Understanding how the Cypress architecture works

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Cypress has become a popular choice for modern web test automation, especially for frontend developers and QA engineers. But what makes Cypress so unique and different from other tools like Selenium is its architecture. In this blog, we'll break down how Cypress works under the hood in simple terms, helping you understand why it's so fast, reliable, and developer friendly.

What is Cypress?

Cypress is a JavaScript-based end-to-end testing framework built specifically for modern web applications. Unlike Selenium, which runs tests outside the browser using WebDriver, Cypress runs directly in the browser, giving it deeper access to everything happening inside your app.

Cypress Architecture: The Big Picture

At a high level, Cypress consists of two main components:

1. Test Runner (Client Side)

This is the UI that opens when you run npx cypress open. It shows your test files, the browser, the test steps, and the time-travel snapshots.

2. Node Server (Backend Process)

This is the server that communicates with the browser, handles file access, test commands, plugins, and more.

Unlike traditional frameworks, Cypress doesn't use WebDriver. Instead, it injects test code directly into the browser where your app runs. This allows Cypress to listen and react in real-time to everything happening in the app just like a user would.

How Test Commands Flow in Cypress

Let's break down what happens behind the scenes when you run a test.

  1. The Cypress Test Code is Parsed by Node
    • Your test file (.cy.js) is executed by Cypress’s Node.js process.
  2. Commands Are Queued
    • Commands like cy.visit(), cy.get(), or cy.click() are queued, not executed immediately.
  3. Cypress Injects Code into the Browser
    • Cypress opens a browser (Chrome, Edge, Firefox) and injects its code into the page.
  4. Execution Inside the Browser
    • The browser runs your app and the Cypress code side-by-side.
    • This gives Cypress the ability to observe, control, and manipulate DOM elements directly.
  5. Real-Time Communication
    • The Node process and browser communicate continuously.
    • Screenshots, logs, API intercepts, and assertions are managed and synced live.
  6. UI Update
    • The Cypress Test Runner shows the test progress with snapshots, making it easy to debug.

Key Architectural Features of Cypress

  1. Runs in the Same Run-Loop as the App
    • Cypress tests run in the same browser context as your app. There's no network latency or protocol overhead.
  2. Time Travel & Snapshots
    • Each command takes a snapshot before and after execution. You can hover over test steps in the runner to see the DOM at that moment in time.
  3. Automatic Waiting
    • Cypress automatically waits for elements to appear, animations to finish, and AJAX calls to complete reducing flakiness.
  4. Network Layer Control
    • Using cy.intercept(), Cypress can spy, stub, or block API requests this is possible because of the deep control it has over browser network traffic.
  5. No External Dependencies
    • Unlike Selenium, Cypress doesn't require additional drivers or servers (like Selenium Grid or WebDriver). It's a single, self-contained tool.

Cypress Limitations

  • Cannot test multiple tabs or browsers at the same time.
  • Limited support for mobile browsers or real device testing.
  • Limited cross-browser support (only Chromium and Firefox-based browsers are supported).

Cypress's architecture is designed for speed, control, and developer happiness. By running inside the browser alongside your app, Cypress gives you direct access to the DOM, real-time network monitoring, and powerful debugging tools all without needing external servers or drivers.