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This codelab is part of the Developing Progressive Web Apps training course, developed by the Google Developers Training team. You will get the most value out of this course if you work through the codelabs in sequence.

For complete details about the course, see the Developing Progressive Web Apps overview.

Introduction

This lab shows you how you can automate tasks with gulp, a build tool and task runner.

What you'll learn

  • How to set up gulp
  • How to create tasks using gulp plugins
  • Ways to automate your development

What you should know

  • Basic JavaScript, HTML, and CSS
  • Some experience using a command line interface
  • Some experience with Node.js and npm is recommended

What you will need

  • Computer with terminal/shell access
  • Connection to the internet
  • A text editor
  • Node.js and npm

Download or clone the pwa-training-labs repository from github and install the LTS version of Node.js, if needed.

Open the gulp-lab/project/ folder in your preferred text editor. The project/ folder is where you will be building the lab.

This folder contains:

  • app/images/ contains sample images
  • app/scripts/main.js is the app's main JavaScript
  • app/styles/main.css is the app's main stylesheet
  • index.html is the home (and only) page for the app
  • gulpfile.js is an empty gulp configuration file, and where you will write most of your code

Gulp and its plugins are available as Node.js packages.

In order to use gulp directly from the command line, the gulp command line interface needs to be installed globally. Run the following command to globally install the gulp command line interface:

npm install gulp-cli --global

Next run the following command from the project/ directory to create a new package.json file, which Node.js uses to track local project dependencies (the -y flag accepts the default configuration):

npm init -y

Note that a package.json file was created. Open the file and inspect it.

From the same directory, run the following command to install the first local package, gulp:

npm install --save-dev gulp@4.0.0

Observe gulp and its dependencies have been installed in a node_modules/ directory, and that the package.json now lists "gulp" as a "devDependency". You should also see that a package-lock.json file was created.

Note: Some text editors hide files and directories that are listed in the .gitignore file. Both node_modules/ and build/ are in the PWA ILT repository .gitignore. If you have trouble viewing these during the lab, just delete the .gitignore file.

Explanation

Both package.json and package-lock.json allow Node.js to track the dependencies that a project uses, so that we (or other developers) can easily reinstall or update project dependencies.

With npm, packages like gulp can be installed locally for our project, in a node_modules/ folder. The --save-dev flag adds the corresponding package (in this case gulp) to package.json, and the @ specifies which version of the package we want to install.

For more information

Package management with npm

Gulp is configured using a gulpfile.js/ file. In this file, gulp "tasks" are defined using regular JavaScript.

In the empty project/gulpfile.js file, add the following code to import the gulp package from the node_modules/ directory.

const gulp = require('gulp');

Next, add the following function to the same file in order to define a copy function:

function copy() {
  return gulp.src([
    'app/*.html',
    'app/**/*.jpg',
    'app/**/*.css',
    'app/**/*.js'
  ])
  .pipe(gulp.dest('build'));
}

Then add the following line to define a copy task:

gulp.task('copy', copy);

From the command line (at the project/ directory), use the following command to run the copy task:

gulp copy

Note: If you get a TypeError: Cannot read property 'apply' of undefined in the command line, run the following command to uninstall the previous version of gulp on your machine: npm uninstall -g gulp. Then reinstall the latest version of the gulp-cli with the following command: npm install gulp-cli --global. You may be prompted to remove a file and retry the command. You should be able to run gulp copy after the gulp-cli has finished reinstalling.

Observe that the files in app/ have been copied to a build/ directory.

Explanation

Gulp tasks can be defined by functions, and are exposed to gulp with the gulp.task method. Once configured, these tasks can be run from the command line using the corresponding task name.

In this example, the src/ files of our app are being "piped" to a dest/ folder (build/). This example is contrived, but is typical of the general flow of gulp tasks. In general gulp is used to read some source files, process them, and output the processed files to a new destination. Examples of actual processing methods are demonstrated in later steps.

Let's use gulp to serve our app during development, so that we can more easily test our changes.

Run the following to install the browser-sync package, which provides a local server with live reloading:

npm install browser-sync --save-dev

Then require the new package in gulpfile.js with the following code:

const browserSync = require('browser-sync');

Add the following serve function below the existing code:

function serve() {
  return browserSync.init({
    server: 'build',
    open: false,
    port: 3000
  });
}

Next, define the buildAndServe task, with slightly different syntax than we saw before:

gulp.task('buildAndServe', gulp.series(copy, serve));

Finally, run gulp buildAndServe from the command line. Observe the command line logs and open localhost:3000/ in your browser to see the app.

Note: Unregister any service workers and clear all service worker caches for localhost so that they do not interfere with the lab. In Chrome DevTools, you can achieve this by clicking Clear site data from the Clear storage section of the Application tab.

Explanation

Similarly to the previous copy task, configuring the buildAndServe task consisted of installing a package, requiring that package in gulpfile.js, and then writing a function to use the package. However, unlike with the copy task, we specified a series of functions to run for the buildAndServe task. Using gulp.series() allows us to run tasks sequentially. In this case, the buildAndServe task actually runs the copy function, followed by serve function, effectively "building" and then serving our app.

Let's take a closer look at processing files with plugins

Compile with babel

Run the following to install the gulp-babel package, which uses babel to compile browser-compatible JavaScript:

npm install --save-dev gulp-babel babel-core babel-preset-env

Note: If the browserSync server is blocking your command line, terminate the process with Ctrl+c. Going forward, do the same for any gulp process that blocks the command line when you need it.

Then require the new package in gulpfile.js with the following code:

const babel = require('gulp-babel');

Add the following processJs function and expose it to gulp with the gulp.task method:

function processJs() {
  return gulp.src('app/scripts/*.js')
  .pipe(babel({
      presets: ['env']
  }))
  .pipe(gulp.dest('build/scripts'));
}

gulp.task('processJs', processJs);

Next, remove the JavaScript files from the copy function, since we are processing them separately in processJs. The updated copy function should look like:

function copy() {
  return gulp.src([
    'app/*.html',
    'app/**/*.jpg',
    'app/**/*.css',
    // 'app/**/*.js' // processed by processJs
  ])
  .pipe(gulp.dest('build'));
}

Finally, run gulp processJs from the command line. Compare app/scripts/main.js and build/scripts/main.js and observe that the arrow function has been compiled to the more broadly supported function syntax.

Explanation

Similarly to the copy task, the processJs task reads some src files (only app/scripts/main.js in our simple app). In this case however, the files are piped through the babel function (defined in the required babel package). This babel function compiles the source JavaScript in our app, and then pipes the output to a dest (build/scripts/main.js).

The babel function also accepts some configuration option (presets), which is common for gulp modules.

Uglify and rename

A single gulp task is not limited to using one package. Let's extend the processJs task to uglify (minify) and rename the apps JavaScript files.

Run the following to install the gulp-uglify and gulp-rename packages:

npm install --save-dev gulp-uglify gulp-rename

Then require the new packages in gulpfile.js with the following code:

const uglify = require('gulp-uglify');
const rename = require('gulp-rename');

Update the processJs function to the following:

function processJs() {
  return gulp.src('app/scripts/*.js')
  .pipe(babel({
      presets: ['env']
  }))
  .pipe(uglify())
  .pipe(rename({
    suffix: '.min'
  }))
  .pipe(gulp.dest('build/scripts'));
}

Now run gulp processJs from the command line. Observe that build/scripts/main.js is now being uglified (minified) and renamed to build/scripts/main.min.js (you can delete the old build/scripts/main.js file).

Explanation

Rather than piping the output of the babel function directly to dest, processJs now pipes the output to another function, uglify. That function pipes its own output to yet another function, rename. The output of rename is what is finally written to dest.

Watch files

Gulp supports a powerful "watch" functionality, which configures tasks to be automatically run when specified files change.

Add the following watch function and corresponding task:

function watch() {
  gulp.watch('app/scripts/*.js', processJs);
}

gulp.task('watch', watch);

Now run gulp watch from the command line. Comment out all the code in app/scripts/main.js and save the file. Observe that build/scripts/main.min.js is automatically updated by the processJs task.

Explanation

Here the watch method is used to "watch" all the .js files in the app/scripts/ directory (in this case, main.js) and run the processJs task anytime there are changes. This automation enables us to see changes during development without the need to re-run tasks in the command line.

Similarly to gulp.src, gulp.dest, and gulp.task, the gulp.watch method is defined in the gulp package itself, so it doesn't require importing a new package.

The remaining exercises are unguided challenges. Try to complete them on your own. You can view the solution/ directory if you get stuck.

Define and expose a processCss task, similar to the processJs task. This task should:

  • read the source CSS files from app/styles/
  • process the files with the gulp-clean-css module
  • rename the files with a ".min" extension
  • write the processed and renamed files to build/styles/

Tips

  • remember to require the gulp-clean-css at the top of gulpfile.js
  • remember to comment out the CSS pattern in the copy function
  • you can delete the old build/styles/main.css to avoid confusion with the new build/styles/main.min.css

Complete the following challenges to combine everything learned so far:

  1. Update the copy task, so that the JavaScript and CSS files are not copied. (These files are processed by processJs and processCss, respectively.)
  2. Update the watch task to run processCss whenever the CSS files in app/styles/ change.
  3. Update the buildAndServe task to run copy, processJs, and processCss before it serves.
  4. In addition, update buildAndServe to call both the serve and watch functions in parallel, rather than only calling the serve function. If configured correctly, the gulp buildAndServe command should run copy, processJs, and processCss, and then call both the serve and watch functions together.
  5. Update app/index.html to reference scripts/main.min.js and styles/main.min.css instead of scripts/main.js and styles/main.css respectively.

Then run gulp buildAndServe. If all the challenges are completed successfully, all files in app/ should be processed into build/, the built app should be served at localhost:3000/, and appropriate files should be re-processed automatically whenever changes are made.

You have learned how to set up gulp, create tasks using plugins, and automate your development!

Resources

To see all the codelabs in the PWA training course, see the Welcome codelab for the course/