How Cypress Works

Most testing tools operate by running outside of the browser and executing remote commands across the network. Cypress is the exact opposite. Cypress is executed in the same run loop as your application. Behind Cypress is a Node.js server process. Cypress and the Node.js process constantly communicate, synchronize, and perform tasks on behalf of each other. Having access to both parts (front and back) gives us the ability to respond to your application’s events in real time, while at the same time work outside of the browser for tasks that require a higher privilege.

Cypress also operates at the network layer by reading and altering web traffic on the fly. This enables Cypress to not only modify everything coming in and out of the browser, but also to change code that may interfere with its ability to automate the browser.

Cypress ultimately controls the entire automation process from top to bottom, which puts it in the unique position of being able to understand everything happening in and outside of the browser. This means Cypress is capable of delivering more consistent results than any other testing tool.

Because Cypress is installed locally on your machine, it can additionally tap into the operating system itself for automation tasks. This makes performing tasks such as taking screenshots, recording videos, general file system operations and network operations possible.

Because Cypress operates within your application, that means it has native access to every single object. Whether it is the window, the document, a DOM element, your application instance, a function, a timer, a service worker, or anything else - you have access to it in Cypress. There is no object serialization, there is no over-the-wire protocol - you have access to everything at your fingertips.

Having ultimate control over your application, the network traffic, and native access to every host object unlocks a new way of testing that has never been possible before. Instead of being ‘locked out’ of your application and not being able to easily control it - Cypress instead lets you alter any aspect of how your application works. Instead of incredibly slow and expensive tests such as creating the state required for a given situation - you can simply create these states artificially like you do in an unit test. For instance you can:

• Stub the browser or your applications functions and force them to behave as needed in your test case.
• Expose data stores (like in Redux) so you can programmatically alter the state of your application directly from your test code.
• Test edge cases like ‘empty views’ by forcing your server to send empty responses.
• Test how your application responds to errors on your server by modifying response status codes to be 500.
• Modify DOM elements directly - like forcing hidden elements to be shown.
• Use 3rd party plugins programmatically. Instead of fussing with complex UI widgets like multi selects, autocompletes, drop downs, tree views or calendars, just call methods directly from your test code to control them.
• Prevent Google Analytics from loading before any of your application code executes when testing.
• Get synchronous notifications whenever your application transitions to a new page or when it begins to unload.
• Control time by moving forward or backward so that timers or polls automatically fire without having to wait for the required time in your tests.
• Add your own event listeners to respond to your application. You could update your application code to behave differently when under tests in Cypress. You can control websocket messages from within Cypress, conditionally load 3rd party scripts, or call functions directly on your application.

Trying to test hard to reach areas of your application? Don’t like the side effects an action creates? Tired of repeating the same repetitive and slow actions over and over again? You can simply skip them all. Cypress prevents you from being forced to always ‘act like a user’ to generate the state of a given situation. With Cypress you can programmatically interact and control your application. You no longer have to use your UI to build up state!

That means you no longer have to visit a login page, type in a username and password and wait for the page to load and/or redirect. Cypress gives you the ability to take shortcuts and programmatically log in. With commands like cy.request(), you can send HTTP requests directly, yet have those requests synchronized with the browser. Cookies are automatically sent and applied back. Worried about CORS? Don’t be, it’s completely bypassed. The power to choose when to test like a user and when to skip slow and repetitive parts is yours.

Cypress knows and understands everything that happens in your application synchronously. It’s notified the moment the page loads, the moment the page unloads. It’s impossible for Cypress to miss elements when it fires events. Cypress even knows how fast an element is animating and will wait for it to stop! Additionally, it automatically waits for elements to become visible, to become enabled, and to stop being covered. When pages begin to transition, Cypress will pause command execution until the following page is fully loaded. You can even tell Cypress to wait on specific network requests to finish.

Cypress executes the vast majority of its commands inside the browser, so there is no network lag. Commands execute and drive your application as fast as its capable of rendering. To deal with modern JavaScript frameworks with complex UI’s, you use assertions to tell Cypress what the desired state of your application is. Cypress will automatically wait for your application to reach this state before moving on. You are completely insulated from fussing with manual waits or retries. Cypress automatically waits for elements to exist and will never yield you stale elements that have been detached from the DOM.

Above all else Cypress has been built for usability. There are hundreds of custom error messages describing the exact reason Cypress failed your test. There is a rich UI which visually shows you the command execution, assertions, network requests, spies, stubs, page loads, or URL changes. Cypress takes snapshots of your application and enables you to time travel back to the state it was in when commands ran. You can use the Dev Tools while your tests run, you can see every console message, every network request. You can inspect elements, and you can even use debugger statements in your spec code or your application code. There is no fidelity loss - you can use all the tools you’re already comfortable with. This enables you to test and develop all at the same time. If you’re using Cypress Cloud, you have access to even more detailed insights on your tests’ performance, helping you to optimize your runs, and ship faster with more confidence.