Script to automatically generate service worker after a GWT build and before packaging the app.

package.json

{
  "name": "gwt-pwa",
  "version": "0.0.1",
  "description": "generates service worker for GWT app",
  "main": "serviceWorkerBuilder.js",
  "dependencies": {
    "handlebars": "^4.0.11"
  },
  "devDependencies": {},
  "scripts": {
    "generate": "node serviceWorkerBuilder.js"
  },
  "repository": {
    "type": "git",
    "url": "git+https://github.com/zak905/gwt-pwa-demo.git"
  },
  "author": "zakaria amine",
  "license": "ISC",
  "bugs": {
    "url": "https://github.com/zak905/gwt-pwa-demo/issues"
  },
  "homepage": "https://github.com/zak905/gwt-pwa-demo#readme"
}

pom.xml

<plugin>
	 <groupId>com.github.eirslett</groupId>
        <artifactId>frontend-maven-plugin</artifactId>
        <version>1.6</version>
		<executions>
			<execution>
				<id>npm install</id>
				<goals>
					<goal>npm</goal>
				</goals>
				<phase>generate-resources</phase>
				<configuration>
					<arguments>install</arguments>
				</configuration>
			</execution>
			<execution>
				<id>npm generate</id>
				<goals>
					<goal>npm</goal>
				</goals>
				<phase>prepare-package</phase>
				<configuration>
					<arguments>run-script generate ${project.build.finalName}</arguments>
				</configuration>
			</execution>
		</executions>
	</plugin>

serviceWorkerBuilder.js

const args = process.argv;
const arguments = args.slice(2);

const buildFolderName = arguments[0];
const gwtModuleName = arguments[1];

if (typeof buildFolderName === 'undefined' || !buildFolderName){
    console.log("please provide the name of the build folder");
    return;
}

const rootFolder = 'target/' + buildFolderName;
const fs = require('fs');
var Handlebars = require('handlebars');

var filesToCache = [];


browseAllFilesInDirectory(rootFolder);

function browseAllFilesInDirectory(folder) {
    const filesRegExp = /\.(html|css|js|gif|png|jpeg)$/i;
    const exceptions = ["WEB-INF", "META-INF", "bower_components", "clear.cache.gif"];

    fs.readdirSync(folder).forEach(fileName => {
        const resource = folder + "/" + fileName;
          if (exceptions.indexOf(fileName) < 0) {        
            if (fileName.match(filesRegExp)) {
                    filesToCache.push(resource.replace(rootFolder+"/", ""));
            } else if (fs.lstatSync(resource).isDirectory()) {
                    browseAllFilesInDirectory(resource);
            }
          } 
    });
}

var swData = {
    cacheName: gwtModuleName || "cache_"+new Date().getTime().toString(),
    filesToCache: filesToCache,
};

fs.readFile("sw_template.js", "utf8", (error, data) => {
    if (error) {
       console.log("Unable to read template file");
    }
    var template = Handlebars.compile(data);
   var serviceWorkerJs = template(swData);

   fs.writeFile(rootFolder+"/sw.js", serviceWorkerJs, (error) => {
     console.log("successfully generated service worker sw.js in " + rootFolder);
   });
});

sw_template.js

var cacheName = '{{cacheName}}';  
var filesToCache = [  {{#each filesToCache}}
                     '{{.}}'{{#unless @last}},{{/unless}}
                      {{/each}}
                     ];


self.addEventListener('install', function(e) {  
  console.log('[ServiceWorker] Install');  
  e.waitUntil(  
    caches.open(cacheName).then(function(cache) {  
      console.log('[ServiceWorker] Caching app shell');  
      return cache.addAll(filesToCache);  
    })  
  );  
});


self.addEventListener('activate', function(e) {  
    console.log('[ServiceWorker] Activate');  
    e.waitUntil(  
      caches.keys().then(function(keyList) {  
        return Promise.all(keyList.map(function(key) {  
          console.log('[ServiceWorker] Removing old cache', key);  
          if (key !== cacheName) {  
            return caches.delete(key);  
          }  
        }));  
      })  
    );  
  });

self.addEventListener('fetch', function(e) {  
    console.log('[ServiceWorker] Fetch', e.request.url);  
    e.respondWith(  
      caches.match(e.request).then(function(response) {  
        return response || fetch(e.request);  
      })  
    );  
  });

Hey kevzlou7979,

sorry for the late reply. The error seems familiar. What maven goal are you running? It works for me using mvn package. Also, the frontend-maven-plugin does not look for node path in your system, but inside /nodes_modules, so in case you tried to avoid the install-node-and-npm , this may cause it. I did not find enough flexibility in the frontend-maven-plugin to allow using the already installed npm and node. If you find a better way, please let me know. As for the code, I think this might be some misinterpretation from IntelliJ, the code works well if you try node serviceWorkerBuilder.js. Here is the full demo project: https://github.com/gwidgets/gwt-pwa-demo