When using npm, it is often the case that we aren't familiar with all of the code in the dependency tree. Building modern JavaScript applications relies on our ability to use the tools built for us by others. As it turns out, most of those tools are also built using other people's tools. One package may be used in another, which is used in another, and another, and so on...
Using npm, we download specific packages of code. If those packages have
dependencies, the dependencies are also downloaded in a recursive manner. For
the purposes of our own application, however, we only need to know about the
node packages we specifically need to get our app working. We don't need to
worry about what packages those packages need. Why? Because every node package
includes a package.json file that lists out all dependencies. This file
lets Node know what to download when we run npm install. Node will download
all the packages, check the package.json files present, then download any
additional packages, and repeat.
We will see in future labs that as the number of packages increases, more and
more happens when we run npm install. All we need to worry about, though, is
the top-level - what is listed in our application's package.json file.
In this code-along, we are going to practice the process of setting up a package.json
file. We will also install an npm package or two and use their functionality in
new code we write.
- Practice initializing a
package.jsonfile - Practice installing an npm package
- Introduce how to import a package into a different JavaScript file
Before we can create a package.json file, we'll need an project and a project
folder to contain all the files. For this code-along, we'll be building out a
clock application that changes color every second.
This lesson already has its own package.json file, so a sub-folder has been
created for us to use, color-clock, that contains some basic starter files for
the project. Change directory into this folder in your terminal by typing the command cd color-clock.
The package.json can be written quickly from scratch, but we actually have a
handy command for creating these files: npm init.
Run npm init and you will be prompted to confirm the information that will
be stored in package.json, starting with the name of the project.
Most prompts will provide a default value. Some are blank and can be left this
way for now. Follow the prompts until the end and a fully constructed
package.json file will appear in the color-clock directory.
In the process of creating the package.json file, you wre prompted to
write a test script. Let's add a working script in to see how this works.
Open the newly created package.json file and look for a section
titled "scripts". Let's replace the default "test" script with
an shell command:
"scripts": {
"test": "echo 'Hello World!'"
}We can now call this script and have it run by using the command npm test in the terminal (if that doesn't work, try npm run test). You
should see a print out of Hello World!.
In all the JavaScript-based labs you've encountered so far, this sort
of script is how we run tests. If you look at the "test" script on
previous labs, most will have something like this:
"test": "mocha -R mocha-multi --reporter-options spec=-,json=.results.json"This is actually a command that you can run in the terminal. This is a call to
the testing package, mocha, along with a second package, mocha-multi that
helps with reporting. When you run learn or learn test in a lab, npm test gets called.
Scripts are often useful for things like testing or to start a necessary process, like a local server.
With package.json set up, we can now add a package we want to include
in our project.
Now, we're building a colorful clock - the project is simple enough that we could build it entirely out of custom code. Here's the thing though — one of the reasons packages exist and are so useful is because programmers often run into the same problems over and over. Node packages are written so we don't have to re-find a solution other programmers have found.
In the case of a colorful clock, we have to deal with formatting time. This is such a common problem, that a package has been created to help us: Moment.js. Moment.js is a handy package that comes with a number of functions that make displaying dates and times simpler than trying to figure out JavaScripts built-in functions.
Let's install Moment.js and incorporate into our clock. To install a package and save it to your package.json file, run npm install
followed by the package name. In our case, that would be:
npm install momentThis command will add the package to the list of dependencies in package.json. When npm install is run, all dependencies are installed. If you were to publish this repository on GitHub, other users would now be able to clone down the repo and install whatever is listed in pakage.json to get the program working.
If package.json file has the correct package, and the node module has been
installed, open index.html and you should see a colorful clock appear!
We've departed from the shore and are now afloat on the sea of code. When building our own applications, we will often rely on existing packages to handle specific pieces of a project.
Although we only installed one package in this lab, there are many layers of dependencies for them, and many of additional dependencies were installed. It isn't necessary to understand how each of these works. The main thing to grasp is how to implement and use the specific dependencies you need.