Introduction
SOLID is a set of five object-oriented design principles introduced by Robert C. Martin (Uncle Bob). These principles help developers build software that is:
- Easy to maintain
- Easy to extend
- Highly reusable
- Loosely coupled
- Easy to test
In enterprise Playwright frameworks, following SOLID principles results in cleaner Page Objects, reusable utilities, scalable workflows, and better test automation architecture.
Why SOLID is Important in Automation Frameworks
Without SOLID principles, frameworks often become:
- Huge Page Objects (1000+ lines)
- Duplicate code
- Difficult to maintain
- Hard to extend
- Strongly coupled
- Fragile when the application changes
With SOLID principles:
- Classes have clear responsibilities
- New features require minimal changes
- Code becomes reusable
- Frameworks scale efficiently
What does SOLID stand for?
| Principle | Full Form |
|---|---|
| S | Single Responsibility Principle |
| O | Open Closed Principle |
| L | Liskov Substitution Principle |
| I | Interface Segregation Principle |
| D | Dependency Inversion Principle |
S — Single Responsibility Principle (SRP)
Definition
A class should have only one reason to change.
Each class should have one responsibility.
Bad Example
A LoginPage doing everything.
class LoginPage {
login() {}
logout() {}
readExcel() {}
generateRandomUser() {}
takeScreenshot() {}
sendEmail() {}
}Problems
- Too many responsibilities
- Difficult maintenance
- Hard to reuse
- Large file
Good Example
Separate responsibilities.
LoginPage
↓
Login onlyExcelUtility
↓
Read ExcelScreenshotUtility
↓
Capture ScreenshotLogger
↓
LoggingEach class has a single purpose.
Enterprise Folder Structure
pages/
LoginPage.ts
utils/
ExcelUtility.ts
Logger.ts
ScreenshotUtility.tsBenefits
✔ Small classes
✔ Easy maintenance
✔ Better readability
✔ Easier testing
Interview Question
Why is SRP important in Playwright?
Answer
It prevents Page Objects from becoming large and difficult to maintain. By separating responsibilities into page objects, utilities, workflows, and services, the framework becomes cleaner, reusable, and easier to extend.
O — Open Closed Principle (OCP)
Definition
Software entities should be open for extension but closed for modification.
Instead of changing existing code, extend it.
Bad Example
if(browser=="chromium"){
}
else if(browser=="firefox"){
}
else if(browser=="webkit"){
}Whenever a new browser is added, this code must be modified.
Better Approach
Use the Strategy Pattern.
Browser Strategy
↓
Chromium
Firefox
WebKitAdding a new browser:
Edge StrategyNo existing code changes.
Playwright Example
Instead of modifying
BrowserLauncher.tsCreate
ChromiumStrategy
FirefoxStrategy
WebKitStrategy
EdgeStrategyBenefits
✔ Easy extension
✔ No modification
✔ Low risk
✔ Better architecture
Interview Question
Explain OCP using Playwright.
Answer
A browser launcher should support new browsers without changing existing code. Using Strategy Pattern, each browser has its own implementation, allowing new browsers to be added by creating new strategy classes rather than modifying the launcher.
L — Liskov Substitution Principle (LSP)
Definition
A derived class should be replaceable with its base class without breaking the application.
Example
Suppose
BasePagecontains
open()
waitForPage()
verifyTitle()Now
LoginPage
DashboardPage
CheckoutPageextend BasePage.
Any of them should work wherever a BasePage is expected.
Good Example
BasePage
↓
LoginPage
↓
DashboardPage
↓
CartPageEach subclass honors the behavior defined by the base class.
Bad Example
Suppose BasePage defines
openPage()but CheckoutPage throws an exception because it cannot open itself.
That violates LSP.
Benefits
✔ Better inheritance
✔ Predictable behavior
✔ Easier maintenance
Interview Question
How does LSP help automation?
Answer
It ensures that all page objects derived from a common base behave consistently. Shared framework code can work with any page object without requiring special handling.
I — Interface Segregation Principle (ISP)
Definition
Clients should not be forced to depend on interfaces they don’t use.
Bad Example
interface Utility{
readExcel();
takeScreenshot();
sendEmail();
uploadFile();
downloadFile();
}Every implementing class must define unnecessary methods.
Better Example
Separate interfaces.
ExcelReader
↓
readExcel()ScreenshotService
↓
takeScreenshot()FileUploader
↓
upload()Benefits
✔ Small interfaces
✔ Easier implementation
✔ Better readability
Interview Question
Why is ISP useful in Playwright?
Answer
It avoids creating large interfaces that force unrelated implementations. Separate interfaces for reporting, screenshots, file handling, and data readers keep the framework modular and easier to maintain.
D — Dependency Inversion Principle (DIP)
Definition
High-level modules should not depend on low-level modules.
Both should depend on abstractions.
Bad Example
class LoginWorkflow{
private login=new LoginPage(page);
}Strong coupling.
Better Example
Inject dependency.
constructor(private loginPage: LoginPage){
}Now
LoginWorkflow
doesn’t create LoginPage.
Someone else provides it.
Architecture
Test
↓
Fixture
↓
Workflow
↓
Page ObjectEach layer receives dependencies instead of creating them.
Playwright Example
Playwright Fixtures naturally support Dependency Injection.
Instead of
const page=new LoginPage(browserPage);Fixtures inject
loginPagedirectly into the test.
Benefits
✔ Loose coupling
✔ Easy testing
✔ Easy mocking
✔ Better scalability
Interview Question
How does Playwright support Dependency Injection?
Answer
Playwright fixtures inject dependencies such as browser instances, pages, authenticated sessions, and page objects into test functions. This reduces object creation inside tests and promotes loose coupling.
SOLID Applied to a Playwright Framework
Test Layer
│
▼
Workflow Layer
│
▼
Page Object Layer
│
▼
Utility/Service Layer
│
▼
Playwright Framework
│
▼
BrowserEach layer follows a specific responsibility.
Before SOLID
LoginPage
1000 Lines
↓
Login
Logout
Excel
JSON
API
Logger
Screenshot
Database
Email
Random DataVery difficult to maintain.
After SOLID
LoginPage
↓
Login OnlyLogger
↓
Logging OnlyAPI Client
↓
API OnlyExcel Utility
↓
Excel OnlyWorkflow
↓
Business ProcessClean architecture.
SOLID + Design Patterns
| SOLID Principle | Design Pattern |
|---|---|
| SRP | Page Object Model |
| OCP | Strategy Pattern |
| LSP | Base Page Inheritance |
| ISP | Small Interfaces |
| DIP | Fixtures + Dependency Injection |
Real Enterprise Example
Suppose you’re automating an e-commerce application.
Without SOLID
CheckoutPage
↓
Login
↓
Search
↓
Add Product
↓
Payment
↓
Database
↓
Email
↓
Report
↓
ScreenshotOne class controls everything.
With SOLID
CheckoutWorkflow
↓
LoginPage
↓
ProductPage
↓
CartPage
↓
PaymentPage
↓
OrderPageUtilities
Logger
API
Screenshot
Excel
JSON
Random DataEvery class has one responsibility.
Enterprise Benefits
Following SOLID results in:
- Smaller Page Objects
- Better readability
- Easier debugging
- Cleaner architecture
- Reusable code
- Easier onboarding for new team members
- Better unit and integration testing
- Faster feature development
- Simpler code reviews
Common SOLID Mistakes in Playwright
❌ Putting business workflows inside page objects
Move business flows to a separate Workflow (Facade) layer.
❌ Creating one huge Utility class
Split it into focused utilities:
- Logger
- DateUtility
- JsonUtility
- ExcelUtility
- ScreenshotUtility
❌ Hardcoding object creation
Avoid:
const loginPage = new LoginPage(page);Prefer dependency injection through Playwright fixtures or a factory.
❌ Large BasePage classes
Keep BasePage limited to truly common functionality such as navigation, waiting, or shared helpers. Don’t force unrelated pages to inherit unnecessary behavior.
❌ Large interfaces
Create small, purpose-specific interfaces rather than one interface containing dozens of unrelated methods.
Senior Playwright Interview Questions
1. What are SOLID principles?
Answer
SOLID is a set of five object-oriented design principles that improve maintainability, extensibility, reusability, and scalability. They help reduce coupling and encourage clean architecture.
2. Which SOLID principle is most important in automation?
Answer
All five are valuable, but Single Responsibility Principle (SRP) is often the most impactful because it prevents oversized Page Objects and encourages separation of concerns across pages, workflows, utilities, and services.
3. How does Playwright support Dependency Inversion?
Answer
Playwright fixtures provide dependency injection by supplying browser instances, contexts, pages, and custom page objects to tests. Tests depend on abstractions provided by fixtures instead of creating concrete objects themselves.
4. How do SOLID principles improve a Playwright framework?
Answer
They make the framework modular, reusable, easier to extend, and simpler to maintain. New functionality can often be added by introducing new classes rather than modifying existing ones, reducing the risk of breaking stable automation.
5. How do SOLID principles relate to design patterns?
Answer
Design patterns are practical implementations of SOLID ideas. For example:
- SRP → Page Object Model, Workflow layer
- OCP → Strategy Pattern
- LSP → Base Page inheritance
- ISP → Focused interfaces
- DIP → Fixtures and Dependency Injection
Using SOLID together with patterns like Factory, Builder, Strategy, and Facade results in an enterprise-grade Playwright framework that is easier to scale and maintain over time.