Why await not right working with reject error

user_11631000 0
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Why promise catch not working with await?

I want catch error when rejected, but I got two differences result

(async () => {
    var t = (new Promise((r1, r2) => r2(12345))); 
    t.catch(e=> console.log(e)); 
    await t; 
})()

Results in error: VM5989:1 Uncaught (in promise)

(async () => {
    var t = (new Promise((r1, r2) => r2(12345))).catch(e=> console.log(e));
    await t; 
})()

Causes no error and returns 12345

demo image:

at node@12 / chrome@75 console , I got the two differences result.

when I use t.catch, I got Unhandled promise rejection.

when I use (new Promise()).catch, I got except result.

why? And how I can use t and get the right result?

promise async-await javascript
2 Answers

In the first example, you create a new promise that handles the exception by calling t.catch(), but you don't assign it to t, so t remains a promise with an unhandled rejection.

In the second example, t references the chain which is a promise with a handled rejection (the .catch() continuation does not throw so it resolves with undefined), so awaiting t will not cause the async function to throw.

However, because t in the first example still contains a rejected promise, awaiting it will cause the async function to throw.

user_15415630

In the first case, since Promise is asynchronous in nature and statement r2 was already executed even before attaching the catch.

That means catch was not present when r2 - reject function - was called.

Whenever you reject while using promise then catch must be present. Otherwise, it's obvious to see an error on reject.

Edit: As answered by @Patrick Roberts,

the error is not due to unhandled catch but because of the awating for rejected t. While in the case of the second statement t (promise) was still pending.

user_37137310
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