Build the safeguards
Eight explained steps, tradeoffs, pitfalls and a checkpoint before moving on.
Read the tutorialTypeScript / React / Runtime boundaries
The compiler said it was fine.
A price becomes “not a number.” Two identifiers trade places. An old response overwrites a new one. Make these bugs happen, then follow the decisions that stop them.
The idea in plain language
TypeScript checks the program before it runs. A runtime validator checks the data that actually arrives. This lab uses both, then protects the workflow and asynchronous results separately.
The React application runs real compiled TypeScript against three invented sample responses. It makes no customer API calls. The public controls choose reviewed examples; they do not execute visitor-supplied code.
Your independent testing environment
Make the failure visible.
Choose “A price sent as text.” Run it without safeguards, then with safeguards. Watch an invalid price become a clear validation message. Reset starts over and stops pending work.
Loading the example controls… The written lesson and code download are available below.
Why the change matters
Show the malformed input first. Explain why the assertion compiled, where validation belongs, how the union changes the interface, and why cancellation and response ordering are separate concerns.
Use the code viewer to inspect the original source that produced this release. Each file is checked against the release manifest before it is displayed. The complete download includes the runnable example, tests, setup instructions and third-party notices.
Keep exploring
Follow the question you have next.
Compare the choices
Choose a pattern and an environment for the problem you actually have.
Compare approachesTry it on your own
A beginner route, a local setup and an extension challenge.
Open the how-to guideQuestions before you start
Do I need to know how to code?
No. Start with the default example, read the trace and compare the two implementations. The tutorial introduces the code after you have seen the result.
Does this use real customer data or send anything?
No. Examples use invented records and simulated downstream work. Each lab keeps its own transient environment; resetting it stops the experiment.
What can this experiment prove?
The React application runs real compiled TypeScript against three invented sample responses. It makes no customer API calls. The public controls choose reviewed examples; they do not execute visitor-supplied code.
Can I use or change the source?
The original lesson and application source are MIT-licensed. Third-party runtimes retain their own licenses. Download the package and follow GETTING_STARTED before changing it.