OAuth 2.0

Authorization framework for secure delegated access.

When to Use

• Third-party authentication
• API authorization
• Single sign-on (SSO)
• Social login integration

Quick Start

// Authorization Code Flow
const authUrl = new URL("https://auth.example.com/authorize");
authUrl.searchParams.set("client_id", CLIENT_ID);
authUrl.searchParams.set("redirect_uri", REDIRECT_URI);
authUrl.searchParams.set("response_type", "code");
authUrl.searchParams.set("scope", "openid profile email");
authUrl.searchParams.set("state", generateState());
authUrl.searchParams.set("code_challenge", generateCodeChallenge());
authUrl.searchParams.set("code_challenge_method", "S256");

window.location.href = authUrl.toString();

Core Concepts

Authorization Code with PKCE

// 1. Generate PKCE verifier and challenge
function generateCodeVerifier(): string {
  const array = new Uint8Array(32);
  crypto.getRandomValues(array);
  return base64UrlEncode(array);
}

async function generateCodeChallenge(verifier: string): Promise<string> {
  const hash = await crypto.subtle.digest(
    "SHA-256",
    new TextEncoder().encode(verifier),
  );
  return base64UrlEncode(new Uint8Array(hash));
}

// 2. Exchange code for tokens
async function exchangeCodeForTokens(code: string, verifier: string) {
  const response = await fetch("https://auth.example.com/token", {
    method: "POST",
    headers: { "Content-Type": "application/x-www-form-urlencoded" },
    body: new URLSearchParams({
      grant_type: "authorization_code",
      code,
      redirect_uri: REDIRECT_URI,
      client_id: CLIENT_ID,
      code_verifier: verifier,
    }),
  });
  return response.json();
}

Token Refresh

async function refreshTokens(refreshToken: string) {
  const response = await fetch("https://auth.example.com/token", {
    method: "POST",
    headers: { "Content-Type": "application/x-www-form-urlencoded" },
    body: new URLSearchParams({
      grant_type: "refresh_token",
      refresh_token: refreshToken,
      client_id: CLIENT_ID,
    }),
  });
  return response.json();
}

Common Patterns

Protected API Calls

async function fetchWithAuth(url: string, accessToken: string) {
  const response = await fetch(url, {
    headers: {
      Authorization: `Bearer ${accessToken}`,
    },
  });

  if (response.status === 401) {
    // Token expired, refresh and retry
    const newTokens = await refreshTokens(getRefreshToken());
    saveTokens(newTokens);
    return fetchWithAuth(url, newTokens.access_token);
  }

  return response.json();
}

Best Practices

Do:

• Use PKCE for all public clients
• Store tokens securely
• Implement token refresh
• Validate state parameter

Don't:

• Store tokens in localStorage
• Use implicit flow
• Skip HTTPS
• Ignore token expiration

Troubleshooting

Issue	Cause	Solution
Invalid grant	Code expired	Retry auth flow
CORS error	Wrong redirect	Check allowed origins
Token rejected	Clock skew	Sync server time

References

• OAuth 2.0 RFC
• PKCE RFC