Is it possible to define a TypeScript class inside a function and access its parameters?

Question

Is it possible to define a TypeScript class inside a function and access its parameters?

For example, in the world of AngularJS, you might see a construction like this:

myApp.factory('MyFactory', function(injectable) {
    return function(param) {
        this.saySomething = function() {
            alert("Param=" + param + " injectable=" +injectable);
        }
    };
});

This code snippet can then be utilized in the following manner:

function(MyFactory) {
    new MyFactory().saySomething();
}

Essentially, when the function provided to the factory method is executed, the injectable parameter is captured and will be accessible to new instances of MyFactory without the need for re-specifying that parameter.

Now, if you are transitioning to TypeScript and aiming to define that your MyFactory is instantiable and contains a saySomething method, how can this be done elegantly?

One possible solution could involve the following code:

class MyFactory {
    constructor(private injectable, private param) {}
    saySomething() {
        alert(...);
    }
}
myApp.factory('myFactory', function(injectable) {
    return function(param) {
        return new MyFactory(injectable, param);
    }
});

However, this revised approach slightly alters the API:

function(myFactory) {
    myFactory().saySomething();
}

Is there a more refined and elegant way to achieve this objective? The addition of "new" distinctly signals the creation of a new unique object, which is the primary purpose of using a factory.

angularjs typescript

Answer 1

Answer №1

** Edit: With TypeScript version 1.6 or higher, the support for class expressions allows for the following syntax:

myApp.factory(injectable: SomeService) {
  class TodoItem {
    ...
  }
}

** Original response:

I encounter the same issue when working with AngularJS and ES5. I appreciate dependency injection without cluttering constructors and the ability to use the new keyword.

ES6 allows for encapsulating a class inside a function, which is not yet fully supported by TypeScript (refer to https://github.com/Microsoft/TypeScript/issues/307).

Here is what I typically do:

class TodoItem {
  title: string;
  completed: boolean;
  date: Date;

  constructor(private injectable: SomeService) { }

  doSomething() {
    alert(this.injectable);
  }
}

class TodoItemFactory() {
  constructor(private injectable: SomeService) { }

  create(): TodoItem {
    return new TodoItem(this.injectable);
  }

  // Mapping JSON data from the server
  createFromJson(data: any): TodoItem {
    var todoItem = new TodoItem(this.injectable);
    todoItem.title = data.title;
    todoItem.completed = data.completed;
    todoItem.date = data.date;
    return todoItem;
  }
}

myApp.service('TodoItemFactory', TodoItemFactory);


class TodosCtrl {
  constructor(private todoItemFactory: TodoItemFactory) { }

  doSomething() {
    var todoItem1 = this.todoItemFactory.create();

    var todoItem2 = this.todoItemFactory.createFromJson(
      {title: "Meet with Alex", completed: false}
    );
  }
}

While this approach is less elegant compared to ES5 and functions (as not utilizing classes with TypeScript is not recommended), the existing solution works.

Here is what I wish I could write instead:

@Factory
@InjectServices(injectable: SomeService, ...)
class TodoItem {
  title: string;
  completed: boolean;
  date: Date;

  // Constructor without DI pollution
  constructor() { }

  saySomething() {
    alert(this.injectable);
  }

  static createFromJson(data: string): TodoItem {
    ...
  }
}

@Controller
@InjectFactories(TodoItem: TodoItem, ...)
class TodosCtrl {
  constructor() { }

  doSomething() {
    var todoItem1 = new TodoItem();

    var todoItem2 = TodoItem.createFromJson({title: "Meet with Alex"});
  }
}

Alternatively, using functions:

myApp.factory(injectable: SomeService) {
  class TodoItem {
    title: string;
    completed: boolean;
    date: Date;

    // Constructor without pollution
    constructor() { }

    saySomething() {
      alert(injectable);
    }

    static createFromJson(data: string): TodoItem {
      ...
    }
  }
}

myApp.controller(TodoItem: TodoItem) {
  class TodosCtrl {
    constructor() { }

    doSomething() {
      var todoItem1 = new TodoItem();

      var todoItem2 = TodoItem.createFromJson({title: "Meet with Alex"});
    }
  }
}

Answer 2

** Edit: With TypeScript version 1.6 or higher, the support for class expressions allows for the following syntax:

myApp.factory(injectable: SomeService) {
  class TodoItem {
    ...
  }
}

** Original response:

I encounter the same issue when working with AngularJS and ES5. I appreciate dependency injection without cluttering constructors and the ability to use the new keyword.

ES6 allows for encapsulating a class inside a function, which is not yet fully supported by TypeScript (refer to https://github.com/Microsoft/TypeScript/issues/307).

Here is what I typically do:

class TodoItem {
  title: string;
  completed: boolean;
  date: Date;

  constructor(private injectable: SomeService) { }

  doSomething() {
    alert(this.injectable);
  }
}

class TodoItemFactory() {
  constructor(private injectable: SomeService) { }

  create(): TodoItem {
    return new TodoItem(this.injectable);
  }

  // Mapping JSON data from the server
  createFromJson(data: any): TodoItem {
    var todoItem = new TodoItem(this.injectable);
    todoItem.title = data.title;
    todoItem.completed = data.completed;
    todoItem.date = data.date;
    return todoItem;
  }
}

myApp.service('TodoItemFactory', TodoItemFactory);


class TodosCtrl {
  constructor(private todoItemFactory: TodoItemFactory) { }

  doSomething() {
    var todoItem1 = this.todoItemFactory.create();

    var todoItem2 = this.todoItemFactory.createFromJson(
      {title: "Meet with Alex", completed: false}
    );
  }
}

While this approach is less elegant compared to ES5 and functions (as not utilizing classes with TypeScript is not recommended), the existing solution works.

Here is what I wish I could write instead:

@Factory
@InjectServices(injectable: SomeService, ...)
class TodoItem {
  title: string;
  completed: boolean;
  date: Date;

  // Constructor without DI pollution
  constructor() { }

  saySomething() {
    alert(this.injectable);
  }

  static createFromJson(data: string): TodoItem {
    ...
  }
}

@Controller
@InjectFactories(TodoItem: TodoItem, ...)
class TodosCtrl {
  constructor() { }

  doSomething() {
    var todoItem1 = new TodoItem();

    var todoItem2 = TodoItem.createFromJson({title: "Meet with Alex"});
  }
}

Alternatively, using functions:

myApp.factory(injectable: SomeService) {
  class TodoItem {
    title: string;
    completed: boolean;
    date: Date;

    // Constructor without pollution
    constructor() { }

    saySomething() {
      alert(injectable);
    }

    static createFromJson(data: string): TodoItem {
      ...
    }
  }
}

myApp.controller(TodoItem: TodoItem) {
  class TodosCtrl {
    constructor() { }

    doSomething() {
      var todoItem1 = new TodoItem();

      var todoItem2 = TodoItem.createFromJson({title: "Meet with Alex"});
    }
  }
}

Answer 3

Answer №2

Here's a possible approach

This is my approach

Is it possible to declare a TypeScript class inside a function

No, it must be at the top level of the file or in a module. Just to note, if it were allowed inside a function, the information would be confined within that function and the type information would be ineffective.

Answer 4

Here's a possible approach

This is my approach

Is it possible to declare a TypeScript class inside a function

No, it must be at the top level of the file or in a module. Just to note, if it were allowed inside a function, the information would be confined within that function and the type information would be ineffective.

Answer 5

Answer №3

Why would you want to create multiple instances of MyFactory instead of having a single instance injected into your dependent code?

After injection, the class declaration you provided would most likely be utilized in this manner:

function(myFactory) {
    myFactory.saySomething();
}

If you require passing a constructor function into your dependent code, it might be necessary to forgo TypeScript classes. This is because classes cannot be defined inside a function, making it challenging to create a closure on an injected variable within such a function.

Alternatively, you can opt for using a function in TypeScript instead of a class. This way, you still get the advantages of strong typing and can call 'new' on it since it ultimately resolves to a .js function. Here is a slightly more TypeScript-oriented version:

myApp.factory('MyFactory', (injectable: ng.SomeService) => {

    return (param: string) => {

        return {
            saySomething: () {
                alert("Param=" + param + " injectable=" +injectable);
            }
        };
    };
});

Answer 6

Why would you want to create multiple instances of MyFactory instead of having a single instance injected into your dependent code?

After injection, the class declaration you provided would most likely be utilized in this manner:

function(myFactory) {
    myFactory.saySomething();
}

If you require passing a constructor function into your dependent code, it might be necessary to forgo TypeScript classes. This is because classes cannot be defined inside a function, making it challenging to create a closure on an injected variable within such a function.

Alternatively, you can opt for using a function in TypeScript instead of a class. This way, you still get the advantages of strong typing and can call 'new' on it since it ultimately resolves to a .js function. Here is a slightly more TypeScript-oriented version:

myApp.factory('MyFactory', (injectable: ng.SomeService) => {

    return (param: string) => {

        return {
            saySomething: () {
                alert("Param=" + param + " injectable=" +injectable);
            }
        };
    };
});

Is it possible to define a TypeScript class inside a function and access its parameters?

Answer №1

Answer №2

Answer №3

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