Check out our Angular Book Series.

Angular Unit Testing: How do I Fix a Platform with a Different Configuration Has Been Created?

I've run into this one a few times. You're setting up Unit Testing of an Angular application and get an error like this:

Uncaught Error: A platform with a different configuration has been created. Please destroy it first.

It's frustrating and a but confusing. If you Google on that error you'll find a bunch of stuff, but no explicit solution.

You've probably set up a TestBed configuration environment as part of your unit testing, probably like this:


import { TestBed } from "@angular/core/testing";
import { BrowserDynamicTestingModule,
platformBrowserDynamicTesting } from "@angular/platform-browser-dynamic/testing";

TestBed.initTestEnvironment(BrowserDynamicTestingModule, platformBrowserDynamicTesting());

However, somewhere in the code, the non-testing modules have been loaded, probably like this:


import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';

import { AppModule } from './app.module';

platformBrowserDynamic().bootstrapModule(AppModule);

The error occurs because of the conflict between the platformBrowserDynamic() and the platformBrowserDynamicTesting(). Make sure that you exclude the source files the import and initialization of the non-test version of the library.

In your karma.conf.js file add an exclude property to the Karma configuration object, something like this:


exclude : "src/app/main.ts",

In my case, today, the error was related to a missing '/' in the excluded directory structure.

I hope this helps someone.

How do I fix IE11 problems with CSS Calc and min-height?

I hate it when you have a problem that you cannot replicate in a simple example. That's exactly where I am. I'm working on an application built with HTML, CSS, and various JavaScript frameworks. The application requires a footer to be aligned at the bottom of the page. If the content is too large for the page, then the the footer should show up under the content. But, if the content is too small for the page, the footer should show up at the bottom of the window, or viewport.

This should be pretty simple to build using CSS and HTML:


<div class="wrapper">
Lots of Main Content and navigation and headers Here
</div>
<footer>
Some Footer Information Here that should always be at the bottom of the page
</footer>

The CSS would be something like this:


html, body {
height: 100vh;
margin : 0;
}

.wrapper {
min-height: calc(100% - 200px);
}

footer {
height : 200px;
background-color : blue;
}

The actual client project code is a lot more complex, but this gives you the gist. Play with the code here. Ignore my design skills.

The problem I was having was that IE11 seemed to have a problem with the min-height. When clicking in the page, or clicking a link to go to another page, it was as if the wrapper's min-height would shrink down on pages where the view height was greater than the actual content. This would cause the footer to jump to the middle of the page.

For the life of me, I haven't been able to create a simple sample to demonstrate this. Some part of the client's custom code seems to be causing this.

After some head banging and conferring with my team, I came up with this solution. It makes use of JavaScript and JQuery to 'hard code' the min-height on the wrapper. Instead of using the CSS Calc. this happens when the page loads and whenever the page is resized.


if(/MSIE \d|Trident.*rv:/.test(navigator.userAgent)){
function onResize(){
     /* The jquery calc code */
     $('.wrapper').css('min-height', '100%').css('min-height', '-=200px');
}

$(window).resize(function() {
onResize()
});

$(window).ready(function(){
onResize()
});

}

I used a regex trick to only execute the code for IE browsers. And I used this answer as a base for my solution.

Part of our problem solving this was my inability to create a simple reproducible case, but pouring over thousands of line of code I could not find the magic style combination that caused the 'footer jump' problem.

Frustrating; but the JS code above seems to solve the issue admirably.

Create TypeScript Modules - - Part 8

This is the last in my series of articles intended to introduce you to Typescript. It is bonus material I wrote for my Angular 4 book. This is the last part of the series. Check out part 1, part 2, Part 3, Part 4, Part 5. and Part 6, and Part 7.

When writing a real-world application, it does not make sense to include all the code in a single file. TypeScript supports that by allowing you to expand different functionality into modules.

Create Interface Module

The first thing we're going to do is create a module for the name interface. I put this in a file named IFile.ts:


export interface name {
firstName: string;
middleInitial? : string;
lastName: string;
getName() : string;
};

This looks exactly like the Interface created in the previous exception with the one addition of the export keyword. Export tells the compiler that this class is available for use inside other classes.

Create Class Modules

Now create the Person class:


export class Person implements name {
firstName: string;
middleInitial : string;
lastName: string;
getName() : string {
return this.firstName + ' ' + this.middleInitial + ' ' + this.lastName;
}
}

This also puts the export keyword in front of the class definition. If you try this you'll notice an immediate error. The name is not defined. To define it we'll need to add an import statement:


import {name} from "./IName";

The import statement tells you that the name entity is imported from the IName file and can be used within this class. The path I used, './IName', tells us that the files are in the same directory, however we can use a more elaborate package setup, and most likely you will do that for main applications.

We can create the Pets.ts module in the same manner:


import {name} from "./IName";

export class Pet implements name {
firstName: string;
lastName: string;
type : string;
getName() : string {
return this.firstName + ' ' + this.lastName + ", " + this.type;
}
}

This code mirrors the Person module, with the primary changes being the use of the export keyword before the class definition and the import of the name interface.

The Echo class needs a similar rework:


import {name} from "./IName";

export class Echo {
static readonly messageIntro : string = "Hello"
subjectArray : name[];
private message : string;
constructor(subjects : name[]){
this.subjectArray = subjects;
}
createMessage():void{
this.message = '';
for (let person of this.subjectArray){
this.message += Echo.messageIntro + " " + person.getName() + "<br/>";
}
}
echo():string{
return this.message;
}
}

The functionality remains unchanged. Like the previous classes it uses an export statement to make the class available elsewhere, and an import statement to make use of the name interface.

Rework Main Application

With all the classes stored in separate files, our primary app has become a lot simpler. First, import all the classes:


import {name} from "./IName";
import {Person} from "./Person";
import {Pet} from "./Pet";
import {Echo} from "./Echo";

Then, create the nameArray:


let nameArray : name[] = [];

Now, populate the nameArray:


let jeffryInstance : Person = new Person();
jeffryInstance.firstName = "Jeffry";
jeffryInstance.middleInitial = "A";
jeffryInstance.lastName = "Houser";
nameArray.push(jeffryInstance);

let hercInstance : Pet = new Pet();
hercInstance.firstName = "Hercules";
hercInstance.lastName = "Houser";
hercInstance.type = "Dog";
nameArray.push(hercInstance);

let zeusInstance : Pet = new Pet();
zeusInstance.firstName = "Isadora";
zeusInstance.lastName = "Houser";
zeusInstance.type = "Dragon";
nameArray.push(zeusInstance);

Create an instance of the Echo class:


let echoInstance : Echo = new Echo(nameArray);

Call the createMessage() function:


echoInstance.createMessage();

Finally, output the results:


document.body.innerHTML = echoInstance.echo();

The changes here was, primarily, removing the class and interface definitions and replacing them with imports.

Setup Module Loader

The import statement is not a native JavaScript statement and does not have an easy parallel. To make code like this work in the browser we'll need to use a module loader. There are a few different types of module loaders such as RequireJS or SystemJS. Many people use code tools like Browserify or WebPack to encapsulate away the complexity.

For the purposes of this sample, I'm going to use RequireJS. Open up the index.html file and add this script statement:


<script data-main="requireconfig"
src="//cdnjs.cloudflare.com/ajax/libs/require.js/2.3.4/require.min.js">

</script>

This loads the RequireJS library from a remote CDN. Note that I have completely removed the script tag that loads the hello.js file. The script tag also has a different attribute named data-main with a value of requireconfig. This property tells Require that whenever it has completed loading it should look for the requireconfig.js file. Create that file next:


requirejs.config({
baseUrl: '',
paths: {
app: ''
}
});

This sets up the baseUrl and the paths to the app. Since all our code is in the main dev directory, I set these values to blank. Now, tell RequireJS to load our main application file:


requirejs(['hello']);

Since there are a few different methods of creating modules, we have to tell our TypeScript compiler which one to use. We want it to use the amd approach. When you compile your application add the module flag and specify amd. Use this at the command line:


tsc module amd hello

You'll see this:

Now load the app in the browser:

Congratulations! This series should have given you all the information you need to know to start using TypeScript to build your applications. Check out our book on Angular 4 which uses TypeScript heavily.

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Create an Interface in TypeScript - - Part 7

I'm writing a series of articles about Typescript as extra material to support my upcoming Angular 4 book. This is the seventh part of that series. Check out part 1, part 2, Part 3, Part 4, Part 5. and Part 6.

This section will talk about interfaces. It is not about User interfaces, but rather object-oriented interfaces. An interface is a definition of properties and methods that must be supported by a class which implements this interface. You can think of an interface as the contract between a class instance and a method argument or variable. If they both match the interface they are interchangeable.

Create the Interface

For this sample, we're going to create a name interface. This interface will represent the name of a person and will contain a first name and last name. It will have an optional middle initial and also a method to retrieve the name. Here is the interface:


interface name {
firstName: string;
middleInitial? : string;
lastName: string;
getName() : string;
}

The interface is set up similar to a class or enum. It uses the keyword interface, and the name of the interface--in this case name. The firstName and lastName properties are as normal properties on an object. The middleInitial is an optional property, and that is distinguished by a question mark. Finally, a single method is defined by the interface, getName(). This will be used to return the name. The interface does not contain an implementation, just a definition of the method. That means different classes which extend the interface can have different implementations.

Rework the Echo Class

Let's rework the Echo class to use the interface instead of strings. First create the method stub with the instance variables:


class Echo {
static readonly messageIntro : string = "Hello"
subjectArray : name[];
private message : string;
}

The main change here is that the subject string was replaced with a subjectArray of name types. The constructor:


constructor(subjects : name[]){
this.subjectArray = subjects;
}

The constructor now accepts an array of name instead of a single string. The createMessage() function now loops over the array:


createMessage():void{
this.message = '';
for (let person of this.subjectArray){
this.message += Echo.messageIntro + " " + person.getName() + "<br/>";
}
}

Inside the loop, the interface's getName() function is used to add an element to the output. Finally, the echo() function:


echo():string{
return this.message;
}

The echo function has not changed, it just returns the object instance variable, message.

Implement the Interface with a Class

I'm going to create two classes which implement our name interface contract. One will be for a Person and another for a Pet. First, the Person class:


class Person implements name {
firstName: string;
middleInitial : string;
lastName: string;
getName(): string {
return this.firstName + ' ' + this.middleInitial + ' ' + this.lastName;
}
}

The class is defined with the class keyword followed by the name of the class. Then comes the implements keyword followed by the name of the interface. If you wanted you could list multiple interfaces here.

The firstName, middleInitial, and lastName are created properties on the object, not much different than how they were with the interface definition. The getName() method includes a definition. It concatenates the firstName, middleInitial, And lastName, returning a single string. Now, let's create another class that implements name. This one is called Pet:


class Pet implements name {
firstName: string;
lastName: string;
type : string;
getName(): string {
return this.firstName + ' ' + this.lastName + ", " + this.type;
}
}

This includes the firstName and lastName defined as variables on the class object. A new property is named type, this will represent the type of pet we have. A class can implement properties or methods not included in an interface. The middleInitial is left out, which is okay because that is marked as an optional property in the interface--and my pets don't have middle initials. The getName() method puts together the firstName and lastName, but appends the type to the end.

Create a name Array

We have an interface and two classes that extend it. Let's put all together. Start by creating a names array:


let nameArray : name[] = [];

Now create a new Person instance. This one represents me:


let jeffryInstance : Person = new Person();
jeffryInstance.firstName = "Jeffry";
jeffryInstance.middleInitial = "A";
jeffryInstance.lastName = "Houser";
nameArray.push(jeffryInstance);

The new jeffryInstance of the Person class is added to the nameArray.

Create two pets:


let hercInstance : Pet = new Pet();
hercInstance.firstName = "Hercules";
hercInstance.lastName = "Houser";
hercInstance.type = "Dog";
nameArray.push(hercInstance);

let zeusInstance : Pet = new Pet();
zeusInstance.firstName = "Isadora";
zeusInstance.lastName = "Houser";
zeusInstance.type = "Dragon";
nameArray.push(zeusInstance);

One is for my pet dog, Hercules, and the other is for my pet dragon, Isadora. Both are added to the same nameArray value without issue even though they have different types. The implementation of the interfaces tells the compiler that the nameArray can accept both Pet and Person instances.

Now, create an instance of the Echo class:


let echoInstance : Echo = new Echo(nameArray);

Create the message:


echoInstance.createMessage();

And output the message:


document.body.innerHTML = echoInstance.echo();

Recompile the app and load it in a browser:

It works as expected.

I have one more entry into this introduction to TypeScript series. It will tell you how to create Modules from your class and interface files.

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Create your first TypeScript Class - - Part 6

I'm writing a series of articles about Typescript as extra material to support my upcoming Angular 4 book. This is the sixth part of that series. Check out part 1, part 2, Part 3, Part 4, and Part 5.

This entry will teach you about creating your own classes. A class is like your own variable type with data and functions to process that data.

Create a Class

Create your class stub:


class Echo {
}

This creates a class named Echo. The purpose of this class is to create and store our welcome message. Let's start by creating some properties on the class:


static readonly messageIntro : string = "Hello"
subject : string;
private message : string;

The subject is a relatively normal variable. It is a string that contains the subject of our welcome message. The message is similar, but has the private distinguisher. This means that the message variable is private to the local message. It will contain the final message once it is put together, but is only accessible inside the class, not from code accessing an instance of the class.

The messageIntro is a bit different. We are not able to define constants inside of a TypeScript class, so instead I used the readonly keyword. This means the value will not be changeable, just like a constant. I also define the messageIntro as a static property, which means it exists on the class and not on an instance of the class.

Every class needs a constructor, so create one:


constructor(subject : string){
this.subject = subject;
}

The constructor will be called automatically when we create a new instance of the class with the new keyword. This constructor accepts one argument, the subject. The code just saves the subject argument into the subject variable.

Now, add a createMessage() function:


createMessage():void{
this.message = Echo.messageIntro + " " + this.subject + "<br/>";
}

This concatenates the read only messageIntro variable with the subject and a line break. We had similar code in previous samples, but not encapsulated into a function. The return type of this function is void, meaning nothing is returned.

We'll add one final function to our class:


echo():string{
return this.message;
}

This function returns the complete message. The message value could be blank if the createMessage() function as not called yet, however this will still work.

Now, create an instance of the Echo class:


var echoInstance : Echo = new Echo('World')

This uses the new keyword to create the new instance. You may have seen this in other languages. You can output the subject for testing purposes:


console.log(echoInstance.subject);

You can also try to output the message:


console.log(echoInstance.message);

Since the message is a private variable, you'll see a compile error:

By the same token, we can access the messageIntro static value:


console.log(echoInstance.messageIntro);
console.log(Echo.messageIntro);

Can you guess which line works before trying to compile the code? The second one is the proper way to access static variables. The first one will throw an error:

Our final code is to output the value to the screen:


document.body.innerHTML = echoInstance.echo();

Compile the code--you should see no errors--and load it in a browser:

In a non-sample application, I might try to make my classes more discrete. Since this one adds a line break at the end of the message, it is conflating data processing and display code, something which is undesirable in real world applications.

Review the Generated Code

This is the generated code:


var Echo = (function () {
function Echo(subject) {
this.subject = subject;
}
Echo.prototype.createMessage = function () {
this.message = Echo.messageIntro + " " + this.subject + "<br/>";
};
Echo.prototype.echo = function () {
return this.message;
};
Echo.messageIntro = "Hello";
return Echo;
}());
var echoInstance = new Echo('World');
echoInstance.createMessage();
document.body.innerHTML = echoInstance.echo();

The Echo class is created using an immediately invoked function expression. The static class property is created as an instance variable inside the class--remember that JavaScript doesn't have the concept of readonly or static properties. The subject and message variables are not defined since they do not have default values. They'll be created on the class when they are needed, which is inside the function.

Although beyond the scope of this tutorial, inheritance is supported with TypeScript, so one class can inherit from another.

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Loops and Arrays in TypeScript - Part 5

I'm writing a longer series of articles about Typescript. This will be extra material to support my upcoming Angular 4 book. This is the fifth part of that series. Check out part 1, part 2, Part 3, and Part 4.

This entry will focus on loops and arrays. We'll create an array of strings, and then loop over them with our echo function to welcome multiple people in our HTML page.

Create an Array

The first step is to create an array. You can use this sytnax:


let personArray : string[] = ["Jeffry", "Tom", "John"];

This creates a variable just like other variables we had seen. The key differentiator is that after the type, I added square brackets. This is what tells the compiler we are creating an array of strings. You can define arrays with any of the native types, such as numbers or Boolean, or with your own custom types.

For-in Loop

The very first time I wanted to loop over an array, I used a for-in loop, like this:


let bodyString : string = "";
for (let person in personArray){
bodyString += echo(messageIntro + " " + person + "<br/>");
}
document.body.innerHTML = bodyString ;

I had used for-in loops in other languages, such as ActionScript so this was my natural impulse.

Let's look at the results:

Instead of outputting the data in the array, it outputted the index. The proper way to use a for-in loop in TypeScript would be like this:


for (let person in personArray){
bodyString += echo(messageIntro + " " + personArray[person] + "<br/>");
}

This isn't much different than a generic for loop that uses a counter, although the syntax is a bit nicer:

All version of ECMAScript after 2015 support for-in loops the same way that TypeScript does. This is supported by most browsers and in fact that generated JS Code uses for-in loops:


for (var person in personArray) {
bodyString += echo(messageIntro + " " + person + "<br/>");
}

But, this result wasn't quite what I was after. Thankfully TypeScript includes another option, the for-of loop.

For-Of Loops

A for-of loop makes one syntactical difference than a for-in loop. Instead of using the keyword in, it uses the keyword on:


for (let person of personArray){
bodyString += echo(messageIntro + " " + person + "<br/>");
}

This type of loop the loop counter, person, will match the value of the personArray instead of the index. After the compilation this is turned into a for loop with an explicit counter:


for (var _i = 0, personArray_1 = personArray; _i < personArray_1.length; _i++) {
var person = personArray_1[_i];
bodyString += echo(messageIntro + " " + person + "<br/>");
}

Compile this code and load it in the browser to see the expected results:

Looping in TypeScript is not much different than looping in other languages.

The next entry in this series will focus on creating classes.

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String Enums in TypeScript - Part 4

I'm writing a longer series of articles about Typescript. This will be extra material to support my upcoming Angular 4 book. This is the fourth part of that series. Check out part 1, part 2, and Part 3. This article will focus on creating String based Enums.

What is an Enum?

Enums are a data type I have not come across in most other languages, but they are inspired by C#. An enum is a way to give friendly names to numerical or string values. They are like an array with indexes, but more specific. I can envision using something like this with a view stack style component and using an enum to handle which view is currently displayed. Or, I might use it with an event class instance to determine what type of event occurred. Or it could be used to determine what color style to apply to certain text.

Create a String based Enum

It is interesting that Enum's can be used to represent textual values too. Look at this:


enum MyColors {
Blue = "#0000FF",
Red = "#FF0000",
Green = "#00FF00",
};

The value of the color name represents the hex value of the actual color. I recently could have used something similar when dynamically applying colors to the header of a site based on which section the user was viewing. Get the individual colors, and output them:


let myColor1 :MyColors = MyColors.Blue;
let myColor2 :MyColors = MyColors.Red;
let myColor3 :MyColors = MyColors.Green;

console.log(myColor1);
console.log(myColor2);
console.log(myColor3);

You'll see something like this:

Enums are an interesting data type, caught somewhere between an array and a class.

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Enums in TypeScript - Part 3

I'm writing a longer series of articles about Typescript. This will be extra material to support my upcoming Angular 4 book. This is the third part of that series. Check out part 1 and part 2. This article will focus on Enums, a TypeScript data type that does not exist in JavaScript.

What is an Enum?

Enums are a data type I have not come across in most other languages, but they are inspired by C#. An enum is a way to give friendly names to numerical values. They are like an array with indexes, but more specific. I can envision using something like this with a view stack style component and using an enum to handle which view is currently displayed. Or, I might use it with an event class instance to determine what type of event occurred. Or it could be used to determine what color style to apply to certain text.

Create a Number based Enum

I'm going to start by creating a simple sample with a number based enum:


enum MyNumbers {
First,
Second,
Third
}

The MyNumbers enum contains four values each one representing a number. Since enums are sort of like arrays, the First item is at the 0 index, the second item will have the value of 1, and so on. We can use MyNumbers like it was it's own variable type:


let myNum :MyNumbers = MyNumbers.First;

Output this value.


console.log(myNum);

What do you think you'll get? Since it is the value of MyNumbers.First and that is the first element of the zero-based index, you'll get number 0:

A benefit of enums is that we can control the number scheme:


enum MyNumbers {
First = 1,
Second,
Third
}

We specified the First item is equal to the number 1. Turn each element into a variable:


let myNum1 :MyNumbers = MyNumbers.First;
let myNum2 :MyNumbers = MyNumbers.Second;
let myNum3 :MyNumbers = MyNumbers.Third;

Then output them:


console.log(myNum1);
console.log(myNum2);
console.log(myNum3);

You'll see:

Now our text numbers match up with the actual text index. We can control the numbers even if they aren't in sequential order. Add a new entry to the MyNumbers enum:


enum MyNumbers {
First = 1,
Second,
Third,
Ten = 10,
}

This is number 10, skipping four through 9. Grab it as a variable and output it:


let myNum10 :MyNumbers = MyNumbers.Ten;
console.log(myNum10);

Combined with our other outputs, you'll see something like this:

Enums provide a lot of flexibility.

Enums can also be created to use a string based index, and we'll discuss that in the next article.

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Creating Variables in TypeScript - Part 2

I'm working on a longer series of articles about Typescript. This will be extra material to support my upcoming Angular 4 book. This is the second part of that series. Check out part 1.

An important part of programming is the ability to create variables. Let's turn our "Hello World" Variable into a variable. First, we'll do it the Javascript way and then we'll modify it with some TypeScript.

Create a Variable

This is the original demo from the previous section:


function echo(message) {
return message;
}
document.body.innerHTML = echo("Hello World");

We can easily add a variable in here. First, create the variable:


var message ="Hello World"

The var keyword is the normal way to create a variable in JavaScript. Now, modify the echo() call to pass the variable instead of the string:


document.body.innerHTML = echo(message);

Recompile the code and it will work just as the previous sample:

Define the Variable with Let instead of Var

TypeScript includes a secondary way to create a variable, using the let keyword instead of the var keyword:


let message ="Hello World";

This simple replaces the var command with the let command. Recompile and you'll see the same results. Let is part of the ECMAScript 6 standard, but for maximum compatibility, you probably want to stick to ECMAScript 5 or lower compatibility.

The reason to use the let command instead of var is because the two scope differently. When you use var the new variable is created as part of the function block. If the variable is not part of the function block it is added to the global scope of the page. This is why it is considered a best practice for a lot of code to be included in an IIFE. This can cause unexpected results in some cases, such as when you access loop counters inside nested for loops, or have one function embedded in another. The let keyword always scopes at the block level, so each for loop would be considered its' own block.

Consider this code:


function scopedemovar(){
for(var i = 0; i <10; i++){
console.log('outer i: ' + i);
for(var i = 0; i <10; i++){
console.log('inner i: ' + i);
}
}
}
scopedemovar();

Although this might be considered nonstandard code because the counter variable for both loops is defined twice and identical it is still perfectly valid. What would you expect the output to be? Try it and run it:

You may expect the outer loop to loop 10 times, but it only loops once. That is because the inner loop changes the value of the counter variable and the stop condition triggers the second time the outer loop tries to run.

Change the var to let:


function scopedemolet(){
for(let i = 0; i <10; i++){
console.log('outer i: ' + i);
for(let i = 0; i <10; i++){
console.log('inner i: ' + i);
}
}
}

Now, each i variable is scoped to its own for block and they do not interfere with each other:

I cut off the full output since it was very long, but you can run the code to see that both the inner and outer loop both execute a full 10 times using the let value.

Add Typing to Variables

An important decision for using TypeScript is to make variables statically typed instead of dynamically typed. We can add a type to the variable definition, like this:


let message : string ="Hello World";

We can change the function definition too:


function echo(message:string ):string {
return message;
}

The function now includes a typed argument and an explicit return type. When we send in the message variable the compiler checks to make sure that we are sending in a string. Recompile the code and run it. You'll see no differences in the output:

Open up the JavaScript file created by the TypeScript compiler for a bit of a surprise:


function echo(message) {
return message;
}
var message = "Hello World";
document.body.innerHTML = echo(message);

All your type statements were stripped out. That is because these types are part of TypeScript, but not JavaScript. They allow for compile time checking of values, but do not change the underlying language or how things run in the browser. This is an important distinction to remember as you learn TypeScript. The language is all about improved tooling and compile time validation; not about a new language in the browser.

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Introduction to TypeScript - Part 1

I'm working on a longer series of articles about Typescript. This will be extra material to support my upcoming Angular 2 book. This is the first part of that series.

What is TypeScript?

TypeScript is a strongly typed language which can be used to build web applications. It come with a special compiler that converts the strongly typed language into JavaScript so it can run in a web browser. Since TypeScript is strongly typed, it can offer better tooling than can be used with simple JavaScript. Angular 2 was built using TypeScript and I use it heavily in my Angular 4 book. This article is intended to give you an introduction to TypeScript.

Setup the Project

The first step to creating a TypeScript application is to install the compiler. I'm going to use the Node compiler. First, you'll need to set up the node project. Run this command:


npm init

And follow the instructions. You'll see something like this:

This will create a package.json file that will look something like this:


{
"name": "01helloworld",
"version": "1.0.0",
"description": "Sample TypeScript Project",
"main": "index.js",
"scripts": {
"test": "echo \"Error: no test specified\" && exit 1"
},
"author": "Jeffry Houser",
"license": "ISC"
}

Now install the TypeScript compiler:


npm install -g typescript

You'll see this:

With TypeScript installed, you're ready to write your first Typescript application.

Create Your First TypeScript File

Create a file named hello.ts:


function echo(message) {
return message;
}
document.body.innerHTML = echo("Hello World");

The purpose of this code is to echo a Hello Word message. The function accepts a string, and returns it. The innerHTML of the document's body tag is set to the results of the function call.

This file would be valid JavaScript, but it works fine since TypeScript is a Superset of JavaScript. We can use JavaScript inside of TypeScript easily. We'll improve on this throughout the article as we introduce more TypeScript specific concepts.

You can compile this file by running this command line:


tsc hello

You'll see something like this:

You aren't given a lot of feedback, but if you check the directory, you'll see a hello.js file:


function echo(message) {
return message;
}
document.body.innerHTML = echo("Hello World");

There isn't a lot of difference between the original file and the compiled file, that is because our main TypeScript file is primarily JavaScript. This is a place to start and over the series we'll expand our TypeScript knowledge.

Test the Application in a Browser

Now it is time to test the application in a browser. Create a page named Index.html:


<html>
<head><title>TypeScript Greeter</title></head>
<body>
<script src="hello.js"></script>
</body>
</html>

This index file loads the hello.js file, which will cause the document.body.innerHTML assignment to be called, which will run function and return the results:

Congratulations! You've created your first TypeScript application.

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