Using a function to make a http request with NodeJS. Passing a previously defined array as argument when running the function but the array is empty when console logged.
var testList = []:
var httpQuery = function(ep, q, lst) {
ep.selectQuery(q).then(function (res) {
return res.json()
}).then(function (result) {
lst = result.results.bindings;
}).catch(function (err) {
console.error(err)
})
};
httpQuery(endpoint, testq, testList);
How can I get the data from the function callback in to my previously defined array and use it outside of the function?
If you were console.logging 'testList' after calling httpQuery, then of course it would be empty. httpQuery is asyncronous, (and reassigning lst inside the fucntion wouldn't have worked anyway).
To take care of asyncronous promise-based functions, you have to return the promise and use .then()
var httpQuery = function(ep, q) {
return ep.selectQuery(q).then(function (res) {
return res.json()
}).then(function (result) {
return result.results.bindings;
}).catch(function (err) {
console.error(err)
})
};
httpQuery(endpoint, testq)
.then(list => {
console.log(list)
});
[edit] Depending on what version of node you're using, you may be able to use async/await. The above code is essentially equivalent to
async function httpQuery(ep, q) {
try {
let res = await ep.selectQuery(q);
let result = await res.json();
return result.results.bindings;
} catch(e) {
console.error(e);
}
}
(async () => {
let testList = await httpQuery(endpoint, testq);
console.log(testList);
})();
Related
Today, when I was working with node, I met some special async functions with "overloads" that accept both promises and callbacks. Like this:
doSomething(result => {
console.log(result)
})
doSomething()
.then(result => console.log(result))
And probably this:
const result = await doSomething()
console.log(result)
I tried to implement this in my code but was unsuccessful. Any help would be appreciated.
You can make a function like this by creating a promise, then chaining on that promise with the argument if there is one, and then returning that chained promise. That will make it call the callback at the appropriate time, as well as giving you access to the promise that will complete when the callback completes. If you want the original promise even when there's a callback (not the chained version), then you can return that instead, by still chaining but then returning the original promise instead.
function f(cb) {
let promise = new Promise((resolve, reject) => {
setTimeout(() => resolve(123), 1000);
});
if (cb) {
return promise.then(cb);
} else {
return promise;
}
}
// usage 1
f(console.log)
// usage 2
f().then(console.log)
Let's say you had a function that was going to read your config file and parse it and you wanted to support both versions. You could do that like this with two separate implementation inside. Note, this has full error handling and uses the nodejs calling convention for the callback that passes parameters (err, result):
function getConfigData(filename, callback) {
if (typeof callback === "function") {
fs.readFile(filename, function(err, data) {
try {
if (err) throw err;
let result = JSON.parse(data);
callback(null, result);
} catch(e) {
callback(err);
}
});
} else {
return fs.promises.readFile(filename).then(data => {
return JSON.parse(data);
}).
}
}
This could then be used as either:
getConfigData('./config.json').then(result => {
console.log(result);
}).catch(err => {
console.log(err);
});
configData('./config.json', (err, data) => {
if (err) {
console.log(err);
} else {
console.log(data);
}
});
Depending upon the specific asynchronous operation, it may be better or more efficient to have two separate implementations internally or to have one implementation that you adapt at the end to either a callback or a promise.
And, there's a useful helper function if you adapt a promise to a callback in multiple places like this:
function callbackHelper(p, callback) {
if (typeof callback === "function") {
// use nodejs calling convention for callbacks
p.then(result => {
callback(null, result);
}, err => {
callback(err);
});
} else {
return p;
}
}
That lets you work up a simpler shared implementation:
function getConfigData(filename, callback) {
let p = fs.promises.readFile(filename).then(data => {
return JSON.parse(data);
});
return callbackHelper(p, callback);
}
I want to use the async function to bring out a particular value from my database to my the function global so I can use it in other parts of my application.
async function dimension() {
const result = await Settings.find({_id : "5d7f77d620cf10054ded50bb"},{dimension:1}, (err, res) => {
if(err) throw new Error(err.message, null);
const holder = res[0].dimension;
return holder;
console.log(holder) /// this print the expected result, that i want to make global
});
return {
result
};
};
console.log(dimension())
but the console.log of the dimension() gives me this
Promise { <pending> }
instead of the same value that
console.log(holder)
gives me nothing.
The problem is you are printing the result of dimension() as soon as you call it, but since this function is async, it returns a promise that is not yet resolved.
You do not need to use async/await here. Settings.find() seems to return a Promise. You can just return directly this Promise and use .then() to do something once that promise is resolved.
Like this :
function dimension () {
return Settings.find({ _id: '5d7f77d620cf10054ded50bb' }, { dimension: 1 }, (err, res) => {
if (err) {
throw new Error(err.message, null);
}
return res[0].dimension;
});
}
dimension().then(result => {
//print the result of dimension()
console.log(result);
//if result is a number and you want to add it to other numbers
var newResult = result + 25 + 45
// the variable "newResult" is now equal to your result + 45 + 25
});
More info on Promises and async/await
You have to await for your result, like this:
const result = await dimension();
console.log(result);
In that case, you don't even to make the original function async, just write it like this:
function dimension() {
return Settings.find({_id : "5d7f77d620cf10054ded50bb"},{dimension:1}, (err, res) => {
if(err) throw new Error(err.message, null);
const holder = res[0].dimension;
return holder;
});
};
async function myGlobalFunc() {
const result = await dimension();
console.log(result);
}
The best way to have this globally available is to just put your function dimension in a file somewhere. Then where you need the value, you just require it and await its value. E.g.
// get-dimension.js
// ...const Settings = require... comes here
module.exports = function getDimension() {
return Settings.find({_id : "5d7f77d620cf10054ded50bb"},{dimension:1}, (err, res) => {
if(err) throw new Error(err.message, null);
const holder = res[0].dimension;
return holder;
});
}
// your other modules, e.g.
// my-service-handler.js
const getDimesion = require('./get-dimension');
async function myServiceHandler() {
const dimension = await getDimension();
// do stuff with dimension.
}
You're using async/await, but you're mixing it with callbacks, this is not desirable as it leads to confusion. It's not clear what you expect to happen in the callback, but return holder; likely doesn't do what you expect it to do, returning from a callback does not work the same way returning from a promise handler works. Your entire implementation should work with promises so that the async/await syntax reads more naturally (as it was intended).
async function dimension() {
// We're already awaiting the result, no need for a callback...
// If an error is thrown from Settings.find it is propagated to the caller,
// no need to catch and rethrow the error...
const res = await Settings.find({_id: "5d7f77d620cf10054ded50bb"}, {dimension: 1});
return {result: res[0].dimension};
}
(async () => {
try {
console.log(await dimension());
} catch (err) {
console.error(err);
}
})();
Use dimension().then() in your code then it will work fine.
async function globalDimension() {
const data = await Users.findOne({ phone: 8109522305 }).exec();
return data.name;
}
globalDimension().then(data => {
console.log(data);
});
My Codes below;
I've a then-catch block. My responseArray is a global variable. i got response from functionName function; but i can't use result out of then block. How can i use then response out of block?
My Codes below;
I've a then-catch block. My responseArray is a global variable. i got response from functionName function; but i can't use result out of then block. How can i use then response out of block?
module.exports = {
foo1: function(param){
return new Promise((resolve,reject) => {
var result = //some code here
resolve(result);
});
},
foo2: function(param){
return new Promise((resolve,reject) => {
this.foo1('abc').then(function(res){
let response = {
'item':'ok',
'result':res.some_field
};
console.log(response); // its ok here.
responseArray.push(response); //its ok here too
}).catch(err =>{
console.log(err);
reject(err);
});
console.log(responseArray); //nothing in array here
resolve(responseArray);
});
}
};
First thing to remember is that promises are asynchronous. Promises are doing exactly what they say, you are essentially signing a contract (promise) that you will get your data (or error) but not synchronously, but at some time in the future when the computations have finished.
In order to access your responseArray you will need to resolve your foo2 promise (inside of .then) and continue the promise chain by calling it, i.e.
module.exports = {
foo1: function(param){
return new Promise((resolve,reject) => {
var result = //some code here
resolve(result);
});
},
foo2: function(param){
return new Promise((resolve,reject) => {
this.foo1('abc').then(function(res){
let response = {
'item':'ok',
'result':res.some_field
};
console.log(response); // its ok here.
responseArray.push(response); //its ok here too
resolve(responseArray) // resolve the promise inside of .then
}).catch(err =>{
console.log(err);
reject(err);
});
});
}
};
foo2('someValue').then(response => {
console.log(response) // this will be your array
})
Also, as a side note, ensure you are not falling into the trap of the promise constructor anti-pattern. This is where you unnecessarily turn synchronous code into asynchronous code just for the sake of using "promises"
For example, a valid use of a promise would be to convert a callback, like so:
const getFile = filename => {
return new Promise((resolve, reject) => {
fs.readFile(filename, 'utf8', (err, data) => {
if (err) reject(err)
resolve(data)
})
})
}
whereas this is unnecessary:
const printData = data => {
return new Promise((resolve, reject) => {
resolve(console.log(data))
})
}
vs
const printData = data => {
console.log(data)
}
Read more here: What is the explicit promise construction antipattern and how do I avoid it?
I'm running NodeJS with pg-promise for accessing my PostgreSQL.
This is my test, which fails with undefined:
function a() {
pgPromise.one('SELECT 1')
.then(row => {
return row;
})
.catch(error => {
console.log(error);
});
}
console.log(a());
I asume it fails with undefined because it's running asyncronized.
I've tried to optimize with:
async function a() {
var output = null;
await pgPromise.one('SELECT 1')
.then(row => {
output = row;
})
.catch(error => {
console.log(error);
});
return output;
}
console.log(a());
But that simply gives me:
[2018-09-13 08:37:09.521] [LOG] Promise { <pending> }
You must await for the result, so that you don't get the promise but its result once it's solved:
console.log(await a());
But your code is way too complicated. Your a function makes no real sense.
Change
async function a() {
var output = null;
await pgPromise.one('SELECT 1')
.then(row => {
output = row;
})
.catch(error => {
console.log(error);
});
return output;
}
to
async function a() {
try {
return await pgPromise.one('SELECT 1')
} catch(error) {
console.log(error);
}
}
If you're getting a syntax error on the await, it probably means you're not in an async function. A typical case in node is when you're at the root. The simplest solution is to wrap it in an async function:
;(async function(){
console.log(await a());
})();
async functions return a Promise, so you need to await it or use .then() in order to get the result.
Remember that await can only be used inside an async function so you need to change the structure of your code.
I recommend you check the documentation and also more information asynchronous programming in Javascript.
Keeping your first code like that, it could be fixed like this (still is a bit weird in my opinion):
function a() {
return pgPromise.one('SELECT 1')
.then(row => {
return row;
})
.catch(error => {
console.log(error);
});
}
a().then(console.log);
If you want to go with async/await, this example could be good for learning purposes:
async function a() {
const row = await pgPromise.one('SELECT 1');
return row;
}
a().then(console.log).catch(console.error);
In this case the error handling is being done outside the function, in the caller side (it could be done inside through try/catch, it depends on the use case).
I need help with ES6 Promises chaining in array processing.
How to process/define each item of array which goes into Promise.all method, when there is other async method inside resolve?
Here is simplified example:
function getData(data, callback) {
let groupPromises = data.map(row => {
var coordinates = getCoordinates(row);
return Promise.resolve({
"place": getPlaces(coordinates), //how to invoke this method
"data": row
};
});
Promise.all(groupPromises)
.then(groups => callback(groups))
.catch(err => console.log(err));
}
}
function getPlaces(coordinates) {
return new Promise(function(resolve, reject) {
if(coordinates == null) {
reject();
}
parameters = {
location: [coordinates.latitude, coordinates.longitude],
rankby: "distance",
};
googlePlaces.searchPlace(parameters, function (error, response) {
if (error) {
reject(error);
};
resolve(response);
});
}
}
You can do it like this where you add a .then() handler to your first promise that gets the place and then when that's available returns the object you want. The resolved results of your Promise.all() will then be the array of objects you want:
function getData(data, callback) {
let groupPromises = data.map(row => {
var coordinates = getCoordinates(row);
// add .then() handler here to convert the place result
// into the object you want it in
return getPlaces(coordinates).then(place => {
return {place: place, data: row};
});
});
return Promise.all(groupPromises)
.then(groups => callback(groups))
.catch(err => {
console.log(err);
throw err;
});
}
}
function getPlaces(coordinates) {
return new Promise(function(resolve, reject) {
if(coordinates == null) {
reject();
}
parameters = {
location: [coordinates.latitude, coordinates.longitude],
rankby: "distance",
};
googlePlaces.searchPlace(parameters, function (error, response) {
if (error) {
reject(error);
};
resolve(response);
});
}
}
FYI, since you're converting over to promises, why not just return the promise from getData() and not use a callback there at all? Your current code has no way of communicating back an error from getData() which is something that comes largely for free with promises.
In fact with pure promises, getData() could be simplified to this:
function getData(data, callback) {
return Promise.all(data.map(row => {
return getPlaces(getCoordinates(row)).then(function(place) {
return {place: place, data: row};
});
}));
}