Using Mocha with Puppeteer

To add more structure to your browser check, you can use the Mocha testing framework in combination with Puppeteer. Mocha is a widely used Javascript test runner. Mocha allows you to order and execute tests and collect the results of those tests.

Run an example script on your local machine

We've created a GitHub repo you can clone run from you local machine to get a feel for writing browser checks with Mocha. Clone the checkly/browser-checks-starter-mocha repo and run the provided example.

git clone
cd browser-checks-starter-mocha
npm install
mocha check_duckduckgo.js

The result should look something like this, if you are working on a Mac or on Linux.

Breaking down the Mocha browser check step-by-step

The piece of code you just ran is a fully featured browser check. You can copy and paste it into Checkly and it would check the search results of Duck Duck Go search every x minutes. Let's step through the code step by step

1. Initial declarations: We first import Assert and Puppeteer libraries. We also declare a “browser” and “page” variable using “let”.

const assert = require('assert')
const puppeteer = require('puppeteer')

let browser
let page

2. Before hook: Inside the Mocha before hook, we launch a browser and a new browser page, which is like a normal browser tab. We do this inside the before hook because we always want this initialization to happen before any checks are done. Also notice the use of the async/await syntax.

Almost everything in the Puppeteer framework is asynchronous: using this syntax saves us from using callbacks or promise chains.

before(async () => {
  browser = await puppeteer.launch()
  page = await browser.newPage()

3. Create a test suite and test case: We create a Mocha test suite and test case using the describe and it syntax. The describe is strictly speaking not necessary, but it helps us organise test cases if multiple tests belong together.

describe('Duck Duck Go Search', () => {
  it('returns Chrome Puppeteer GitHub repo as first search result', async () => {

4. Search for a term: There's a lot going on here:

  • We request the Duck Duck Go site and wait till the page is loaded and most network activity has died down. This is the extra waitUntil option.
  • We type the sentence “chrome puppeteer” into the input box that has the HTML id attribute search_form_input_homepage
  • Then we click on the button with the id search_button_homepage. This kicks of the search!
  • Lastly, we wait until the results page has loaded and the element with id r1-0 has loaded.
    await page.goto('', { waitUntil: 'networkidle2' })
    await page.type('input#search_form_input_homepage', 'chrome puppeteer', { delay: 50 })
    await page.waitForSelector('.results--main #r1-0')

5. Validate the results: We now have a page with search results. To check if the results are as expected, we do two things:

  1. We parse the link from the first result using the page.evaluate function. This executes a piece of code in the context of the results page. In this case we grab the first anchor tag a with CSS class result__a. We're interest in the contents of that element. For good measure we trim off any leading or trailing spaces.
  2. Using the assert function, we check if the link is what we expect it to be. If this fails, our check fails.

For good measure, we also take a screen shot of the current page. This always helps when debugging.

    const githubLink = await page.evaluate(() => document.querySelector('a.result__a').textContent.trim())
    assert(githubLink, '')
    await page.screenshot({ path: 'duckduckgo.png' })

6. After hook: We clean up the browser in the Mocha after hook. This makes sure our checks ends the instant all test cases are done.

after(async () => {
  await browser.close()