) {
        this.context.user = user
        return
      }
    }

    this.context.fail({ message: "AuthenticateUser.failure" })
  }
}

It would be very difficult to test this interactor in isolation and even if you did, as soon as you change your ORM or your encryption algorithm, your interactors (business concerns) break.

Draw clear lines.

Contributions

Interactor is open source and contributions from the community are encouraged! No contribution is too small.