CODEWITHSUNDEEP
javascriptecmascript-6promisees6-promise
Root-Core
Root-Core

Reputation: 61

Promise rejection not passed onto catch, if stored in variable

I have just come along a curiosity of promises. If I reject directly in an chain, I can catch on the variable later.

If I reject on the variable, I can not catch at all. The promise is always considered to be resolved in this case:

let proMISS = Promise.resolve();
proMISS.then(() => console.log('THEN 1'))
       .then(() => Promise.reject())
       .then(() => console.log('THEN 2'));

setTimeout(() => {
    proMISS.catch(() => console.log('CATCH'));
}, 1000);

This indeed does work:

let PROmiss = Promise.resolve()
                     .then(() => console.log('THEN 1'))
                     .then(() => Promise.reject())
                     .then(() => console.log('THEN 2'));

setTimeout(() => {
    PROmiss.catch(() => console.log('CATCH'));
}, 1000);

This does not seem to be deterministically

Upvotes: 0

Views: 193

Answers (3)

Md Eftakhairul Islam
Md Eftakhairul Islam

Reputation: 198

let proMISS = Promise.resolve();
let promissTwo = proMISS.then(() => console.log('THEN 1'))
       .then(() => Promise.reject())
       .then(() => console.log('THEN 2'));

setTimeout(() => {
    promissTwo.catch(() => console.log('CATCH'));
}, 1000);
You need to take the output of the first promise chain in a variable and only then you can handle the rejection. I took proMISS chain's output in a new varible promissTwo and then handled it

Your second example works based on the same concept.

Upvotes: 0

Bergi
Bergi

Reputation: 664538

You're essentially doing

let promise1 = Promise.resolve();
let promise2 = promise1.then(() => console.log('THEN 1'))
       .then(() => Promise.reject())
       .then(() => console.log('THEN 2'));
setTimeout(() => {
    promise1.catch(() => console.log('CATCH'));
    promise2.catch(() => console.log('CATCH'));
}, 1000);

Yes, promise1 is different from promise2. The first one is fulfilled with undefined, while the second is getting rejected.

Upvotes: 1

Root-Core
Root-Core

Reputation: 61

I just found the answer, I guess.

The promise in the variable is resolved, but the chain is not. Therefore, if you catch on the variable, it has to be resolved.

You would have to save the last member of the chain, every-time you add a member.

Upvotes: 1

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