Lessons · TypeScript · catching a promise
Catching a rejection
A promise that rejects with nobody listening is an unhandled rejection: often silent, sometimes fatal. Attach .catch, or await inside try/catch.
Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.
What it is for
A network call that fails at 2am must be seen somewhere. Unhandled rejections are how a page quietly stops working with a blank spot where the data should be.
How to think about it
Every promise chain ends in .catch; every await that can fail sits in a try. Report the error where a person will see it, then decide whether to retry or degrade.
Worked example
async function risky() { throw new Error("bad"); }Rejects.risky().catch(e => console.log("caught", e.message));caught bad: without the catch, the rejection is unhandled.async function safe() { try { await risky(); } catch (e) { return "handled"; } }await rethrows the rejection, so try/catch works.safe().then(console.log);handled.
Your turn
Report any failure of the fetch.
fetchData().(err => report(err));
Solve one with the tests running
The trap
.then(ok, fail) does not catch errors thrown inside ok. Put .catch after the then, where it covers both.