What is a headless browser?
A headless browser is a web browser without a graphical user interface (GUI). Instead of a typical user interface with displays, buttons and links, a headless browser runs efficiently in the background, controlled programmatically through code. It operates much more quickly and uses fewer resources than a traditional browser.
The term headless comes from the server world, where servers run without a display. In almost any data center, a monitor for each server would be impractical. Instead, one display interfaces with all the servers controlled remotely or programmatically.
Why are headless browsers important?
Despite lacking a GUI, headless browsers load and navigate webpages, interact with elements on the page, execute JavaScript code and render content. Some of the more common uses for headless browsers include the following:
- Testing web applications to simulate user interaction to ensure functionality and compatibility across various web browsers and devices such as smartphones.
- Automating the process of extracting data from websites without GUI concerns to quickly gather large amounts of information.
- Automating repetitive tasks using scripts to save time compared with manual scraping.
- Server-side rendering to generate webpages on the server to improve search engine optimization and ensure the page is readable to search engines.
- Generating screenshots to monitor the network and obtain information for changes to the page or create reports.
- Producing high-quality PDFs by converting webpages or specific content for archiving or distribution.
Putting a face to headless browsers
There are many headless browsers used for various functions across systems. Among the most popular are the following:
- Headless Chrome, a built-in mode in the Google Chrome browser controlled through command-line flags.
- Puppeteer, a Node.js-based library specifically designed to control Headless Chrome.
- Playwright, another Node.js library that controls Firefox Headless, WebKit-based browsers and Chrome.
- Chromium, a Chrome rendering engine used in Brave and Microsoft Edge.
- Safari Headless for use on Macs.
- Splash, a Python-based headless browser built on WebKit, the same rendering engine used in Safari.
- Zombie.js, a JavaScript-based headless browser focused on simplicity and speed, primarily used for basic scraping tasks.
- TrifleJS, another lightweight JavaScript headless browser.
What is headless browser testing?
Headless browser testing is the process of running automated tests of web applications without the need for a GUI. It's no different from the tests run on a standard browser with a GUI.
However, headless testing means faster, more efficient tests because there is no rendering of graphics or Hypertext Markup Language. Also, less resources being consumed increases opportunities for scalability, meaning more tests can be run on the same machine with resources that otherwise would be devoted to the GUI.
Developers can easily incorporate testing into continuous integration/continuous delivery (CI/CD) pipelines used in a DevOps environment. These CI/CD pipelines quickly perform automated testing with every code change.
Multiple types of testing can be done this way, starting with regression testing to ensure an application's integrity after any code changes. It also allows for the following:
- Functional testing to analyze specific features and user flows for proper behavior.
- Performance testing.
- Visual regression testing to examine a website for unexpected layout changes.
When to use -- and avoid -- headless browsers
Headless browser testing is applicable only in certain scenarios, such as the following:
- Constrained resources. Without the heavy graphical overhead of a traditional browser, headless browsers are ideal for testing in resource-constrained environments and for testing performance in a constrained environment, such as a smartphone.
- Testing specific functions. Headless browsers are especially good at simulating user interactions such as form submissions, button clicks and JavaScript execution, which must be repeated many times to complete a thorough test. Headless browsers perform hundreds of tests in a very short period.
- Application programming interface (API) testing. Headless browsers test the interaction between a website's APIs directly, bypassing the unnecessary GUI.
- Website scraping. It's not testing per se, but headless browsers can be used for website scraping to extract specific data either from a personal website or other websites for analysis or comparison.
The benefits of headless browsers are obvious, but like everything, there are downsides as well. The lack of a GUI leads to multiple challenges, including the following:
- Minimal visual feedback. It's difficult to troubleshoot an error or failure when it can't be seen. Finding and reading error messages and debugging code is time-consuming.
- Constrained testing scope and coverage. Because headless browsers lack UI interactions, things such as user activity flows and other visual aspects are not properly tested. Instead, a comprehensive and complete evaluation requires manual testing with a traditional browser.
- Restricted debugging tools. Most browser debugging tools are specifically designed for testing a browser with a GUI. This can limit their efficacy. For example, diagnosing front-end test failures in headless browsers requires a manual review of the logs since the issue cannot be visually confirmed.
- Limited testing of visual and UI aspects. Because a headless browser lacks output, the ability to fully assess layout, interactions, responsiveness and the overall user experience is significantly curbed and usually requires manual testing with a full browser.
- Inadequate testing of complex interactions. Websites relying heavily on advanced interactions -- animations, intricate user interactions or drag-and-drop functionality -- typically need a traditional browser to test the application fully.