If we are developing any application using TypeScript, be it a small-scale or a large-scale application, we will use classes to manage our properties and methods. Prior to ES 2015, JavaScript did not have the concept of classes, and we used functions to create class-like behavior. TypeScript introduced classes as part of its initial release, and now we have classes in ES6 as well. The behavior of classes in TypeScript and JavaScript ES6 closely relates to the behavior of any object-oriented language that you might have worked on, such as Java or C#.

Object-oriented programming allows us to represent our code in the form of objects, which themselves are instances of classes holding properties and methods. Classes form the container of related properties and their behavior. Modeling our code in the form of classes allows us to achieve various features of object-oriented programming, which helps us write more intuitive, reusable, and robust code. Features such as encapsulation, polymorphism, and inheritance are the result of implementing classes.

TypeScript, with its implementation of classes and interfaces, allows us to write code in an object-oriented fashion. This allows developers coming from traditional languages, such as Java and C#, feel right at home when learning TypeScript.

Prior to ES 2015, JavaScript developers did not have any concept of classes; the best way they could replicate the behavior of classes was with functions. The function provides a mechanism to group together related properties and methods. The methods can be either added internally to the function, or using the prototype keyword. The following is an example of such a function:

function Name (firstName, lastName) {
  this.firstName = firstName;
  this.lastName = lastName;
  this.fullName = function() {
  return this.firstName + ' ' + this.lastName ;
  };
}

In this preceding example, we have the fullName method encapsulated inside the Name function. Another way of adding methods to functions is shown in the following code snippet with the prototype keyword:

function Name (firstName, lastName) {
  this.firstName = firstName;
  this.lastName = lastName;
}
Name.prototype.fullName = function() {
  return this.firstName + ' ' + this.lastName ;
};

These features of functions did solve most of the issues of not having classes, but most of the dev community has not been comfortable with these approaches.

Classes make this process easier. Classes provide an abstraction on top of common behavior, thus making code reusable. The following is the syntax for defining a class in TypeScript:

The syntax of the class should look very similar to readers who come from an object-oriented background. To define a class, we use a class keyword followed by the name of the class. The News class has three member properties and one method. Each member has a type assigned to it, and has an access modifier to define the scope. On line 10, we create an object of a class with the new keyword. Classes in TypeScript also have the concept of a constructor, where we can initialize some properties at the time of object creation. We will look at constructors and other concepts of classes in Chapter 3, Sports News Combinator – Adding Features.

Once the object is created, we can access the public members of the class with the dot operator. Note that we cannot access the author property with the espn object because this property is defined as private. TypeScript provides three types of access modifiers.

Any property defined with the public keyword will be freely accessible outside the class. As we saw in the previous example, all the variables marked with the public keyword were available outside the class in an object. Note that TypeScript assigns public as a default access modifier if we do not assign any explicitly. This is because the default JavaScript behavior is to have everything public.

When a property is marked as private, it cannot be accessed outside of the class. The scope of a private variable is only inside the class when using TypeScript. In JavaScript, as we do not have access modifiers, private members are treated similarly to public members.

The protected keyword behaves similarly to private, with the exception that protected variables can be accessed in the derived classes. The following is one such example:

class base{
  protected id: number;
}
class child extends base{
  name: string;
  details():string{
  return `${name} has id: ${this.id}`
  }
}

In the preceding code, we extend the child class with the base class, and have access to the id property inside the child class. If we create an object of the child class, we will still not have access to the id property outside.

As the name suggests, a property with a readonly access modifier cannot be modified after the value has been assigned to it. The value assigned to a readonly property can only happen at the time of variable declaration or in the constructor. We will look at constructors in Chapter 3, Sports News Combinator – Adding Features; here, we will see an example of a readonly property when initialized at the time of declaration:

In the above code, line 5 gives an error stating that property name is readonly, and cannot be an assigned value.

While learning TypeScript, it is important to remember that TypeScript is a superset of JavaScript, and not a new language on its own. Browsers can only understand JavaScript, so it is important for us to understand the JavaScript that is transpiled by TypeScript. TypeScript provides an option to generate JavaScript based on the ECMA standards. You can configure TypeScript to transpile into ES5 or ES6 (ES 2015) and even ES3 JavaScript by using the flag target in the tsconfig.json file. We discussed this setting in Chapter 1, Getting Started with TypeScript. The biggest difference between ES5 and ES6 is with regard to the classes, let, and const keywords which were introduced in ES6.

So, the JavaScript that's generated will be different based on the target setting. Here, we will take an example of class in TypeScript and generate JavaScript for both ES5 and ES6. The following is the class definition in TypeScript:

This is the same code that we saw when we introduced classes in the Understanding Classes section. Here, we have a class named News that has three members, two of which are public and one private. The News class also has a format method, which returns a string concatenated from the member variables.

Then, we create an object of the News class in line 10 and assign values to public properties. In the last line, we call the format method to print the result.

Now let's look at the JavaScript transpiled by TypeScript compiler for this class.

ES6, also known as ES 2015, is the latest version of JavaScript, which provides many new features on top of ES5. Classes are one such feature; JavaScript did not have classes prior to ES6. The following is the code generated from the TypeScript class, which we saw previously:

If you compare the preceding code with TypeScript code, you will notice minor differences. This is because classes in TypeScript and JavaScript are similar, with just types and access modifiers additional in TypeScript. In JavaScript, we do not have the concept of declaring public members. The author variable, which was defined as private and was initialized at its declaration, is converted to a constructor initialization in JavaScript. If we had not have initialized author, then the produced JavaScript would not have added author in the constructor.

ES5 is the most popular JavaScript version supported in browsers, and if you are developing an application that has to support the majority of browser versions, then you need to transpile your code to the ES5 version. This version of JavaScript does not have classes, and hence the transpiled code converts classes to functions, and methods inside the classes are converted to prototypically defined methods on the functions.

The following is the code transpiled when we have the target set as ES5 in the TypeScript compiler options:

As discussed earlier, the basic difference is that the class is converted to a function. The interesting aspect of this conversion is that the News class is converted to an immediately invoked function expression (IIFE). An IIFE can be identified by the parenthesis at the end of the function declaration, as we see in line 9 in the preceding code snippet. IIFEs cause the function to be executed immediately, and help to maintain the correct scope of a function rather than declaring the function in a global scope. Another difference was how we defined the method format in the ES5 JavaScript. The prototype keyword is used to add the additional behavior to the function, which we see here.

A couple of other differences you may have noticed include the change of the let keyword to var, as let is not supported in ES5. All variables in ES5 are defined with the var keyword. Also, the format method now does not use a template string, but standard string concatenation to print the output.

TypeScript does a good job of transpiling the code to JavaScript while following recommended practices. This helps in making sure we have a robust and reusable code with minimum error cases.