16 min read

One of the primary benefits of compiled languages is that they provide a more plain syntax for the developer to work with before the code is eventually converted to machine code. TypeScript is able to bring this advantage to JavaScript development by wrapping several different patterns into language constructs that allow us to write better code. Every explicit type annotation that is provided is simply syntactic sugar that will be removed during compilation, but not before their constraints are analyzed and any errors are caught. In this article by Christopher Nance, the author of TypeScript Essentials, we will explore this type system in depth. We will also discuss the different language structures that TypeScript introduces. We will look at how these structures are emitted by the compiler into plain JavaScript. This article will contain a detailed at into each of these concepts:

(For more resources related to this topic, see here.)

  • Types
  • Functions
  • Interfaces
  • Classes

Types

These type annotations put a specific set of constraints on the variables being created. These constraints allow the compiler and development tools to better assist in the proper use of the object. This includes a list of functions, variables, and properties available on the object. If a variable is created and no type is provided for it, TypeScript will attempt to infer the type from the context in which it is used. For instance, in the following code, we do not explicitly declare the variable hello as string; however, since it is created with an initial value, TypeScript is able to infer that it should always be treated as a string:

var hello = "Hello There";

The ability of TypeScript to do this contextual typing provides development tools with the ability to enhance the development experience in a variety of ways. The type information allows our IDE to warn us of potential errors in our code, or provide intelligent code completion and suggestion. As you can see from the following screenshot, Visual Studio is able to provide a list of methods and properties associated with string objects as well as their type information:

TypeScript Essentials

When an object’s type is not given and cannot be inferred from its initialization then it will be treated as an Any type. The Any type is the base type for all other types in TypeScript. It can represent any JavaScript value and the minimum amount of type checking is performed on objects of type Any. Every other type that exists in TypeScript falls into one of three categories: primitive types, object types, or type parameters. TypeScript’s primitive types closely mirror those of JavaScript. The TypeScript primitive types are as follows:

  • Number: var myNum: number = 2;
  • Boolean: var myBool: boolean = true;
  • String: var myString: string = “Hello”;
  • Void: function(): void { var x = 2; }
  • Null: if (x != null) { alert(x); }
  • Undefined: if (x != undefined) { alert(x); }

All of these types correspond directly to JavaScript’s primitive types except for Void. The Void type is meant to represent the absence of a value. A function that returns no value has a return type of void. Object types are the most common types you will see in TypeScript and they are made up of references to classes, interfaces, and anonymous object types. Object types are made up of a complex set of members. These members fall into one of four categories: properties, call signatures, constructor signatures, or index signatures.

Type parameters are used when referencing generic types or calling generic functions. Type parameters are used to keep code generic enough to be used on a multitude of objects while limiting those objects to a specific set of constraints. An early example of generics that we can cover is arrays. Arrays exist just like they do in JavaScript and they have an extra set of type constraints placed upon them. The array object itself has certain type constraints and methods that are created as being an object of the Array type, the second piece of information that comes from the array declaration is the type of the objects contained in the array. There are two ways to explicitly type an array; otherwise, the contextual typing system will attempt to infer the type information:

var array1: string[] = [];
var array2: Array = [];

Both of these examples are completely legal ways of declaring an array. They both generate the same JavaScript output and they both provide the same type information. The first example is a shorthand type literal using the [ and ] characters to create arrays. The resulting JavaScript for each of these arrays is shown as follows:

var array1 = [];
var array2 = [];

Despite all of the type annotations and compile-time checking, TypeScript compiles to plain JavaScript and therefore adds absolutely no overhead to the run time speed of your applications. All of the type annotations are removed from the final code, providing us with both a much richer development experience and a clean finished product.

Functions

If you are at all familiar with JavaScript you will be very familiar with the concept of functions. TypeScript has added type annotations to the parameter list as well as the return type. Due to the new constraints being placed on the parameter list, the concept of function overloads was also included in the language specification. TypeScript also takes advantage of JavaScript’s arguments object and provides syntax for rest parameters. Let’s take a look at a function declaration in TypeScript:

function add(x: number, y: number): number {
   return x + y;
}

As you can see, we have created a function called add. It takes two parameters that are both of the type number, one of the primitive types, and it returns a number. This function is useful in its current form but it is a little limited in overall functionality. What if we want to add a third number to the first two? Then we have to call our function multiple times. TypeScript provides a way to provide optional parameters to functions. So now we can modify our function to take a third parameter, z, that will get added to the first two numbers, as shown in the following code:

function add(x: number, y: number, z?: number) {
   if (z !== undefined) {
       return x + y + z;
   }
   return x + y;
}

As you can see, we have a third named parameter now but this one is followed by ?. This tells the compiler that this parameter is not required for the function to be called.

Optional parameters tell the compiler not to generate an error if the parameter is not provided when the function is called. In JavaScript, this compile-time checking is not performed, meaning an exception could occur at runtime because each missing parameter will have a value of undefined. It is the responsibility of the developer to write code that verifies a value exists before attempting to use it.

So now we can add three numbers together and we haven’t broken any of our previous code that relied on the add method only taking two parameters. This has added a little bit more functionality but I think it would be nice to extend this code to operate on multiple types. We know that strings can be added together just the same as numbers can, so why not use the same method? In its current form, though, passing strings to the add function will result in compilation errors. We will modify the function’s definition to take not only numbers but strings as well, as shown in the following code:

function add(x: string, y: string): string;
function add(x: number, y: number): number;
function add(x: any, y: any): any {
   return x + y;
}

As you can see, we now have two declarations of the add function: one for strings, one for numbers, and then we have the final implementation using the any type. The signature of the actual function implementation is not included in the function’s type definition, though. Attempting to call our add method with anything other than a number or string will fail at compile time, however, the overloads have no effect on the generated JavaScript. All of the type annotations are stripped out, as well as the overloads, and all we are left with is a very simple JavaScript method:

function add(x, y) {
 return x + y;
}

Great, so now we have a multipurpose add function that can take two values and combine them together for either strings or numbers. This still feels a little limited in overall functionality though. What if we wanted to add an indeterminate number of values together? We would have to call our add method over and over again until we eventually had only one value. Thankfully, TypeScript includes rest parameters, which is essentially an unbounded list of optional parameters. The following code shows how to modify our add functions to include a rest parameter:

function add(arg1: string, ...args: string[]): string;
function add(arg1: number, ...args: number[]): number;
function add(arg1: any, ...args: any[]): any {
   var total = arg1;
   for (var i = 0; i 

A rest parameter can only be the final parameter in a function’s declaration. The TypeScript compiler recognizes the syntax of this final parameter and generates an extra bit of JavaScript to generate a shifted array from the JavaScript arguments object that is available to code inside of a function. The resulting JavaScript code shows the loop that the compiler has added to create the array that represents our indeterminate list of parameters:

function add(arg1) {
   var args = [];
   for (var _i = 0; _i 

Now adding numbers and strings together is very simple and is completely type-safe. If you attempt to mix the different parameter types, a compile error will occur. The first two of the following statements are legal calls to our Add function; however, the third is not because the objects being passed in are not of the same type:

alert(add("Hello ", "World!"));
alert(add(3, 5, 9, 120, 42));
//Error
alert(add(3, "World!"));

We are still very early into our exploration of TypeScript but the benefits are already very apparent. There are still a few features of functions that we haven’t covered yet but we need to learn more about the language first. Next, we will discuss the interface construct and the benefits it provides with absolutely no cost.

Interfaces

Interfaces are a key piece of creating large-scale software applications. They are a way of representing complex types about any object. Despite their usefulness they have absolutely no runtime consequences because JavaScript does not include any sort of runtime type checking. Interfaces are analyzed at compile time and then omitted from the resulting JavaScript. Interfaces create a contract for developers to use when developing new objects or writing methods to interact with existing ones. Interfaces are named types that contain a list of members. Let’s look at an example of an interface:

interface IPoint {
   x: number;
   y: number;
}

As you can see we use the interface keyword to start the interface declaration. Then we give the interface a name that we can easily reference from our code.

Interfaces can be named anything, for example, foo or bar, however, a simple naming convention will improve the readability of the code. Interfaces will be given the format I and object types will just use , for example, IFoo and Foo.

The interfaces’ declaration body contains just a list of members and functions and their types. Interface members can only be instance members of an object. Using the static keyword in an interface declaration will result in a compile error.

Interfaces have the ability to inherit from base types. This interface inheritance allows us to extend existing interfaces into a more enhanced version as well as merge separate interfaces together. To create an inheritance chain, interfaces use the extends clause. The extends clause is followed by a comma-separated list of types that the interface will merge with.

interface IAdder {
   add(arg1: number, ...args: number[]): number;
}
interface ISubtractor {
   subtract(arg1: number, ...args: number[]): number;
}
interface ICalculator extends IAdder, ISubtractor {
   multiply(arg1: number, ...args: number[]): number;
   divide(arg1: number, arg2: number): number;
}

Here, we see three interfaces:

  • IAdder, which defines a type that must implement the add method that we wrote earlier
  • ISubtractor, which defines a new method called subtract that any object typed with ISubtractor must define
  • ICalculator, which extends both IAdder and ISubtractor as well as defining two new methods that perform operations a calculator would be responsible for, which an adder or subtractor wouldn’t perform

These interfaces can now be referenced in our code as type parameters or type declarations. Interfaces cannot be directly instantiated and attempting to reference the members of an interface by using its type name directly will result in an error. In the following function declaration the ICalculator interface is used to restrict the object type that can be passed to the function. The compiler can now examine the function body and infer all of the type information associated with the calculator parameter and warn us if the object used does not implement this interface.

function performCalculations(calculator: ICalculator, num1, num2) {
   calculator.add(num1, num2);
   calculator.subtract(num1, num2);
   calculator.multiply(num1, num2);
   calculator.divide(num1, num2);
   return true;
}

The last thing that you need to know about interface definitions is that their declarations are open-ended and will implicitly merge together if they have the same type name. Our ICalculator interface could have been split into two separate declarations with each one adding its own list of base types and its own list of members. The resulting type definition from the following declaration is equivalent to the declaration we saw previously:

interface ICalculator extends IAdder {
   multiply(arg1: number, ...args: number[]): number;
}
interface ICalculator extends ISubtractor {
   divide(arg1: number, arg2: number): number;
}

Creating large scale applications requires code that is flexible and reusable. Interfaces are a key component of keeping TypeScript as flexible as plain JavaScript, yet allow us to take advantage of the type checking provided at compile time. Your code doesn’t have to be dependent on existing object types and will be ready for any new object types that might be introduced. The TypeScript compiler also implements a duck typing system that allows us to create objects on the fly while keeping type safety. The following example shows how we can pass objects that don’t explicitly implement an interface but contain all of the required members to a function:

function addPoints(p1: IPoint, p2: IPoint): IPoint {
   var x = p1.x + p2.x;
   var y = p1.y + p2.y;
   return { x: x, y: y }
}
//Valid
var newPoint = addPoints({ x: 3, y: 4 }, { x: 5, y: 1 });
//Error
var newPoint2 = addPoints({ x: 1 }, { x: 4, y: 3 });

Classes

In the next version of JavaScript, ECMAScript 6, a standard has been proposed for the definition of classes. TypeScript brings this concept to the current versions of JavaScript. Classes consist of a variety of different properties and members. These members can be either public or private and static or instance members.

Definitions

Creating classes in TypeScript is essentially the same as creating interfaces. Let’s create a very simple Point class that keeps track of an x and a y position for us:

class Point {
   public x: number;
   public y: number;
   constructor(x: number, y = 0) {
       this.x = x;
       this.y = y;
   }
}

As you can see, defining a class is very simple. Use the keyword class and then provide a name for the new type. Then you create a constructor for the object with any parameters you wish to provide upon creation. Our Point class requires two values that represent a location on a plane.

The constructor is completely optional. If a constructor implementation is not provided, the compiler will automatically generate one that takes no parameters and initializes any instance members.

We provided a default value for the property y. This default value tells the compiler to generate an extra JavaScript statement than if we had only given it a type. This also allows TypeScript to treat parameters with default values as optional parameters. If the parameter is not provided then the parameter’s value is assigned to the default value you provide. This provides a simple method for ensuring that you are always operating on instantiated objects. The best part is that default values are available for all functions, not just constructors. Now let’s examine the JavaScript output for the Point class:

var Point = (function () {
   function Point(x, y) {
       if (typeof y === "undefined") { y = 0; }
       this.x = x;
       this.y = y;
   }
   return Point;
})();

As you can see, a new object is created and assigned to an anonymous function that initializes the definition of the Point class. As we will see later, any public methods or static members will be added to the inner Point function’s prototype. JavaScript closures are a very important concept in understanding TypeScript. Classes, modules, and enums in TypeScript all compile into JavaScript closures. Closures are actually a construct of the JavaScript language that provide a way of creating a private state for a specific segment of code. When a closure is created it contains two things: a function, and the state of the environment when the function was created. The function is returned to the caller of the closure and the state is used when the function is called.

For more information about JavaScript closures and the module pattern visit http://www.adequatelygood.com/JavaScript-Module-Pattern-In-Depth.html.

The optional parameter was accounted for by checking its type and initializing it if a value is not available. You can also see that both x and y properties were added to the new instance and assigned to the values that were passed into the constructor.

Summary

This article has thoroughly discussed the different language constructs in TypeScript.

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