The Boundary

Validate what arrives, once, at the edge. Your own API is not exempt. It will change, it will have a bug, and a deploy will one day return something the frontend did not expect.

In TypeScript this is the module 9 lesson: annotating a response checks nothing, because types are erased. Take it as unknown, check it with a guard or a schema, and trust the types completely behind that line.

Handle the network honestly. Requests are slow, fail, and come back out of order. You built all of this in React and in APIs & Datasets: loading before asking, an error with a retry, a guard against a late response overwriting a newer one, and debouncing so typing is not one request per keystroke.

None of it is new. This is where you assemble it.