I have an array of methods and I running them by Promise.mapSeries:
Promise.mapSeries(functions.map(function => {
return new Promise((resolve, reject) => {
function(req, res, function(err) {
if (success) {
return something(); //if we here, break mapSeries
}
resolve();
});
})
}))
.then(() => {
return something();
})
How I can break from mapSeries, to prevent execution rest of functions?
Since you are returning something() you won't execute resolve(); so maybe try to resolve the promise before?
Promise.mapSeries(functions.map(function => {
return new Promise((resolve, reject) => {
function(req, res, function(err) {
if (success) {
return Promise.resolve(something()); //if we here, break mapSeries
}
});
})
}))
.then(() => {
return something();
})
Related
im trying to write a promise but seems to be missing something. here is my code:
const myPromise = new Promise(() => {
setTimeout(() => {
console.log("getting here");
return setinputs({ ...inputs, images: imageAsUrl });
}, 100);
});
myPromise
.then(() => {
console.log("getting here too");
firebase.database().ref(`collection/${idNode}`).set(inputs);
})
.then(() => {
console.log("all is set");
})
.catch((err) => {
console.log(err);
});
if i run the program, the first part of the promise is executing but all .then() functions arent executing. how do i fix this?
In this scheme, the promise callback has one (resolve) or two (resolve,reject) arguments.
let p = new Promise((resolve, reject)=> {
//do something
//resolve the promise:
if (result === "ok") {
resolve(3);
}
else {
reject("Something is wrong");
}
});
p.then(res => {
console.log(res); // "3"
}).catch(err => {
console.error(err); //"Something is wrrong
});
Of course, nowadays you can use async + await in a lot of cases.
You need to resolve the promise, using resolve() and also return the promise from firebase so the next .then in the chain works properly.
const myPromise = new Promise((resolve, reject) => {
setTimeout(() => {
console.log("getting here");
// You have to call resolve for all `.then` methods to be triggered
resolve({ ...inputs, images: imageAsUrl });
}, 100);
});
myPromise
.then((inputs) => {
console.log("getting here too");
// You have to return a promise in a .then function for the next .then to work properly
return firebase.database().ref(`collection/${idNode}`).set(inputs);
})
.then(() => {
console.log("all is set");
})
.catch((err) => {
console.log(err);
});
Reading other posts here the consensus is the forEach should be synchronous and blocking.
However I must have done something strange in my code because it doesn't appear that way:
var noDupes = false; // should be true but force no inserts for now
console.log('forEach');
courses.forEach((course) =>
{
const promiseNoDupe = new Promise((resolve, reject) =>
{
dbo.collection("courses").findOne({ id: course.id }, (err, result) =>
{
if (err) throw err;
if (result) { console.log('dupe'); return reject('dupe'); }
console.log('nodupe');
resolve('nodupe');
});
});
noDupes &= promiseNoDupe.then(() =>
{
console.log('true promise');
return true;
}).catch(() =>
{
console.log('false promise');
return false;
});
});
console.log('End forEach');
if (noDupes)
{
console.log('Inserting many');
dbo.collection("courses").insertMany(courses, (err, result) =>
{
if (err) return res.status(400).send(error.details[0].message);
res.send(courses);
});
}
else
{
console.log('No Dupes allowed');
res.status(400).send('Inserting duplicate ID not Allowed!');
}
Console output:
forEach
End forEach
No Dupes allowed
nodupe
true promise
nodupe
true promise
The end forEach is executed before the promise is completed and before any of the internal processing is conducted! Subsequently the logic waiting on the promise is processing ahead of time.
I'm not sure what is going wrong but I'm trying to wait for the completion of all checks in the forEach before committing any new records.
Thanks to charlietfl for steering me towards map() and Promise.all().
Here is the working code:
var dupePromises = courses.map((course) =>
{
return new Promise((resolve, reject) =>
{
dbo.collection("courses").findOne({ id: course.id }, (err, result) =>
{
if (err) throw err;
if (result) return reject(false);
resolve(true);
});
}).then(() =>
{
return true;
}).catch(() =>
{
return false;
});
});
Promise.all(dupePromises).then((results) =>
{
if (results.every((isnotDupe) => { return isnotDupe /* == true */ }))
{
dbo.collection("courses").insertMany(courses, (err, result) =>
{
if (err) return res.status(400).send(error.details[0].message);
res.send(courses);
});
}
else{
res.status(400).send('Inserting duplicate ID not Allowed!');
}
});
}
I have this following piece of code
new Promise((resolve, reject) => {
resolve(apiRequest(data))
reject(console.log('Error'))
}).then(response)
Both methods (resolve and reject) are being fired but I want to call reject only when something goes wrong.
How can I throw an error if something goes wrong on that case?
I checked that but it seems like I can not use an If statement to do that check.
new Promise((resolve, reject) => {
const printResult = apiRequest(data)
console.log(printResult) //Outputs Promise {<pending>}
resolve(printResult) //Then it works
reject(console.log('Error'))
}).then(response)
What would be the correct approach to reject a promise?
The easiest way would be with an if condition. i.e
new Promise((resolve, reject) => {
// do something...
if(somethingGoodHappened) {
resolve(data)
} else {
reject(error)
}
})
But usually when dealing with async requests, the thing you are calling will often be returning a promise, so you can attach the then and catch callbacks there.
apiRequest(data)
.then((result) => {
// all good
})
.catch((err) => {
console.log(err)
})
const mock_api = () => new Promise((res, rej) => {
const number = Math.floor((Math.random() * 100) + 1);
setTimeout(() => {
if (number%2==0) return res('randomly RESOLVED')
return rej('randomly REJECTED')
}, 2000)
})
const async_promise = () => new Promise(async (resolve, reject) => {
try {
const resolvedPromise = await mock_api()
resolve(resolvedPromise)
} catch (e) {
reject(e)
}
})
const classicPromise = () => new Promise((resolve, reject) => {
mock_api()
.then(resolve)
.catch(reject)
})
const makeAsyncRequest = async () => {
try {
const data = await async_promise()
console.log('ASYNC AWAIT RESOLVE', data)
} catch (e) {
console.log('ASYNC AWAIT ERR', e)
}
}
makeAsyncRequest()
classicPromise()
.then(r => console.log('PROMISE CHAIN RESOLVE', r))
.catch(e => console.log('PROMISE CHAIN ERR', e))
Because of you resolve before reject so it cannot run into reject,
You can use:
if (printResult) {
resolve(printResult)
} else {
reject(console.log('Error'))
}
You can catch exceptions and return them as rejected Promises
function asyncFunc() {
try {
doSomethingSync();
return doSomethingAsync()
.then(result => {
···
});
} catch (err) {
return Promise.reject(err);
}
}
Always check for err if there is any err return a promise (example below)
// Return new promise
return new Promise(function(resolve, reject) {
// Do async job
request.get(options, function(err, resp, body) {
if (err) {
reject(err);
} else {
resolve(JSON.parse(body));
}
})
})
I need to test function testMe using Mocha. But there is trouble when my unit test throw an error. Here is simpified example
function testMe(callback) {
new Promise((resolve, reject) => {
setTimeout(() => resolve([1,2,3]), 1000);
}).then((result) => {
callback(null, result);
}).catch((error) => {
callback(error, null)
});
}
testMe((err, result) => {
if(err) throw new Error();
if(result.length < 5) throw new Error();
});
In this example after throw runs catch block. But I need to run catch block only after reject.
EDIT:
In this case the script never stop. I don't understand why.
function testMe(callback) {
new Promise((resolve, reject) => {
setTimeout(() => resolve([1,2,3]), 1000);
}).then((result) => {
callback(null, result);
}, (error) => {
callback(error, null)
}).catch(() => {
console.log('Do not throw an error but still running');
});
}
testMe((err, result) => {
if(err) throw new Error();
if(result.length < 5) throw new Error();
});
When you work with promises then return the promises from functions instead of taking callbacks.
For example, instead of:
function testMe(callback) {
new Promise((resolve, reject) => {
// ...
});
}
use:
function testMe(callback) {
return new Promise((resolve, reject) => {
// ...
});
}
that way you will have the promise available to the caller of the function.
If you need to mix both styles, i.e. returning promises and taking callbacks, consider using a reliable library to handle that for you especially if you have trouble coding the translation between those style yourself:
http://bluebirdjs.com/docs/api/ascallback.html
http://bluebirdjs.com/docs/api/promise.promisify.html
You can simply return the promise from the test:
function testMe() {
// ^^ drop the callback
return new Promise((resolve, reject) => {
// ^^^^^^ return the promise
setTimeout(() => resolve([1,2,3]), 1000);
});
}
var p = testMe().then(result) => {
// ^^^^^ use the promise
if(result.length < 5) throw new Error();
});
return p; // to mocha
I want to stop promise chain after it resolved via some conditions. Below code is might useful to understand what am I saying.
function update(id, data) {
return new Promise((resolve, reject) => {
let conn;
pool.get()
.then((db) => {
conn = db;
if(Object.keys(data).length === 0) {
return resolve({ updated: 0 });
}
else {
return generateHash(data.password);
}
})
.then((hash) => {
conn.query("UPDATE ... ", (err, queryResult) => {
if(err) {
throw err;
}
resolve({ updated: queryResult.affectedRows });
});
})
.catch((err) => { ... })
});
}
Note that pool.get() is promise wrapped API for getting connection pool from MySQL module that I made.
What I'm trying to do is updating user data. And for save server resources, I avoided to update if no data to update(Object.keys(data).length === 0).
When I tried this code, second then(updating db) is always happening even if no data to update!
I read this post, but it didn't worked. Why the promise chain wasn't stopped when I called "return resolve();"? And how to I stop it properly? I really like using Promises, but sometimes, this kind of things make me crazy. It will be very appreciate to help me this problem. Thanks!
P.S. I'm using node v6.2.2 anyway.
Why the promise chain wasn't stopped when I called "return resolve();"?
You've returned from the current then callback and fulfilled the outer promise. But that doesn't "stop" anything, then then chain still will continue by resolving with the return value of the callback.
And how to I stop it properly?
You need to put the then call inside the if to have the condition apply to it:
pool.get()
.then((db) => {
…
if (Object.keys(data).length === 0) {
…({ updated: 0 });
} else {
return generateHash(data.password)
.then((hash) => {
conn.query("UPDATE ... ", (err, queryResult) => {
…
});
})
}
})
.catch((err) => { ... })
And in any case, you should avoid the Promise constructor antipattern! You should only promisify the query method:
function query(conn, cmd) {
return new Promise((resolve, reject) => {
conn.query(cmd, (err, queryResult) => {
if (err) reject(err); // Don't throw!
else resolve(queryResult);
});
});
}
and then use that:
function update(id, data) {
return pool.get()
.then(conn => {
if (Object.keys(data).length === 0) {
conn.close(); // ???
return { updated: 0 };
} else {
return generateHash(data.password)
.then(hash => {
return query(conn, "UPDATE ... ")
}).then(queryResult => {
conn.close(); // ???
return { updated: queryResult.affectedRows };
}, err => {
…
conn.close(); // ???
});
}
});
}
Notice that it might not make sense to get a connection from the pool if you can know beforehand that no query will be made, so probably you should put the if on the top level:
function update(id, data) {
if (Object.keys(data).length === 0) {
return Promise.resolve({ updated: 0 });
} else {
return pool.get()
.then(conn => {
return generateHash(data.password)
.then(hash => {
return query(conn, "UPDATE ... ")
}).then(queryResult => {
conn.close(); // ???
return { updated: queryResult.affectedRows };
}, err => {
…
conn.close(); // ???
});
});
}
}
This would be a good situation to use an if statement:
function update(id, data) {
if (Object.keys(data).length === 0) {
return Promise.resolve({ updated: 0 });
}
let conn;
return pool.get()
.then((db) => {
conn = db;
return generateHash(data.password);
})
.then((hash) => {
return new Promise(function (resolve, reject) {
conn.query("UPDATE ... ", (err, queryResult) => {
if(err) {
reject(err);
}
resolve({ updated: queryResult.affectedRows });
});
});
})
.catch((err) => { ... })
}