There are two powerful tools for Angular developers in TypeScript and SystemJS. They are both in the Angular 2 quick start and they work just as well with AngularJS 1.x for building maintainable Angular applications.
However, getting started using these tools can be tricky. You need to align both tools' configurations so they work together and having a basic understanding of what happens under the hood can save you hours of confusion and frustration. Once you understand the core concepts and setup the configuration, this workflow helps to make your code more decoupled and maintainable – and maybe even more fun.
This post walks through the basics of using SystemJS and TypeScript in Visual Studio for development. You learn the minimum configuration that matters, how it interacts with the rest of the system, and what to expect when compiling and running your application.
Before You Begin
The example project is a component called ListMaker which allows the user to make a list by typing in a textbox and pressing ENTER. This component is made up of two child components AddItemTextbox and List, and includes basic CSS styles. The main script bootstraps the ListMaker component and everything is brought together in index.html.
SystemJS is the only library used in the example. Adding dependencies with TypeScript and SystemJS, including Angular, is another level of complexity that is omitted here intentionally. There are enough pieces to understand using SystemJS and TypeScript together so watch for more posts on this topic.
Required Tools and Frameworks
This walkthrough uses the following tools and frameworks:
The demo is built using Visual Studio 2015 Community. Visual Studio provides a seamless experience to compile and run your application on a development server. It also has first class support for TypeScript.
TypeScript Add-In for Visual Studio
This add-in enables TypeScript support in Visual Studio. The demo uses TypeScript 1.8.36. Install the add-in through Visual Studio's Extensions and Updates manager or from the web.
If you don't use Visual Studio, don't fret. You can port the code to another web project as there are no Visual Studio-specific dependencies. Be sure you know how to setup a development server and TypeScript support in your environment of choice.
When using these tools together, your workflow requires a compilation step. The Visual Studio TypeScript add-in automatically handles the compilation when you build or run your project. If you aren't in the habit of using front-end compilation tools such as TypeScript, this may be new to you. Here is an overview:
While the default TypeScript configuration in Visual Studio works, there are a couple options to discuss. First, you should consider adding a tsconfig.json file to your project. By default, you find the TypeScript properties in the Visual Studio project properties, however using a tsconfig.json file has advantages.
Most importantly, the project properties do not show all of the options that are available in tsconfig.json so if at some point you want to set other options, you will be limited by the tooling in the project properties. Also, the tsconfig.json file is a standard configuration for every editor and compiler that supports TypeScript. Therfore if someone joins your team that knows TypeScript, they will know where to look for the configuration no matter which tools they use.
Note: you should install the latest update of Visual Studio 2015. Earlier versions did not properly recognize the tsconfig.json file.
The first related configuration option is
"system" as it is the native format of the loader but you may find that
"amd" provides greater compatibility with other compilation tools.
The other option is
"es3" which gives you the broadest range of browser support. It's worth noting that should you eventually want to emit native ECMAScript module syntax, this option will need to be set to
Your tsconfig.json file should at least have:
Implement ECMAScript 2015 Module Syntax
Take a look at the ListMaker component without the ECMAScript 2015 module syntax:
The first thing to note is that
ListMaker is in the global scope as it is meant to be re-used within a given application. The
ListMaker implementation, however, is not in global scope and is instead wrapped in an immediately invoked function expression (IIFE). If you look closely, you also see that
ListMaker depends on two other constructor functions not defined in this code:
List. These constructors are also defined in global scope and furthermore you have to ensure that
List are loaded in the browser before trying to create a
The fundamental mechanism for modules are
export statements. The
import statements define the module's dependencies and the
export statements define the objects that are available to other modules. Take a look at how the ListMaker code changes when using ECMAScript modules:
The module's dependencies are clearly defined in the
import statment and you can easily determine where to obtain the source code for these objects looking at its URL. Also, because module scope is separated from global scope by default, the use of an IIFE is unnecessary leading to less code and more readable code.
Finally notice the
export statement and the use of the
default keyword. While ECMAScript modules can define multiple exports, only one object is defined as the
default export. Similarly, the
import statements for this module are explicitly importing the
default export from the target modules.
This post doesn't go into all the ways you can write your
export statements. TypeScript supports various options for writing these definitions so read more about them here.
SystemJS Configuration and Reconciling Import URLs
This is likely the most important part configuring SystemJS and TypeScript to work together. Both SystemJS and TypeScript evaluate the
import statements. When TypeScript looks at the
import statement, it is looking for a TypeScript file so that the compiler can use the types defined in that file. Using
import './Foo', TypeScript looks in the current directory for Foo.ts and then uses the information contained in that file for compilation and type checking.
SystemJS on the other hand looks at
import './Foo' and then attempts to make a request to the server for <current-directory>/Foo and without any special configuration, this request will produce a 404 resource not found error. What SystemJS needs to request instead is <current-directory>/Foo.js.
Luckily, SystemJS has a configuration option to append a file extension to requests. You configure this by setting up a specific path in your application using the
packages option. In the demo, all of the TypeScript files are in the components folder and SystemJS needs to add a .js extension to these requests.
Create a systemjs.config.js file in your application and add the following configuration:
You can optionally configure this behavior for the whole application by using an empty string
'' to set the root of your application as the path. Also remember this configuration only applies to requests made by SystemJS and other requests still require explicit file extensions. There are many useful options in the SystemJS configuration so be sure to read up on its capabilities.
Bootstrap the Application
There is bootstrapping logic for the application in main.ts. The browser requests this file first and the script's purpose is to access the Document Object Model (DOM) and render the root component which in this case is
ListMaker. It's important to understand using module loaders that the first module retrieved is the last one to execute its contents. For example, even though main.ts is the entry point of the application, all of the other dependent scripts execute prior to main.ts executing.
Once you determine the main script, you bring the configuration and bootstrapping together on the root HTML page. Reference SystemJS, followed by the SystemJS configuration, and then use the
System.import syntax from SystemJS to load the post-compiled version of main,
'scripts/main'. Remember that even though the extension is not present, you have configured SystemJS to add the extension and request scripts/main.js.
main.js is loaded, all of the other
import statements are evaluated and the target modules are loaded as well to compose the application. This diagram illustrates the dependencies and the order of execution when loading.
At this point, you have configured enough for your application to run. Remember to compile the TypeScript first and all of the modules will run and execute in the correct order. Be sure to check out the complete project for all of the details.
Only the Beginning
This is the first step using ECMAScript 2015 modules with SystemJS and TypeScript and this workflow works quite well as a development workflow. Once you understand the configuration and basics of using the tools, you can get up and running rather quickly.
But you need more. As an Angular developer, you need to bring in third-party libraries and seamlessly integrate with your application code. This is the topic of the next post in this series.
At some point, you will want to bundle these scripts together into one request. You may even want to create several bundles that are loaded dynamically via a route change or a certain screen size. The good news is that by separating your code into modules that have explicit dependency references, you have made this configuration much easier to attain.
Stay tuned for more on this topic. If there is anything else you want to know about these tools or if you also have experience with them, please share in the comments below.