Getters and setters (also known as accessors) were introduced to JavaScript when ECMAScript 5 (2009) was released. The thing is, there's a lot of confusion about their utility and why you would ever even want to use them. I came across this reddit thread where the discussion was about if they were an anti-pattern.

TypeScript has abstract classes, which are classes that have partial implementation of a class and in which other classes can be derived from. They can’t be instantiated directly. Unlike interfaces, abstract classes can have implementation details for their members. To declare an abstract class, we can use the abstract keyword.
Abstract methods and properties are a staple of other object-oriented languages and give you more power when defining the base type of a class hierarchy. The implementation in TypeScript is, however, sufficiently familiar to OO developers that I'm going to leave that for last and begin by looking at private and protected constructors.

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January 2015

Volume 30 Number 1

By Peter Vogel | January 2015

In many ways, it’s useful to think of TypeScript on its own merits. The TypeScript language specification refers to TypeScipt as “a syntactic sugar for JavaScript.” That’s true and probably an essential step in reaching to the language’s target audience—client-side developers currently using JavaScript.

And you do need to understand JavaScript before you can understand TypeScript. In fact, the language specification (you can read it at bit.ly/1xH1m5B) often describes TypeScript constructs in terms of the resulting JavaScript code. Best tools for mac os x. But it’s equally useful to think of TypeScript as a language on its own that shares features with JavaScript.

For example, like C#, TypeScript is a data-typed language, which gives you IntelliSense support and compile-time checking, among other features. Like C#, TypeScript includes generic and lambda expressions (or their equivalent).

But TypeScript, of course, is not C#. Understanding what’s unique about TypeScript is as important as understanding what TypeScript shares with the server-side language you’re currently using. The TypeScript type system is different (and simpler) than C#. TypeScript leverages its understanding of other object models in a unique way and executes inheritance differently than C#. And because TypeScript compiles to JavaScript, TypeScript shares many of its fundamentals with JavaScript, unlike C#.

The question then remains, “Would you rather write your client-side code in this language or in JavaScript?”

TypeScript Is Data-Typed

TypeScript doesn’t have many built-in data types you can use to declare variables—just string, number and Boolean. Those three types are a subtype of the any type (which you can also use when declaring variables). You can set or test variables declared with those four types against the types null or undefined. You can also declare methods as void, indicating they don’t return a value.

This example declares a variable as string:

You can extend this simple type system with enumerated values and four kinds of object types: interfaces, classes, arrays and functions. For example, the following code defines an interface (one kind of object type) with the name ICustomerShort. The interface includes two members: a property called Id and a method called CalculateDiscount:

As in C#, you can use interfaces when declaring variables and return types. This example declares the variable cs as type ICustomerShort:

You can also define object types as classes, which, unlike interfaces, can contain executable code. This example defines a class called CustomerShort with one property and one method:

Like more recent versions of C#, it’s not necessary to provide implementation code when defining a property. The simple declaration of the name and type is sufficient. Classes can implement one or more interfaces, as shown in Figure 1, which adds my ICustomerShort interface, with its property, to my CustomerShort class.

Figure 1 Add an Interface to a Class

As Figure 1 shows, the syntax for implementing an interface is as simple in TypeScript as in C#. To implement the interface’s members you simply add members with the same name instead of tying the interface name to the relevant class’ members. In this example, I simply added Id and CalculateDiscount to the class to implement ICustomerShort. TypeScript also lets you use object type literals. This code sets the variable cst to an object literal containing one property and one method:

This example uses an object type to specify the return value of the UpdateStatus method:

Typescript Abstract Property Default Value

Besides object types (class, interface, literal and array), you can also define function types that describe a function’s signature. The following code rewrites CalculateDiscount from my CustomerShort class to accept a single parameter called discountAmount:

That parameter is defined using a function type that accepts two parameters (one of string, one of boolean) and returns a number. If you’re a C# developer, you might find that the syntax looks much like a lambda expression.

A class that implements this interface would look something like Figure 2.

Typescript Class Type

Figure 2 This Class Implements the Proper Interface

Like the recent versions of C#, TypeScript also infers the datatype of a variable from the value to which the variable is initialized. In this example, TypeScript will assume the variable myCust is of CustomerShort:

Like C#, you can declare variables using an interface and then set the variable to an object that implements that interface:

Typescript Static Method

Finally, you can also use type parameters (which look suspiciously like generics in C#) to let the invoking code specify the data type to be used. This example lets the code that creates the class set the datatype of the Id property:

Torrent ntfs for mac. This code sets the datatype of the Id property to a string before using it:

To isolate classes, interfaces and other public members and avoid name collisions, you can declare these constructs inside modules much like C# namespaces. You’ll have to flag those items you want to make available to other modules with the export keyword. The module in Figure 3 exports two interfaces and a class.

Figure 3 Export Two Interfaces and One Class

To use the exported components, you can prefix the component name with the module name as in this example:

Or you can use the TypeScript import keyword to establish a shortcut to the module:

TypeScript Is Flexible About Data Typing

All this should look familiar if you’re a C# programmer, except perhaps the reversal of variable declarations (variable name first, data type second) and object literals. However, virtually all data typing in TypeScript is optional. The specification describes the data types as “annotations.” If you omit data types (and TypeScript doesn’t infer the data type), data types default to the any type.

TypeScript doesn’t require strict datatype matching, either. TypeScript uses what the specification calls “structural subtyping” to determine compatibility. This is similar to what’s often called “duck typing.” In TypeScript, two classes are considered identical if they have members with the same types. For example, here’s a CustomerShort class that implements an interface called ICustomerShort:

Here’s a class called CustomerDeviant that looks similar to my CustomerShort class:

Thanks to structural subtyping, I can use CustomerDevient with variables defined with my CustomerShort class or ICustomerShort interface. These examples use CustomerDeviant interchangeably with variables declared as CustomerShort or ICustomerShort:

This flexibility lets you assign TypeScript object literals to variables declared as classes or interfaces, provided they’re structurally compatible, as they are her