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));

The trap

.then(ok, fail) does not catch errors thrown inside ok. Put .catch after the then, where it covers both.

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