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Basic CRUD application

While using Socket.IO (or plain WebSockets) for a basic CRUD application might sound a bit overkill, the ability to easily notify all users is really powerful.

In this guide we will create a basic CRUD (standing for Create/Read/Update/Delete) application, based on the awesome TodoMVC project:

Video of the application in action

We will cover the following topics:

Let's start!


The code can be found in the examples directory of the main repository:

git clone

You should see two directories:

  • server/: the server implementation
  • angular-client/: a client implementation based on Angular

Running the frontend#

The project is a basic Angular application which was created with the Angular CLI.

To run it:

cd angular-clientnpm installnpm start

Then if you open http://localhost:4200 in your browser, you should see:

Screenshot of the application

So far, so good.

Running the server#

Let's focus on the server now:

cd ../servernpm installnpm start

You can now open several tabs, and the list of todos should magically be synced between them:

Video of the application in action

How it works#

Server structure#

├── lib│ ├── index.ts│ ├── app.ts│ ├── events.ts│ ├── todo-management│ │ ├── todo.handlers.ts│ | └── todo.repository.ts│ └── util.ts├── package.json├── test│ └── todo-management│   └── todo.tests.ts└── tsconfig.json

Let's detail the duty of each file:

  • index.ts: the entrypoint of the server which creates the components and initializes the application
  • app.ts: the application itself, where the Socket.IO server is created, and the handlers are registered
  • events.ts: the types of each event exchanged between the server and the client (this is the only file that is specific to TypeScript users)
  • todo.handlers.ts: the handlers of the operations on the Todo entities
  • todo.repository.ts: the repository for persisting/retrieving the Todo entities from the database
  • util.ts: some common utility methods that are used in the project
  • todo.tests.ts: the integration tests


First, let's focus on the createApplication method in the lib/app.ts file:

const io = new Server<ClientEvents, ServerEvents>(httpServer, serverOptions);

We create the Socket.IO server with the following options:

{  cors: {    origin: ["http://localhost:4200"]  }}

So the frontend application, which is served at http://localhost:4200, is allowed to connect.


The <ClientEvents, ServerEvents> part is specific to TypeScript users. It allows to explicitly specify the events that are exchanged between the server and the client, so you get autocompletion and type checking:

Screenshot of the IDE autocompletion Screenshot of the IDE type checking

Back to our application! We then create our handlers by injecting the application components:

const {    createTodo,    readTodo,    updateTodo,    deleteTodo,    listTodo,} = createTodoHandlers(components);

And we register them:

io.on("connection", (socket) => {  socket.on("todo:create", createTodo);  socket.on("todo:read", readTodo);  socket.on("todo:update", updateTodo);  socket.on("todo:delete", deleteTodo);  socket.on("todo:list", listTodo);});

Documentation: Listening to events

Note: the event suffixes (:create, :read, ...) replace the usual HTTP verbs in a REST API:

  • POST /todos => todo:create
  • GET /todos/:id => todo:read
  • PUT /todos/:id => todo:update
  • ...

Event handler#

Let's focus on the createTodo handler now, in the lib/todo-management/todo.handlers.ts file:

First, we retrieve the Socket instance:

createTodo: async function (  payload: Todo,  callback: (res: Response<TodoID>) => void) {  const socket: Socket<ClientEvents, ServerEvents> = this;  // ...}

Please note that using an arrow function (createTodo: async () => {}) wouldn't work here, since the this wouldn't point to the Socket instance.

Then, we validate the payload thanks to the great joi library:

const { error, value } = todoSchema.tailor("create").validate(payload, {  abortEarly: false, // return all errors and not just the first one  stripUnknown: true, // remove unknown attributes from the payload});


If there are validation errors, we just call the acknowledgement callback and return:

if (error) {  return callback({    error: Errors.INVALID_PAYLOAD,    errorDetails: error.details,  });}

And we handle the error on the client side:

// angular-client/src/app/store.ts
this.socket.emit("todo:create", { title, completed: false }, (res) => {  if ("error" in res) {    // handle the error  } else {    // success!  }});

Documentation: Acknowledgements

If the payload successfully matches the schema, we can generate a new ID and persist the entity: = uuid();
try {  await;} catch (e) {  return callback({    error: sanitizeErrorMessage(e),  });}

If there is an unexpected error (for example, if the database is down), we call the acknowledgement callback with a generic error message (in order not to expose the internals of our application).

Else, we just call the callback with the new ID:

callback({  data:,});

And finally (that's the magic part), we notify all the other users for the creation:

socket.broadcast.emit("todo:created", value);

Documentation: Broadcasting events

On the client-side, we register a handler for this event:

// angular-client/src/app/store.ts
this.socket.on("todo:created", (todo) => {  this.todos.push(mapTodo(todo));});

And voilà!


Since we are quite reasonable developers, we'll now add a few tests for our handler. Let's open the test/todo-management/todo.tests.ts file:

The application is created in the beforeEach hook:

beforeEach((done) => {  const partialDone = createPartialDone(2, done);
  httpServer = createServer();  todoRepository = new InMemoryTodoRepository();
  createApplication(httpServer, {    todoRepository,  });
  // ...});

And we create two clients, one for sending the payload and the other for receiving the notifications:

httpServer.listen(() => {  const port = (httpServer.address() as AddressInfo).port;  socket = io(`http://localhost:${port}`);  socket.on("connect", partialDone);
  otherSocket = io(`http://localhost:${port}`);  otherSocket.on("connect", partialDone);});

Important note: those two clients are explicitly disconnected in the afterEach hook, so they don't prevent the process from exiting.


Our first test (the happy path) is quite straightforward:

describe("create todo", () => {  it("should create a todo entity", (done) => {    const partialDone = createPartialDone(2, done);
    // send the payload    socket.emit(      "todo:create",      {        title: "lorem ipsum",        completed: false,      },      async (res) => {        if ("error" in res) {          return done(new Error("should not happen"));        }        expect("string");
        // check the entity stored in the database        const storedEntity = await todoRepository.findById(;        expect(storedEntity).to.eql({          id:,          title: "lorem ipsum",          completed: false,        });
        partialDone();      }    );
    // wait for the notification of the creation    otherSocket.on("todo:created", (todo) => {      expect("string");      expect(todo.title).to.eql("lorem ipsum");      expect(todo.completed).to.eql(false);      partialDone();    });  });});

Let's test with an invalid payload too:

describe("create todo", () => {  it("should fail with an invalid entity", (done) => {    const incompleteTodo = {      completed: "false",      description: true,    };
    socket.emit("todo:create", incompleteTodo, (res) => {      if (!("error" in res)) {        return done(new Error("should not happen"));      }      expect(res.error).to.eql("invalid payload");      // check the details of the validation error      expect(res.errorDetails).to.eql([        {          message: '"title" is required',          path: ["title"],          type: "any.required",        },      ]);      done();    });
    // no notification should be received    otherSocket.on("todo:created", () => {      done(new Error("should not happen"));    });  });});

You can run the full test suite with npm test:

Screenshot of the test results

That's all folks! The other handlers are quite similar to the first one, and will not be detailed here.

Next steps#

Thanks for reading!