How Cypress Works Behind the Scenes

Cypress is a popular testing tool used for end-to-end testing of web applications. It's fast, easy to set up, and gives developers real-time feedback. But have you ever wondered what’s happening behind the scenes when Cypress runs your tests?

Let’s break it down in a simple way.

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1. What Is Cypress?

Cypress is a JavaScript-based test automation tool built mainly for testing modern web applications. It runs directly in the browser, just like your actual app does.

🧪 It is mainly used for:

• End-to-end testing

• UI testing

• Integration testing

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2. The Key Difference: Cypress vs Selenium

Cypress does not use WebDriver like Selenium.
Instead, Cypress runs inside the browser itself, which gives it powerful control over:

• The DOM (Document Object Model)

• Network requests

• Browser behavior

💡 This is why Cypress tests are faster and more stable compared to Selenium.

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3. How Cypress Executes a Test

Let’s say you run a simple test:

cy.visit('https://example.com');
cy.get('input[name="email"]').type('test@example.com');
cy.get('button[type="submit"]').click();

Here’s what happens behind the scenes:

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✅ Step 1: Test Runner Starts

When you run npx cypress open or npx cypress run, Cypress:

• Loads the test files from the cypress/integration folder

• Starts a local Node.js server

• Launches the browser (Chrome, Firefox, etc.)

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✅ Step 2: Cypress Injects Scripts into the Browser

Cypress injects its own JavaScript code into the web page. This gives it full control of:

• The DOM

• Window objects

• Cookies and storage

• Network calls (XHR, fetch)

This is how Cypress interacts directly with your web page from inside the browser.

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✅ Step 3: Commands Are Queued

All Cypress commands (like .visit(), .type(), .click()) are asynchronous.
They don’t run immediately. Instead, they are added to a command queue.

Cypress processes one command at a time and waits for it to finish before moving on.

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✅ Step 4: Cypress Uses “Command Execution Engine”

The engine handles the execution flow, including:

• Waiting for elements to appear

• Retrying commands automatically

• Handling timeouts

• Capturing screenshots or videos

This built-in smart waiting avoids the need to add sleep() or manual waits in your tests.

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✅ Step 5: Events & Hooks Work Together

Cypress also supports lifecycle events and hooks like:

beforeEach(() => { ... });
after(() => { ... });

These help you run setup or cleanup code automatically before or after each test.

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✅ Step 6: Real-Time Preview and Debugging

Cypress shows the test running in a real browser window, with:

• Step-by-step test preview

• DOM snapshot on each action

• Error messages if something fails

• Stack traces for debugging

This is possible because Cypress runs both:

• Node.js (outside the browser) for file system, configs, etc.

• Browser context (inside the browser) for direct control of the web app

These two parts talk to each other using a WebSocket connection.

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4. Architecture Summary

+-------------------------+
|      Your Test File     |
+-------------------------+
            |
            v
+-------------------------+
|  Cypress Command Queue  |
+-------------------------+
            |
            v
+-----------------------------+
| Browser with Cypress Hooks |
| (runs your app + test code)|
+-----------------------------+
            |
            v
+------------------+     <-->    +----------------------+
|  Cypress Node.js |  WebSocket  |  File System, Plugins |
+------------------+     <-->    +----------------------+

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5. How Cypress Handles APIs & Network Calls

Cypress can intercept and stub API calls using cy.intercept():

• You can monitor requests

• Mock responses without hitting the server

• Block or delay calls

This helps you test edge cases and speed up testing without depending on the backend.

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6. Advantages of Cypress Architecture

✅ Fast execution (runs inside the browser)
✅ Automatic wait/retry logic
✅ Direct access to DOM and browser behavior
✅ Great debugging tools
✅ Simple to install and use

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7. Limitations to Know

❌ Only works with JavaScript-based frontends
❌ Runs in the browser, so it can’t test multiple tabs or desktop apps
❌ Cannot test real native mobile apps
❌ Less suited for large-scale, multi-browser remote testing

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Conclusion

Cypress is powerful because of its unique architecture:

• It runs your tests in the same browser as your app

• Uses its own command queue and engine

• Gives real-time feedback and automatic waiting

• Connects Node.js and browser code with WebSockets

Understanding how Cypress works behind the scenes helps you write faster, more reliable tests.

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Read More 

Installing Cypress: Step-by-Step Guide

How to Handle Exceptions in Java REST APIs

Why Choose Java for Selenium Automation

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