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Why does .catch () not catch reject () inside a Promise constructor in a loop on an async function if no error is passed?

Considering

(async () => {
 const p = await new Promise((resolve, reject) => setTimeout(() => {
    reject(new Error(1))
  }, Math.floor(Math.random() * 1000))); return p})()
.then(data => console.log(data))
.catch(err => console.error(err));
Run code


Error()

recorded in .catch()

If we extend the template to use a loop, it Error

registers with.catch() 

const fn = async(res, ...props) => {
  for (let prop of props) res.push(await prop())
  return res
}

const arr = [
  () =>
    new Promise((resolve, reject) => 
      setTimeout(() =>
        reject(new Error(1))
      , Math.floor(Math.random() * 1000))
    ),
  () => 
    new Promise((resolve, reject) => 
      setTimeout(() => 
        resolve(1) 
      , Math.floor(Math.random() * 1000))
    )
  ];

fn([], ...arr)
.then(data => console.log(data))
.catch(err => console.log(err));
Run code


If we use a loop to call several functions that return Promise

and do not explicitly pass Error()

to the reject()

Promise

constructor, the resolver

function .catch()

does not catch the error, and the array res

does not return, only the value Promise

passed to resolve()

is available in

const fn = async(res, ...props) => {
  for (let prop of props) res.push(await prop())
  return res
}

const arr = [
  () =>
    new Promise((resolve, reject) => 
      setTimeout(() =>
        reject(1)
      , Math.floor(Math.random() * 1000))
    ),
  () => 
    new Promise((resolve, reject) => 
      setTimeout(() => 
        resolve(1) 
      , Math.floor(Math.random() * 1000))
    )
  ];

fn([], ...arr)
// what happened to our array `res` returned from `fn`?
.then(data => console.log(data))
// what happened to our rejected Promise?
.catch(err => console.log(err));
Run code


Questions:

  • Why is reject()

    n't the call propagated to .catch()

    when Error()

    not explicitly passed to reject()

    inside the constructor Promise

    in a loop using a function async

    ?

  • Why only the value is Promise

    returned in .then()

    , although the array is returned from the function async

    , when one of the objects Promise

    iterated over in the loop function is reject()

    called inside the Promise

    constructor resolver

    function?

+3


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2 answers


If we [...] do not explicitly pass Error()

in reject()

the Promise constructor recognizer function we will not catch an .catch()

error



Of course, because then there is no mistake. But catch

will catch whatever you go to reject

, i.e. the value 1

that registers correctly.

+4


source


When trying to code, for example, where used Promise

and expected to log an error in .catch()

, explicitly pass Error()

, preferably with the appropriate message as a parameter, in reject()

from the Promise

constructor or Promise.reject()

; and replace it with the help of console.error()

on console.log()

on .catch()

to distinguish between error, which you would expect to be written as Error()

, but are not allowed value Promise

, where the allowed value and the deviation may prove the same.



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