ES6 Modules
Introduction
Separate from the module pattern that we discussed in an earlier lesson, "modules" is a feature that arrived with ES6. Browser support for this feature is quite slim at this point, but is slowly improving and until all modern browsers support it, we can make it work using an external module bundler. ES6 modules are starting to appear in many code bases around the net and getting them up and running will give us a chance to explore some new parts of the JavaScript ecosystem, so it's going to be a worthy excursion!
Don't be fooled! We're going to cover much more than just the new module syntax in this lesson! Before we can really use these modules, we're going to have to learn about npm and webpack which are both topics that will be very useful to you even beyond this lesson. In the end, the modules themselves are simple to implement, so we're going to take this chance to learn about a few other things.
Learning outcomes
After completing this lesson, you will be able to:
Explain what npm is and where it was commonly used before being adopted on the frontend.
Describe what
npm init
does and whatpackage.json
is.Know how to install packages using npm.
Describe what a JavaScript module bundler like webpack is.
Explain what the concepts "entry" and "output" mean as relates to webpack.
Briefly explain what a development dependency is.
Explain what "transpiling code" means and how it relates to frontend development.
Briefly describe what a task runner is and how it's used in frontend development.
Describe how to write an npm automation script.
Explain one of the main benefits of writing code in modules.
Explain "named exports" and "default exports".
The History of JavaScript
Why do we even need or want this stuff? What do you gain from all of this added complexity? These are good questions.. with good answers.
npm
The node package manager is a command line tool that gives you access to a gigantic repository of plugins, libraries and tools. If you have done our Fundamentals course, you will probably have encountered it when you installed the Jasmine testing framework to do our exercises.
Yarn
Webpack
To get us started we are going to refer to the official documentation.
There are a couple of key concepts to understanding how webpack works - entry and output. In this example, we rearranged the files into a src
and dist
folder. Technically we could have called those folders anything, but those names are typical. src
is our source directory. In other words, src
is where we write all of the code that webpack is going to bundle up for us. When webpack runs, it goes through all of our files looking for any import
statements and then compiles all of the code we need to run our site into a single file inside of the dist
folder (short for distribution). Our entry file, then is the main application file that links (either directly or indirectly) to all of the other modules in our project. In this example, it is /src/index.js
. The output file is the compiled version - dist/main.js
.
ES6 Modules
Now that we (sorta) understand what webpack is doing it's time to discuss the module syntax. There are only 2 components to it - import and export.
Of course, the import statement is the same thing that you used during the webpack tutorial! These things are simple to use.
There are many benefits to writing your code in modules. One of the most compelling is code reuse. If, for instance, you have written some functions that manipulate the DOM in a specific way, putting all of those into their own file as a 'module' means that you can copy that file and re-use it very easily!
Other benefits include all of the benefits to wrapping your code in factory functions or using the module pattern (the module pattern and ES6 modules are not the same things.. this naming convention is frustrating). By using ES6 modules you can keep different parts of your code cleanly separated, which makes writing and maintaining your code much easier and less error-prone. Keep in mind that you can definitely export constructors, classes and factory functions from your modules.
To pull it all together, let's write a simple module and then include it in our code. We are going to continue from where the webpack tutorial left off. Before beginning your file directory should look something like this:
and you should be able to bundle and run webpack by simply typing npx webpack
in the terminal.
Add a new file to the src
directory called myName.js
with the following contents:
and then in src/index.js
import and use your new function.
Easy! Now, if you run npx webpack
in your project directory your page should show our new function being used.
There are 2 different ways to use exports in your code: named exports and default exports. Which option you use depends on what you're exporting. As a general rule if you want to export multiple functions use named exports with this pattern:
and to import them:
Using this pattern gives you the freedom to only import the functions you need in the various files of your program. So it's perfectly fine to only import functionOne
if that's the only one you need.
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