Sorry C# and Java developers, this is not how TypeScript works

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Cover photo by Lina Trochez on Unsplash.

So you took a look at TypeScript. Classes and a C-like syntax. Seems easy enough.

Your manager asks you to rush the edit todo item feature in your brand new TypeScript application.

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On the server-side you have this C# class.

// TodoItem.cspublic class TodoItem{    public string Id { get; set; }    public bool IsDone { get; set; }    public string Title { get; set; }    public async Task Save()    {        // Write to database    }}

C#: Todo item.

On the client-side, you create a similar class.

// todo-item.tsclass TodoItem {  id: string;  isDone: boolean;  title: string;  save(): Promise<void> {    return fetch("/todo/" + this.id, {      body: JSON.stringify(this),      method: "POST",    })      .then(() => undefined);  }}

TypeScript: Todo item.

Not too bad.

We have a view for editing a todo item. The view class reads the todo item from the server using fetch which returns an HttpResponse.

// edit-todo-item-view.tsclass EditTodoItemView {  todoItem: TodoItem;  onInitialize(id: string): Promise<void> {    return this.readTodoItem(id)      .then(todoItem => this.todoItem = todoItem)      .then(() => undefined);  }  readTodoItem(id: string): Promise<TodoItem> {    return fetch("/todo/" + id)      .then((response: HttpResponse) => response.json());  }      saveTodoItem(): Promise<void> {    return this.todoItem.save();  }}

TypeScript: Edit todo item view.

HttpResponses can be parsed as JSON by using the HttpResponse#json method.

We add the TodoItem type to the returned promise of the readTodoItem method.

The application transpiles to JavaScript without errors, so we deploy it on a web server.

We tell our manager that the edit todo item feature is done and move on to the next task.

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Everything is fine… Until we start getting bug reports from users telling that they edited a todo item and saved it. But when they navigated back to the todo list, the todo item was not updated.

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But… It compiled! Did TypeScript let us down?

JavaScript is a loosely typed programming language and TypeScript does not change that; even if it seems that way.

TypeScript was not lying to us. We were lying to TypeScript. It is easy to miss, but we told TypeScript to give the JSON object the TodoItem type.

The problem is that the JSON object was never constructed from the TodoItem class with the new keyword. It was actually an anonymous object without access to the TodoItem prototype.

To fix the bug, we have to make a few changes.

// todo-item.tsclass TodoItem {  id: string;  isDone: boolean;  title: string;  constructor(properties) {    this.id = properties.id;    this.isDone = properties.isDone;    this.title = properties.title;  }  save(): Promise<void> {    return fetch("/todo/" + this.id, {      body: JSON.stringify(this),      method: "POST",    })      .then(() => undefined);  }}

TypeScript: Todo item with constructor.

We add a constructor that we can pass the JSON object to and get back an instance of TodoItem.

// edit-todo-item-view.tsclass EditTodoItemView {  todoItem: TodoItem;  onInitialize(id: string): Promise<void> {    return this.readTodoItem(id)      .then(todoItem => this.todoItem = todoItem)      .then(() => undefined);  }  readTodoItem(id: string): Promise<TodoItem> {    return fetch("/todo/" + id)      .then((response: HttpResponse) => response.json())      .then(json => new TodoItem(json));  }      saveTodoItem(): Promise<void> {    return this.todoItem.save();  }}

TypeScript: Edit todo item view using the new keyword.

After reading the JSON from the server, we pass it to the TodoItem constructor and get an actual instance of the class back.

We transpile the code, deploy it to the web server and this time we remember to test it… In production of course 🤪

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Dedicated to all the hard-working back-end developers who are forced to learn client-side web development.