Angular 2: TypeScript

Angular 2 is built in TypeScript

Angular 2 is built in a Javascript-like language called TypeScript.

You might be skeptical of using a new language just for Angular, but it turns out, there are a lot of great reasons to use TypeScript instead of plain Javascript.

TypeScript isn’t a completely new language, it’s a superset of ES6. If we write ES6 code, it’s perfectly valid and compilable TypeScript code. Here’s a diagram that shows the relationship between the languages:

ES5, ES6, and TypeScript

What is ES5? What is ES6? ES5 is short for “ECMAScript 5”, otherwise known as “regular Javascript”. ES5 is the normal Javascript we all know and love. It runs in more-or-less every browser. ES6 is the next version of Javascript, which we talk more about below.

At the publishing of this book, very few browsers will run ES6 out of the box, much less TypeScript. To solve this issue we have transpilers (or sometimes called transcompiler). The TypeScript transpiler takes our TypeScript code as input and outputs ES5 code that nearly all browsers understand.

For converting TypeScript to ES5 there is a single transpiler written by the core TypeScript team. However if we wanted to convert ES6 code (not TypeScript) to ES5 there are two major ES6-to-ES5 transpilers: traceur by Google and babel created by the JavaScript community. We’re not going to be using either directly for this book, but they’re both great projects that are worth knowing about.

We installed TypeScript in the last chapter, but in case you’re just starting out in this chapter, you can install it like so:

npm install -g typescript

TypeScript is an official collaboration between Microsoft and Google. That’s great news because with two tech heavyweights behind it we know that it will be supported for a long time. Both groups are committed to moving the web forward and as developers we win because of it.

One of the great things about transpilers is that they allow relatively small teams to make improvements to a language without requiring everyone on the internet upgrade their browser.

One thing to point out: we don’t have to use TypeScript with Angular2. If you want to use ES5 (i.e. “regular” JavaScript), you definitely can. There is an ES5 API that provides access to all functionality of Angular2. Then why should we use TypeScript at all? Because there are some great features in TypeScript that make development a lot better.

What do we get with TypeScript?

There are five big improvements that TypeScript bring over ES5:

Let’s deal with these one at a time.

Types

The major improvement of TypeScript over ES6, that gives the language its name, is the typing system.

For some people the lack of type checking is considered one of the benefits of using a language like JavaScript. You might be a little skeptical of type checking but I’d encourage you to give it a chance. One of the great things about type checking is that

  1. it helps when writing code because it can prevent bugs at compile time and
  2. it helps when reading code because it clarifies your intentions

It’s also worth noting that types are optional in TypeScript. If we want to write some quick code or prototype a feature, we can omit types and gradually add them as the code becomes more mature.

TypeScript’s basic types are the same ones we’ve been using implicitly when we write “normal” JavaScript code: strings, numbers, booleans, etc.

Up until ES5, we would define variables with the var keyword, like var fullName;.

The new TypeScript syntax is a natural evolution from ES5, we still use var but now we can optionally provide the variable type along with its name:

var fullName: string;

When declaring functions we can use types for arguments and return values:

function greetText(name: string): string {
  return "Hello " + name;
}

In the example above we are defining a new function called greetText which takes one argument: name. The syntax name: string says that this function expects name to be a string. Our code won’t compile if we call this function with anything other than a string and that’s a good thing because otherwise we’d introduce a bug.

Notice that the greetText function also has a new syntax after the parentheses: : string {. The colon indicates that we will specify the return type for this function, which in this case is a string. This is helpful because 1. if we accidentally return anything other than a string in our code, the compiler will tell us that we made a mistake and 2. any other developers who want to use this function know precisely what type of object they’ll be getting.

Let’s see what happens if we try to write code that doesn’t conform to our declared typing:

function hello(name: string): string {
    return 12;
}

If we try to compile it, we’ll see the following error:

$ tsc compile-error.ts
compile-error.ts(2,12): error TS2322: Type 'number' is not assignable to type 'string'.

What happened here? We tried to return 12 which is a number, but we stated that hello would return a string (by putting the ): string { after the argument declaration).

In order to correct this, we need to update the function declaration to return a number:

function hello(name: string): number {
    return 12;
}

This is one small example, but already we can see that by using types it can save us from a lot of bugs down the road.

So now that we know how to use types, how can we know what types are available to use? Let’s look at the list of built-in types, and then we’ll figure out how to create our own.

Trying it out with a REPL

To play with the examples on this chapter, let’s install a nice little utility called TSUN (TypeScript Upgraded Node):

$ npm install -g tsun

Now start tsun:

$ tsun
TSUN : TypeScript Upgraded Node
type in TypeScript expression to evaluate
type :help for commands in repl
  >

That little > is the prompt indicating that TSUN is ready to take in commands.

In most of the examples below, you can copy and paste into this terminal and follow along.

Built-in types

String

A string holds text and is declared using the string type:

var fullName: string = 'Nate Murray';

Number

A number is any type of numeric value. In TypeScript, all numbers are represented as floating point. The type for numbers is number:

var age: number = 36;

Boolean

The boolean holds either true or false as the value.

var married: boolean = true;

Array

Arrays are declared with the Array type. However, because an Array is a collection, we also need to specify the type of the objects in the Array.

We specify the type of the items in the array with either the Array<type> or type[] notations:

var jobs: Array<string> = ['IBM', 'Microsoft', 'Google'];
var jobs: string[] = ['Apple', 'Dell', 'HP'];

Or similarly with a number:

var chickens: Array<number> = [1, 2, 3];
var chickens: number[] = [4, 5, 6];

Enums

Enums work by naming numeric values. For instance, if we wanted to have a fixed list of roles a person may have we could write this:

enum Role {Employee, Manager, Admin};
var role: Role = Role.Employee;

The default initial value for an enum is 0, though you can set the starting enum number like this:

enum Role {Employee = 3, Manager, Admin};
var role: Role = Role.Employee;

In the code above, instead of Employee being 0, Employee is 3. The value of the enum increments from there, which means Manager is 4 and Admin is 5, and we can even set individual values:

enum Role {Employee = 3, Manager = 5, Admin = 7};
var role: Role = Role.Employee;

You can also look up the name of a given enum by using its value:

enum Role {Employee, Manager, Admin};
console.log('Roles: ', Role[0], ',', Role[1], 'and', Role[2]);

Any

any is the default type if we omit typing for a given variable. Having a variable of type any allows it to receive any kind of value:

var something: any = 'as string';
something = 1;
something = [1, 2, 3];

Void

Using void means there’s no type expected. This is usually in functions with no return value:

function setName(name: string): void {
  this.fullName = name;
}

Classes

In Javascript ES5 object oriented programming was accomplished by using prototype-based objects. This model doesn’t use classes, but instead relies on prototypes.

A number of good practices have been adopted by the JavaScript community to compensate the lack of classes. A good summary of those good practices can be found in Mozilla Developer Network’s JavaScript Guide, and you can find a good overview on the Introduction to Object-Oriented Javascript page.

However, in ES6 we finally have built-in classes in Javascript.

To define a class we use the new class keyword and give our class a name and a body:

class Vehicle {
}

Classes may have properties, methods, and constructors.

Properties

Properties define data attached to an instance of a class. For example, a class named Person might have properties like first_name, last_name and age.

Each property in a class can optionally have a type. For example, we could say that the first_name and last_name properties are strings and the age property is a number.

The declaration for a Person class that looks like this:

class Person {
  first_name: string;
  last_name: string;
  age: number;
}

Methods

Methods are functions that run in context of an object. To call a method on an object, we first have to have an instance of that object.

To instantiate a class, we use the new keyword. Use new Person() to create a new instance of the Person class, for example.

If we wanted to add a way to greet a Person using the class above, we would write something like:

    class Person {
      first_name: string;
      last_name: string;
      age: number;

      // leanpub-start-insert
      greet() {
        console.log("Hello", this.first_name);
      }
      // leanpub-end-insert
    }

Notice that we’re able to access the first_name for this Person by using the this keyword and calling this.first_name.

When methods don’t declare an explicit returning type and return a value, it’s assumed they can return anything (any type). However, in this case we are returning void, since there’s no explicit return statement.

Note that a void value is also a valid any value.

In order to invoke the greet method, you would need to first have an instance of the Person class. Here’s how we do that:

 
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