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ViewsHow to catch a rejected promises in same time order as resolves?
Promises which resolve() or reject() can be handled in order with .then() regardless if they are resolved or rejected:
const getRecordById = (id) => {
return new Promise((resolve, reject) => {
const randNum = Math.floor(Math.random() * 2) + 1;
if (randNum === 1) {
resolve({ id, firstName: 'John', lastName: 'Doe' });
} else {
reject(`user id ${id} not found`);
}
});
};
for (let id = 1; id <= 10; id++) {
getRecordById(id)
.then(
(record) => { console.log(`you got record #${record.id}`); },
(errMessage) => { console.log(`error was: ${errMessage}`); }
);
}
But I read in articles like this one that e.g.
many libraries and frameworks assume that promises are always rejected with an error
and so one should instead reject with an error like this: reject(new Error('Oops!')); instead of rejecting with a string.
But when I reject with an Error object, why do all the rejections complete after the resolves?
const getRecordById = (id) => {
return new Promise((resolve, reject) => {
const randNum = Math.floor(Math.random() * 2) + 1;
if (randNum === 1) {
resolve({ id, firstName: 'John', lastName: 'Doe' });
} else {
reject (new Error(`user id ${id} not found`));
}
});
};
for (let id = 1; id <= 10; id++) {
getRecordById(id)
.then((record) => { console.log(`you got record #${record.id}`); })
.catch((errMessage) => { console.log(`error was: ${errMessage}`); });
}
1 answers
Answer question0
In the second example the errors are shown at the bottom is not because reject was an error object. In the first code you have handled error inside .then but in the second code snippet there is .catch for handling exception. Second code will log in same order as first if you handle error inside .then.
const getRecordById = (id) => {
return new Promise((resolve, reject) => {
const randNum = Math.floor(Math.random() * 2) + 1;
if (randNum === 1) {
resolve({ id, firstName: 'John', lastName: 'Doe' });
} else {
reject (new Error(`user id ${id} not found`));
}
});
};
for (let id = 1; id <= 10; id++) {
getRecordById(id)
.then((record) => { console.log(`you got record #${record.id}`)},
(errMessage) => { console.log(`error was: ${errMessage}`); })
}