Lesson overview · Free interview practice

CORS

Master cross-origin communication techniques.

Topics in the full lesson
  • Introduction to Cross-Origin Communication
  • Cross-Origin Resource Sharing (CORS)
  • Using the postMessage API
  • Cross-Origin Communication Use Cases
  • Practical Examples
  • Best Practices
  • Exercises

Practice Problems

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Completion marks record your own progress, not an automatically checked result. The task type does not determine whether it is optional.

TheoryMedium

CORS Headers: How Servers Opt In to Cross-Origin Access

Question

How does CORS work, and what headers does a server send to allow cross-origin access?

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Explain your reasoning or try an implementation before comparing answers.

TheoryMedium

The Same-Origin Policy: Browser's Core Isolation Boundary

Question

What is the Same-Origin Policy, what does it restrict, and what does it NOT restrict?

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Explain your reasoning or try an implementation before comparing answers.

TheoryMedium

Preflight Requests: The OPTIONS Negotiation

Question

What is a preflight request, when is it required, and how does it work?

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Explain your reasoning or try an implementation before comparing answers.

TheoryMedium

Cross-Origin Messaging with postMessage

Question

How does window.postMessage work, and what security checks must you implement?

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Explain your reasoning or try an implementation before comparing answers.

TheoryMedium

CORS Troubleshooting Catalog

Question

What are the most common CORS errors, and how do you systematically troubleshoot them?

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Explain your reasoning or try an implementation before comparing answers.

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