Frontend Security Best Practices

Learn how to protect your web applications from common security vulnerabilities

Why Frontend Security Matters

Frontend security is often overlooked, with many developers assuming that security concerns are primarily a backend responsibility. However, frontend code is directly exposed to users and can be a significant attack vector. Implementing proper security measures in your frontend code is essential for protecting:

• User data and privacy
• Application functionality
• Backend systems
• Your organization’s reputation

Security is a shared responsibility across the entire application stack. Even the most secure backend can be compromised if the frontend contains vulnerabilities.

Common Frontend Security Vulnerabilities

Cross-Site Scripting (XSS)

Attackers inject malicious scripts that execute in users’ browsers.

Impact: Can steal cookies, session tokens, and sensitive data, or perform actions on behalf of the user.

Cross-Site Request Forgery (CSRF)

Forces authenticated users to perform unwanted actions on a web application.

Impact: Can lead to unauthorized transactions, data changes, or account compromise.

Clickjacking

Tricks users into clicking on something different from what they perceive.

Impact: Can lead to unwanted actions, downloads, or data exposure.

Sensitive Data Exposure

Exposing sensitive data in frontend code or browser storage.

Impact: Can lead to data breaches, identity theft, or account takeover.

Insecure Dependencies

Using libraries or frameworks with known vulnerabilities.

Impact: Can introduce various security issues depending on the vulnerability.

Client-Side Logic Vulnerabilities

Relying solely on client-side validation or exposing sensitive business logic.

Impact: Can lead to data manipulation, bypassing restrictions, or business logic abuse.

Preventing Cross-Site Scripting (XSS)

XSS remains one of the most common web application vulnerabilities. Here’s how to prevent it:

Content Security Policy (CSP)

CSP is a powerful defense against XSS attacks. It restricts the sources from which various types of content can be loaded.

<!-- Example CSP header -->
<meta http-equiv="Content-Security-Policy" content="default-src 'self'; script-src 'self' https://trusted-cdn.com; style-src 'self' https://trusted-cdn.com; img-src 'self' https://trusted-cdn.com data:; connect-src 'self' https://api.example.com">

You can also set CSP via HTTP headers (preferred method):

Content-Security-Policy: default-src 'self'; script-src 'self' https://trusted-cdn.com; style-src 'self' https://trusted-cdn.com; img-src 'self' https://trusted-cdn.com data:; connect-src 'self' https://api.example.com

Avoid using unsafe-inline and unsafe-eval in your CSP as they significantly reduce its effectiveness against XSS attacks.

Output Encoding

Always encode user-generated content before inserting it into the DOM.

// Unsafe: Direct insertion of user input
const userName = getUserInput();
document.getElementById('greeting').innerHTML = 'Hello, ' + userName;

// Safe: Encode user input
const userName = getUserInput();
const encodedUserName = encodeHTML(userName);
document.getElementById('greeting').innerHTML = 'Hello, ' + encodedUserName;

// Simple HTML encoding function
function encodeHTML(str) {
  return str
    .replace(/&/g, '&')
    .replace(/</g, '&lt;')
    .replace(/>/g, '&gt;')
    .replace(/"/g, '&quot;')
    .replace(/'/g, '&#039;');
}

Many frameworks provide built-in protection:

React

React automatically escapes values in JSX:

// Safe in React - values in curly braces are automatically escaped
const userName = getUserInput();
return <div>Hello, {userName}</div>;

// Dangerous - bypasses React's automatic escaping
const userName = getUserInput();
return <div dangerouslySetInnerHTML={{ __html: userName }} />;

Only use dangerouslySetInnerHTML when absolutely necessary and with proper sanitization.

Vue.js

Vue automatically escapes values in templates:

<!-- Safe in Vue - values in mustaches are automatically escaped -->
<div>Hello, {{ userName }}</div>

<!-- Dangerous - bypasses Vue's automatic escaping -->
<div v-html="userName"></div>

Only use v-html when absolutely necessary and with proper sanitization.

Angular

Angular automatically escapes values in templates:

<!-- Safe in Angular - values in interpolation are automatically escaped -->
<div>Hello, {{ userName }}</div>

<!-- Dangerous - bypasses Angular's automatic escaping -->
<div [innerHTML]="userName"></div>

Only use [innerHTML] when absolutely necessary and with proper sanitization.

Input Validation

Validate user input on both client and server sides.

// Client-side validation example
function validateUsername(username) {
  // Only allow alphanumeric characters and underscores
  const pattern = /^[a-zA-Z0-9_]+$/;
  
  if (!pattern.test(username)) {
    throw new Error('Username can only contain letters, numbers, and underscores');
  }
  
  return username;
}

// Usage
try {
  const validatedUsername = validateUsername(userInput);
  // Proceed with the validated input
} catch (error) {
  // Handle validation error
  displayError(error.message);
}

DOM XSS Prevention

Be careful with DOM manipulation methods that can execute JavaScript.

// Dangerous methods that can lead to XSS:
document.write(userInput);
element.innerHTML = userInput;
element.outerHTML = userInput;
element.insertAdjacentHTML('beforebegin', userInput);
element.insertAdjacentHTML('afterbegin', userInput);
element.insertAdjacentHTML('beforeend', userInput);
element.insertAdjacentHTML('afterend', userInput);

// Safer alternatives:
element.textContent = userInput; // For text
element.setAttribute('value', userInput); // For attributes (except event handlers)

Using Sanitization Libraries

When you need to allow some HTML, use a sanitization library.

// Using DOMPurify
import DOMPurify from 'dompurify';

const userHtml = getUserGeneratedHtml();
const sanitizedHtml = DOMPurify.sanitize(userHtml);

// Now safe to insert
element.innerHTML = sanitizedHtml;

Preventing Cross-Site Request Forgery (CSRF)

CSRF attacks trick users into performing unwanted actions on a site where they’re authenticated.

CSRF Tokens

Implement CSRF tokens for state-changing operations.

<!-- Form with CSRF token -->
<form action="/api/update-profile" method="POST">
  <input type="hidden" name="csrf_token" value="random-token-value">
  <!-- Other form fields -->
  <button type="submit">Update Profile</button>
</form>

// Including CSRF token in AJAX requests
const csrfToken = document.querySelector('meta[name="csrf-token"]').getAttribute('content');

fetch('/api/update-profile', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    'X-CSRF-Token': csrfToken
  },
  body: JSON.stringify(profileData)
});

SameSite Cookies

Use SameSite cookie attribute to prevent CSRF attacks.

Set-Cookie: sessionid=abc123; Path=/; Secure; HttpOnly; SameSite=Strict

SameSite values:

• Strict: Cookies are only sent in a first-party context
• Lax: Cookies are sent when navigating to the site (default in modern browsers)
• None: Cookies are sent in all contexts (requires Secure attribute)

Custom Headers

Add custom headers to AJAX requests that browsers won’t include in cross-site requests.

fetch('/api/update-profile', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    'X-Requested-With': 'XMLHttpRequest' // Custom header
  },
  body: JSON.stringify(profileData)
});

Preventing Clickjacking

Clickjacking attacks use transparent or disguised UI elements to trick users into clicking on something different from what they perceive.

X-Frame-Options Header

Prevent your site from being embedded in frames on other sites.

X-Frame-Options: DENY

Or allow only specific origins:

X-Frame-Options: ALLOW-FROM https://trusted-site.com

Content Security Policy (CSP) frame-ancestors

Modern alternative to X-Frame-Options with more flexibility.

Content-Security-Policy: frame-ancestors 'none';

Or allow specific origins:

Content-Security-Policy: frame-ancestors 'self' https://trusted-site.com;

Frame-busting Code

As a fallback for older browsers, include frame-busting code.

// Basic frame-busting
if (window !== window.top) {
  window.top.location = window.location;
}

More robust version:

// Advanced frame-busting
(function() {
  // If we're in a frame
  if (window !== window.top) {
    // Try to break out
    window.top.location = window.location;
    
    // If we're still in a frame after attempting to break out
    // (can happen if the parent page uses security measures)
    setTimeout(function() {
      if (window !== window.top) {
        // Make the UI unusable
        document.body.innerHTML = 'This site cannot be displayed in a frame.';
        document.body.style.backgroundColor = '#FFF';
        document.body.style.color = '#000';
        document.body.style.padding = '20px';
        document.body.style.position = 'fixed';
        document.body.style.top = '0';
        document.body.style.left = '0';
        document.body.style.right = '0';
        document.body.style.bottom = '0';
        document.body.style.zIndex = '9999999';
      }
    }, 1);
  }
})();

Protecting Sensitive Data

Frontend code should never expose sensitive data or secrets.

Avoid Storing Sensitive Data in JavaScript

// BAD: Hardcoded API key
const apiKey = 'sk_live_abcdef123456';

// GOOD: Get API key from backend
async function fetchDataWithAuth() {
  // Get a short-lived token from your backend
  const { token } = await fetch('/api/get-auth-token').then(r => r.json());
  
  // Use the token
  return fetch('/api/data', {
    headers: {
      'Authorization': `Bearer ${token}`
    }
  });
}

Secure Local Storage Usage

Browser storage mechanisms (localStorage, sessionStorage) are not secure for sensitive data.

// BAD: Storing sensitive data in localStorage
localStorage.setItem('creditCard', '1234-5678-9012-3456');

// BETTER: Store only non-sensitive data
localStorage.setItem('userPreferences', JSON.stringify({ theme: 'dark', fontSize: 'large' }));

// For sensitive data that must be persisted client-side, consider encrypting it
// (though this has limitations and should be used carefully)
import CryptoJS from 'crypto-js';

function secureStore(key, data, secretKey) {
  const encrypted = CryptoJS.AES.encrypt(JSON.stringify(data), secretKey).toString();
  sessionStorage.setItem(key, encrypted);
}

function secureRetrieve(key, secretKey) {
  const encrypted = sessionStorage.getItem(key);
  if (!encrypted) return null;
  
  const decrypted = CryptoJS.AES.decrypt(encrypted, secretKey).toString(CryptoJS.enc.Utf8);
  return JSON.parse(decrypted);
}

Secure Cookie Usage

When using cookies, set appropriate security flags.

Set-Cookie: sessionid=abc123; Path=/; Secure; HttpOnly; SameSite=Strict

• Secure: Only send cookie over HTTPS
• HttpOnly: Prevent JavaScript access to the cookie
• SameSite: Control when cookies are sent with cross-site requests
• Set appropriate expiration times

Managing Dependencies Securely

Third-party dependencies can introduce security vulnerabilities.

Regular Dependency Auditing

Regularly check for vulnerabilities in your dependencies.

# npm
npm audit

# yarn
yarn audit

# To fix vulnerabilities
npm audit fix
yarn audit fix

Subresource Integrity (SRI)

When loading scripts from CDNs, use SRI to ensure they haven’t been tampered with.

<script 
  src="https://cdn.example.com/library.js" 
  integrity="sha384-oqVuAfXRKap7fdgcCY5uykM6+R9GqQ8K/uxy9rx7HNQlGYl1kPzQho1wx4JwY8wC" 
  crossorigin="anonymous">
</script>

Minimize Dependencies

Each dependency increases your attack surface.

// Instead of importing a large library for a simple function
// BAD
import _ from 'lodash';
const reversed = _.reverse([1, 2, 3]);

// GOOD: Use native methods when possible
const reversed = [1, 2, 3].reverse();

// If you need specific functions, import only what you need
// BETTER than importing the whole library
import reverse from 'lodash/reverse';
const reversed = reverse([1, 2, 3]);

Secure Authentication Practices

Authentication is a critical security component.

Implement Proper Token Handling

// Store tokens securely
function storeAuthToken(token) {
  // For SPA, store in memory when possible
  // This token will be lost on page refresh, but that's often acceptable
  // for better security
  const authState = {
    token,
    expiresAt: calculateExpiry(token)
  };
  
  // Store in a closure instead of global variable
  return {
    getToken: () => {
      // Check if token is expired
      if (Date.now() >= authState.expiresAt) {
        return null;
      }
      return authState.token;
    },
    clearToken: () => {
      authState.token = null;
      authState.expiresAt = 0;
    }
  };
}

// Usage
const auth = storeAuthToken(receivedToken);

// When making authenticated requests
fetch('/api/data', {
  headers: {
    'Authorization': `Bearer ${auth.getToken()}`
  }
});

// On logout
auth.clearToken();

Implement Proper Logout

async function logout() {
  try {
    // 1. Invalidate the token on the server
    await fetch('/api/logout', {
      method: 'POST',
      headers: {
        'Authorization': `Bearer ${auth.getToken()}`
      }
    });
    
    // 2. Clear client-side auth state
    auth.clearToken();
    
    // 3. Clear any sensitive application state
    store.dispatch({ type: 'RESET_STATE' });
    
    // 4. Redirect to login page
    window.location.href = '/login';
  } catch (error) {
    console.error('Logout failed', error);
    // Still clear client-side state even if server logout fails
    auth.clearToken();
    store.dispatch({ type: 'RESET_STATE' });
    window.location.href = '/login';
  }
}

Protecting Against Other Common Attacks

JSON Hijacking Protection

Prevent sensitive data in JSON responses from being stolen via script tags.

// Server-side: Prefix JSON responses with a character that makes it invalid JavaScript
app.get('/api/user-data', (req, res) => {
  const userData = { /* sensitive user data */ };
  res.send(`)]}',\n${JSON.stringify(userData)}`);
});

// Client-side: Strip the prefix before parsing
fetch('/api/user-data')
  .then(response => response.text())
  .then(text => {
    // Remove the anti-hijacking prefix
    const json = text.replace(/^\)\]\}',\n/, '');
    return JSON.parse(json);
  })
  .then(data => {
    // Use the data safely
  });

Preventing Prototype Pollution

Prototype pollution can lead to security vulnerabilities in JavaScript applications.

// Unsafe object merging
function mergeUnsafe(target, source) {
  for (const key in source) {
    if (typeof source[key] === 'object') {
      target[key] = target[key] || {};
      mergeUnsafe(target[key], source[key]);
    } else {
      target[key] = source[key];
    }
  }
  return target;
}

// Safer object merging with prototype pollution protection
function mergeSafe(target, source) {
  for (const key in source) {
    // Skip __proto__ and constructor to prevent prototype pollution
    if (key === '__proto__' || key === 'constructor') continue;
    
    if (source[key] && typeof source[key] === 'object') {
      target[key] = target[key] || {};
      mergeSafe(target[key], source[key]);
    } else {
      target[key] = source[key];
    }
  }
  return target;
}

Preventing DOM-based Vulnerabilities

Be careful with DOM APIs that can lead to security issues.

// Dangerous: Using location.href with user input
const userInput = document.getElementById('search').value;
window.location.href = '/search?q=' + userInput; // Vulnerable to JavaScript injection

// Safe: Encode parameters properly
const userInput = document.getElementById('search').value;
window.location.href = '/search?q=' + encodeURIComponent(userInput);

// Dangerous: Using eval or new Function
const userInput = document.getElementById('expression').value;
const result = eval(userInput); // Never do this!

// Safe: Use alternatives to eval
// For JSON parsing:
const jsonString = getJsonString();
const data = JSON.parse(jsonString);

// For dynamic math calculations:
const expression = document.getElementById('expression').value;
// Use a math expression parser library instead of eval

Security Headers

Implement security headers to enhance your application’s security posture.

# Content-Security-Policy: Restricts resource loading
Content-Security-Policy: default-src 'self'; script-src 'self' https://trusted-cdn.com; style-src 'self' https://trusted-cdn.com; img-src 'self' https://trusted-cdn.com data:; connect-src 'self' https://api.example.com

# X-Content-Type-Options: Prevents MIME type sniffing
X-Content-Type-Options: nosniff

# X-Frame-Options: Prevents clickjacking
X-Frame-Options: DENY

# Strict-Transport-Security: Enforces HTTPS
Strict-Transport-Security: max-age=31536000; includeSubDomains; preload

# X-XSS-Protection: Additional XSS protection for older browsers
X-XSS-Protection: 1; mode=block

# Referrer-Policy: Controls the Referer header
Referrer-Policy: strict-origin-when-cross-origin

# Permissions-Policy: Controls browser features
Permissions-Policy: camera=(), microphone=(), geolocation=(self), payment=()

Security in Modern Frontend Frameworks

React Security Best Practices

// 1. Prevent XSS by avoiding dangerouslySetInnerHTML
// Bad
function Comment({ comment }) {
  return <div dangerouslySetInnerHTML={{ __html: comment }} />;
}

// Good
function Comment({ comment }) {
  return <div>{comment}</div>; // React automatically escapes this
}

// 2. Use React's built-in protection against injection attacks
function SearchQuery({ query }) {
  // Safe - React escapes variables in JSX
  return <div>You searched for: {query}</div>;
}

// 3. Sanitize HTML when you must use dangerouslySetInnerHTML
import DOMPurify from 'dompurify';

function RichTextComment({ comment }) {
  const sanitizedComment = DOMPurify.sanitize(comment);
  return <div dangerouslySetInnerHTML={{ __html: sanitizedComment }} />;
}

// 4. Protect against CSRF in forms
function ProfileForm() {
  const csrfToken = useCsrfToken(); // Custom hook to get token
  
  return (
    <form action="/api/update-profile" method="POST">
      <input type="hidden" name="csrf_token" value={csrfToken} />
      {/* Form fields */}
      <button type="submit">Update Profile</button>
    </form>
  );
}

Vue.js Security Best Practices

<template>
  <!-- 1. Prevent XSS by avoiding v-html -->
  <!-- Bad -->
  <div v-html="comment"></div>
  
  <!-- Good -->
  <div>{{ comment }}</div> <!-- Vue automatically escapes this -->
  
  <!-- 2. Sanitize HTML when you must use v-html -->
  <div v-html="sanitizedComment"></div>
  
  <!-- 3. Protect against CSRF in forms -->
  <form action="/api/update-profile" method="POST">
    <input type="hidden" name="csrf_token" :value="csrfToken">
    <!-- Form fields -->
    <button type="submit">Update Profile</button>
  </form>
</template>

<script>
import DOMPurify from 'dompurify';

export default {
  props: ['comment'],
  data() {
    return {
      csrfToken: document.querySelector('meta[name="csrf-token"]').getAttribute('content')
    };
  },
  computed: {
    sanitizedComment() {
      return DOMPurify.sanitize(this.comment);
    }
  }
};
</script>

Angular Security Best Practices

import { Component, Input } from '@angular/core';
import { DomSanitizer, SafeHtml } from '@angular/platform-browser';

@Component({
  selector: 'app-comment',
  template: `
    <!-- 1. Prevent XSS by avoiding [innerHTML] -->
    <!-- Bad -->
    <div [innerHTML]="comment"></div>
    
    <!-- Good -->
    <div>{{ comment }}</div> <!-- Angular automatically escapes this -->
    
    <!-- 2. Use Angular's built-in sanitization when needed -->
    <div [innerHTML]="sanitizedComment"></div>
    
    <!-- 3. Protect against CSRF in forms -->
    <form action="/api/update-profile" method="POST">
      <input type="hidden" name="csrf_token" [value]="csrfToken">
      <!-- Form fields -->
      <button type="submit">Update Profile</button>
    </form>
  `
})
export class CommentComponent {
  @Input() comment: string;
  csrfToken: string;
  sanitizedComment: SafeHtml;
  
  constructor(private sanitizer: DomSanitizer) {
    this.csrfToken = document.querySelector('meta[name="csrf-token"]').getAttribute('content');
  }
  
  ngOnChanges() {
    // Angular's built-in sanitizer helps protect against XSS
    this.sanitizedComment = this.sanitizer.bypassSecurityTrustHtml(this.comment);
    
    // For more control, you can use DOMPurify before Angular's sanitizer
    // this.sanitizedComment = this.sanitizer.bypassSecurityTrustHtml(
    //   DOMPurify.sanitize(this.comment)
    // );
  }
}

Security Testing for Frontend Applications

Automated Security Testing

1. Static Application Security Testing (SAST)

   Use tools to analyze your code for security vulnerabilities.

   # ESLint with security plugins
   npm install --save-dev eslint eslint-plugin-security
   
   # Add to your .eslintrc.json
   {
     "plugins": ["security"],
     "extends": ["plugin:security/recommended"]
   }
   
   # Run ESLint
   npx eslint src/
   
2. Dependency scanning

   Regularly scan dependencies for known vulnerabilities.

   # npm audit
   npm audit
   
   # Snyk
   npm install -g snyk
   snyk test
   
   # OWASP Dependency-Check
   # (Requires Java)
   npm install -g dependency-check
   dependency-check --project "My Project" --scan node_modules
   
3. Dynamic Application Security Testing (DAST)

   Test your running application for security issues.

   Tools:

   • OWASP ZAP (Zed Attack Proxy)
   • Burp Suite
   • Arachni

   # Example using OWASP ZAP CLI
   zap-cli quick-scan --self-contained --start-options "-config api.disablekey=true" https://your-app.com
   

Manual Security Testing

XSS Testing

Test your application for XSS vulnerabilities by trying to inject script tags and other malicious content into all input fields.

Common test payloads:

<script>alert('XSS')</script>
<img src="x" onerror="alert('XSS')">
<a href="javascript:alert('XSS')">Click me</a>

Test in different contexts:

• URL parameters
• Form inputs
• Headers
• File uploads (especially names and metadata)
• JSON payloads

CSRF Testing

Test if your application is vulnerable to CSRF by creating a simple HTML page that submits a form to your application.

<!-- test-csrf.html -->
<!DOCTYPE html>
<html>
<body onload="document.forms[0].submit()">
  <form action="https://your-app.com/api/update-profile" method="POST">
    <input type="hidden" name="name" value="Hacked Name">
    <!-- Other form fields with malicious values -->
  </form>
</body>
</html>

If this form successfully updates the profile when opened in a browser where the user is logged in to your application, you have a CSRF vulnerability.

Sensitive Data Exposure Testing

Check your application for exposed sensitive data:

1. Inspect the browser’s local storage and session storage
2. Check cookies for sensitive information
3. Examine the network tab in developer tools for sensitive data in responses
4. View the page source and JavaScript files for hardcoded secrets
5. Check browser cache for sensitive information

Tools like Retire.js can help identify JavaScript libraries with known vulnerabilities:

npm install -g retire
retire .

Security Headers Testing

Test your application’s HTTP security headers.

Online tools:

• Security Headers
• Mozilla Observatory

Command line:

curl -I https://your-app.com

Check for these important headers:

• Content-Security-Policy
• X-Content-Type-Options
• X-Frame-Options
• Strict-Transport-Security
• Referrer-Policy
• Permissions-Policy

Security Checklist

Use this checklist to ensure your frontend application follows security best practices:

1. XSS Prevention

   • Implement Content Security Policy (CSP)
   • Encode user-generated content before inserting into the DOM
   • Use framework’s built-in XSS protections
   • Sanitize HTML when allowing rich text
   • Validate user input on both client and server sides
   • Avoid dangerous DOM methods with user input
2. CSRF Protection

   • Implement CSRF tokens for state-changing operations
   • Use SameSite cookie attribute
   • Add custom headers to AJAX requests
   • Validate the origin and referrer headers on the server
3. Clickjacking Protection

   • Set X-Frame-Options header
   • Use CSP frame-ancestors directive
   • Implement frame-busting code as a fallback
4. Sensitive Data Protection

   • Avoid storing sensitive data in JavaScript
   • Use secure cookie flags (HttpOnly, Secure, SameSite)
   • Don’t store sensitive data in localStorage or sessionStorage
   • Implement proper token handling
   • Implement proper logout functionality
5. Dependency Management

   • Regularly audit dependencies for vulnerabilities
   • Use Subresource Integrity (SRI) for CDN resources
   • Minimize dependencies
   • Keep dependencies updated
6. Security Headers

   • Implement Content-Security-Policy
   • Set X-Content-Type-Options: nosniff
   • Configure X-Frame-Options
   • Enable Strict-Transport-Security
   • Set appropriate Referrer-Policy
   • Configure Permissions-Policy
7. Security Testing

   • Perform static code analysis
   • Scan dependencies for vulnerabilities
   • Conduct dynamic application security testing
   • Perform manual security testing
   • Test security headers

Resources

Security Guidelines and Standards

• OWASP Top 10
• OWASP Frontend Security Cheat Sheet
• Content Security Policy (CSP)
• Web Security Guidelines by Mozilla

Security Testing Tools

• OWASP ZAP (Zed Attack Proxy)
• Snyk
• ESLint Security Plugin
• DOMPurify
• Security Headers

Learning Resources

• Web Security Academy by PortSwigger
• Frontend Security - The OWASP Top 10 for Frontend Developers
• MDN Web Security

Next Steps

Now that you understand frontend security best practices, you can:

• Learn about Performance Optimization to make your sites faster
• Explore Accessibility best practices to make your sites usable by everyone
• Study Testing strategies to ensure your applications work correctly