add comments to puppeteer script helper

subjects/ephemeris/README.md

@@ -40,31 +40,48 @@ const browser = await puppeteer.launch({
   devtools: true, // to be commented or removed when project is submitted
 })
 const [page] = await browser.pages()
-await page.goto('https://www.google.com/')
-// other actions...
+// actions...
 await browser.close() // you can comment this during you tests
+process.exit(0) // terminate the process as succeeded (you can comment this during you tests)
 ```
 
 **Puppeteer helper - script model**
 
 ```javascript
-if ((await page.$('button#myId')) !== null) {
-  await page.click('button#myId')
-}
+// navigate to a specific url
+await page.goto('https://www.google.com/')
+
+// wait an element in the page
 await page.waitForSelector('input#myId')
-await page.type(`input#myId`, 'myText')
-await page.waitForSelector('div#myId')
+
+// find an element in the page:
+const button = await page.$('button#myId')
+// check the element was found
+if (button) {
+  // use the element
+  await button.click()
+} else {
+  // do something else: for example treat the exception as an error
+  console.error('Button not found !')
+  // and terminate the process as failed (code 1)
+  process.exit(1)
+}
+
+// navigate in a website
 await Promise.all([
   page.waitForNavigation(),
   page.click('div#myId'),
 ])
-await page.waitForSelector('div#myId')
+
+// get the textContent of an element
 const myText = await page.$eval(
   'div#myId',
-  (s) => s.textContent,
+  (elem) => elem.textContent,
 )
-```
 
+// type something in an input
+await page.type(`input#myId`, 'some text')
+```
 
 ### Optional