declaring a function to wait for a promise - javascript

I have the following function:
function JSON_to_buffer(json) {
let buff = Buffer.from(json);
if ( json.length < constants.min_draft_size_for_compression) {
return buff;
}
return zlib.deflate(buff, (err, buffer) => {
if (!err) {
return buffer;
} else {
return BPromise.reject(new VError({
name: 'BufferError',
}, err));
}
});
}
I want to be able to run this but have it wait if it goes to the unzip call. I'm currently calling this within a promise chain and it's going back to the chain without waiting for this and returns after the previous promise chain has completed.

You can put that logic into a Promise along with an async function
function JSON_to_buffer(json) {
return new Promise(function (resolve, reject) {
let buff = Buffer.from(json);
if (json.length < constants.min_draft_size_for_compression) {
return resolve(buff);
}
zlib.deflate(buff, (err, buffer) => {
if (!err) {
resolve(buffer);
} else {
reject(err);
/*return BPromise.reject(new VError({
name: 'BufferError',
}, ));*/
}
});
});
}
async function main() {
try {
let buffer = await JSON_to_buffer(myJSON);
} catch (e) {
console.log(e.message);
}
}

Related

how to get return value of promise

Here is a function to find mx records of a service and i need to save the one value(with the lowest priority) to make a request to it. How can I save and return this value?
const dns = require('dns');
const email = '...#gmail.com'
let res = email.split('#').pop();
function getMxRecords(domain) {
return new Promise(function(resolve, reject) {
dns.resolveMx(domain, function(err, addresses) {
if (err) {
//console.log(err, err.stack)
resolve(null);
} else {
//console.log(addresses);
let copy = [...addresses];
//console.log(copy);
let theone = copy.reduce((previous, current) => {
if (previous.priority < current.priority) {
return current;
}
return previous;
});
resolve(theone);
}
});
});
}
let a = getMxRecords(res);
console.log(a);
Yeah, so i need to export this module to make a request to it like below;
let socket = net.createConnection(25, request(email), () => {})
so for this my function should request me or array or object with only one value, when i'm trying it doesn't work, i always get this:
Promise { } //HERE IS RETURN FROM MY FUNCTION (WITH .THEN)
Error in socket connect ECONNREFUSED 127.0.0.1:25
A Promise is mostly an asynchronous call. It returns an Promise-Object that will resolve or reject the Promise. To access the result, you will call some functions:
function getMxRecords(domain) {
return new Promise(function(resolve, reject) {
dns.resolveMx(domain, function(err, addresses) {
if (err) {
//console.log(err, err.stack)
resolve(null);
} else {
//console.log(addresses);
let copy = [...addresses];
//console.log(copy);
let theone = copy.reduce((previous, current) => {
if (previous.priority < current.priority) {
return current;
}
return previous;
});
resolve(theone);
}
});
});
}
getMxRecords(res)
.then(yourResolveValueProvided => {
// Your code if the promise succeeded
})
.catch(error => {
// Your code if the promises reject() were called. error would be the provided parameter.
})

Callback function is not executed within returned Promise

I have two functions that return promise. The first one provide host value, and the second one use the host value to get IP address. I can see that the first function is running without any issue. But looks like the callback function side getHostIps is not executed at all. Not sure why it happens....what's wrong with my promise function?
my promise chain:
getHostedZoneId(dns)
.then(hostZoneId => {
getHostIps(dns, hostZoneId);
})
.then(hostIps => {
logger.Info(hostIps); //hostIps is undefined
})
.catch(err => logger.error(err));
getHostedZoneId:
var getHostedZoneId = function(dns) {
var params = {
DNSName: dns,
};
return new Promise((resolve, reject) => {
findHostZoneByDNS(params, function(err, data) {
if(err) {
reject(err);
}
else {
resolve(data);
}
});
});
}
getHostIps:
var getHostIps = function(dns, hostZoneId) {
var params = {
HostedZoneId: hostZoneId,
StartRecordName: dns,
};
return new Promise((resolve, reject) => {
findHostIps(params, function(err, data) {
//logger.info("get there");
if(err) {
reject(err);
}
else {
resolve(data);
}
});
});
}
I logged hostIps and err and data, all of them are defined. So I am sure that the callback function inside promise is not executed. But not sure how to fix it.
Any feedback is appreciated! Thanks!
You have to return the promise from your then statement to complete the chain.
getHostedZoneId(dns)
.then(hostZoneId => {
return getHostIps(dns, hostZoneId); // Add return
})
.then(hostIps => {
logger.Info(hostIps);
})
.catch(err => logger.error(err));

Calling a promise function recursively

I'm trying to call a promise function recursively.
The following call service.getSentenceFragment() returns upto 5 letters from a sentence i.e. 'hello' from 'helloworld. Providing a nextToken value as a parameter to the call returns the next 5 letters in the sequence. i.e. 'world'. The following code returns 'hellohelloworldworld' and does not log to the console.
var sentence = '';
getSentence().then(function (data)) {
console.log(sentence);
});
function getSentence(nextToken) {
return new Promise((resolve, reject) => {
getSentenceFragment(nextToken).then(function(data) {
sentence += data.fragment;
if (data.nextToken != null && data.nextToken != 'undefined') {
getSentence(data.NextToken);
} else {
resolve();
}
}).catch(function (reason) {
reject(reason);
});
});
}
function getSentenceFragment(nextToken) {
return new Promise((resolve, reject) => {
service.getSentenceFragment({ NextToken: nextToken }, function (error, data) {
if (data) {
if (data.length !== 0) {
resolve(data);
}
} else {
reject(error);
}
});
});
}
Cause when you do this:
getSentence(data.NextToken);
A new Promise chain is started, and thecurrent chain stays pending forever. So may do:
getSentence(data.NextToken).then(resolve, reject)
... but actually you could beautify the whole thing to:
async function getSentence(){
let sentence = "", token;
do {
const partial = await getSentenceFragment(token);
sentence += partial.fragment;
token = partial.NextToken;
} while(token)
return sentence;
}
And watch out for this trap in getSentenceFragment - if data is truthy but data.length is 0, your code reaches a dead end and the Promise will timeout
// from your original getSentenceFragment...
if (data) {
if (data.length !== 0) {
resolve(data);
}
/* implicit else: dead end */
// else { return undefined }
} else {
reject(error);
}
Instead, combine the two if statements using &&, now our Promise will always resolve or reject
// all fixed!
if (data && data.length > 0)
resolve(data);
else
reject(error);
You could recursively call a promise like so:
getSentence("what is your first token?")
.then(function (data) {
console.log(data);
});
function getSentence(nextToken) {
const recur = (nextToken,total) => //no return because there is no {} block so auto returns
getSentenceFragment(nextToken)
.then(
data => {
if (data.nextToken != null && data.nextToken != 'undefined') {
return recur(data.NextToken,total + data.fragment);
} else {
return total + data.fragment;
}
});//no catch, just let it go to the caller
return recur(nextToken,"");
}
function getSentenceFragment(nextToken) {
return new Promise((resolve, reject) => {
service.getSentenceFragment({ NextToken: nextToken }, function (error, data) {
if (data) {
if (data.length !== 0) {
resolve(data);
}
} else {
reject(error);
}
});
});
}

How to stop promise chain after resolve?

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) => { ... })
}

Blue bird return promise missing warning

I use the following code and I got the warning
Warning: a promise was created in a handler but was not returned from
it
var Promise = require("bluebird");
var glob = Promise.promisify(require('glob'));
glob("files/*.js")
.then(function (files) {
files.forEach(function (file) {
...
invoke(data,response,fn,entity,req);
});
}).catch(function (err) {
console.log('error: ', err);
}
)
what should I do to avoid it?
I read the documentation of BB and it seems that I should return the promise
my question is where? I guess that inside the invoke function but not sure how
This is the inovke function
var invoke = function (data,res,fn,entity,req) {
....
for (var data in entity.app[0]) {
var name = entity.app[0].function;
try {
fn[name](req, res);
}
catch (err) {
res.status(500).send("Error" + err);
}
if (!res.headerSent) {
res.end("done", 200);
}
}
}
}
};
in addition I've tried to return the promise like following which doesnt work
var invoke = function (data,res,fn,entity,req) {
....
return new Promise(function (resolve, reject) {
for (var data in entity.app[0]) {
var name = entity.app[0].function;
try {
resolve(fn[name](req, res));
}
catch (err) {
res.status(500).send("Error" + err);
}
if (!res.headerSent) {
res.end("done", 200);
}
}
}
}
};
I dont want to suppress the warning I want to understand how to solve it...
If I need to add some additional info please let me know, Im stuck here:(
I didn't test the following code but you should have an approach like this:
var Promise = require("bluebird");
var glob = Promise.promisify(require('glob'));
glob("files/*.js").then(function (files) {
return Promise.all(files.map(function (file) {
return invoke(data,response,fn,entity,req);
})
);
}).catch(function (err) {
console.log('error: ', err);
res.status(500).send("Error" + err);
}).finally(function(){
if (!res.headerSent) {
res.end("done", 200);
}
});
var invoke = function (data,res,fn,entity,req) {
return new Promise(function (resolve, reject) {
for (var data in entity.app[0]) {
var name = entity.app[0].function;
try {
fn[name](req, res);
}
catch (err) {
reject(err);
}
}
}
};

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