Why aren't functions included when casting from a JSON literal to a specific type?

Question

Why aren't functions included when casting from a JSON literal to a specific type?

In my model class, the structure looks like this:

export class Task {

    public name: string;
    public status: string = "todo";

    public completeTask(): void {
        this.status = "done";
    }   
}

There is also a service responsible for retrieving tasks:

export class TaskService {

    constructor(private _http: Http) {}

    public getTask(): Observable<Task> {
        return this._http
            .get("url")
            .map(res => res.json().data as Task)
    }
}

However, when attempting to execute the completeTask function on the task object, an error message occurs:

TypeError: task.completeTask() is not a function

The same error is encountered when trying to convert a JSON literal object to a task instance.

let task: Task = <Task>{ name: "Make some coffee" }
task.completeTask(); // This line triggers an error

I am wondering if I made a mistake somewhere. How can I ensure that functions are properly included in my task objects?

javascript typescript angular

Answer 1

Answer №1

When performing a cast in TypeScript, you are not actually creating an instance of the type being cast to. Instead, you are simply declaring an object with the expected structure, not an actual instance of that type.

The purpose of casting in TypeScript is to ensure that the object being cast follows the structure defined by the type. This check occurs during design and compile time, rather than at runtime.

If you need to access methods or properties of a specific type, you will have to instantiate a new object that matches the required structure:

public getTask(): Observable<Task> {
    return this._http
        .get("url")
        .map(res => {
          let content = res.json().data;
          let task = new Task();
          task.name = content.name;
          task.status = content.status;
          return task;
        });
}

For more information on this topic, you can refer to the following question:

Example Question Title

Additional Note:

As suggested by @ssube, you could define the Task class with a constructor that accepts an object with the required properties:

export class Task {
  (...)

  constructor(obj:{name:string, status:string}) {
    this.name = obj.name;
    this.status = obj.status;
  }

  (...)
}

This approach simplifies the instantiation process:

public getTask(): Observable<Task> {
    return this._http
        .get("url")
        .map(res => new Task(res.json().data));
}

Answer 2

When performing a cast in TypeScript, you are not actually creating an instance of the type being cast to. Instead, you are simply declaring an object with the expected structure, not an actual instance of that type.

The purpose of casting in TypeScript is to ensure that the object being cast follows the structure defined by the type. This check occurs during design and compile time, rather than at runtime.

If you need to access methods or properties of a specific type, you will have to instantiate a new object that matches the required structure:

public getTask(): Observable<Task> {
    return this._http
        .get("url")
        .map(res => {
          let content = res.json().data;
          let task = new Task();
          task.name = content.name;
          task.status = content.status;
          return task;
        });
}

For more information on this topic, you can refer to the following question:

Example Question Title

Additional Note:

As suggested by @ssube, you could define the Task class with a constructor that accepts an object with the required properties:

export class Task {
  (...)

  constructor(obj:{name:string, status:string}) {
    this.name = obj.name;
    this.status = obj.status;
  }

  (...)
}

This approach simplifies the instantiation process:

public getTask(): Observable<Task> {
    return this._http
        .get("url")
        .map(res => new Task(res.json().data));
}

Why aren't functions included when casting from a JSON literal to a specific type?

Answer №1

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