Simplify sample, fix unit test (#746)

Author: steren

appengine/headless-chrome/app.js

@@ -22,39 +22,36 @@ const app = express();
 
 let browser;
 
-async function init () {
- // [START browser]
- browser = await puppeteer.launch({
- args: ['--no-sandbox']
- });
- // [END browser]
-
- const server = app.listen(process.env.PORT || 8080, async err => {
- if (err) {
- await browser.close();
- return console.error(err);
- }
- const port = server.address().port;
- console.info(`App listening on port ${port}`);
- });
-}
-
-init();
-
 app.use(async (req, res) => {
  const url = req.query.url;
 
  if (!url) {
  return res.send('Please provide URL as GET parameter, for example: <a href="/?url=https://example.com">?url=https://example.com</a>');
  }
 
+ if (!browser) {
+ // [START browser]
+ browser = await puppeteer.launch({
+ args: ['--no-sandbox']
+ });
+ // [END browser]
+ }
+
  let page = await browser.newPage();
  await page.goto(url);
  const imageBuffer = await page.screenshot();
 
  res.set('Content-Type', 'image/png');
  res.send(imageBuffer);
 });
+
+const server = app.listen(process.env.PORT || 8080, err => {
+ if (err) {
+ return console.error(err);
+ }
+ const port = server.address().port;
+ console.info(`App listening on port ${port}`);
+});
 // [END full_sample]
 
 module.exports = app;