I'm using the popular node library, got, to make simple GET requests to a JSON API.
I have a function that abstracts the request, like so:
function performRequest(url) {
got(url, {
json: true
}).then(function (response) {
return formatResponse(response.body);
}).catch(function (error) {
console.log(error.response.body);
});
}
formatResponse is a simple synchronous method that modifies the JSON returned from the API.
I would like to be able to call performRequest from another function and then use the return value (once resolved). Currently, as performRequest is not recognized as an async method, my code is calling it and then proceeding immediately.
function myBigFunction() {
var url = composeUrl();
var res = performRequest(url);
doMoreStuffWithResponse(res);
}
I know that I need to utilize a Promise, however, I'm always unclear as to how to use a Promise in conjunction with a built-in library function that is already using a Promise (like in this case).
I'm also completely open to the possibility that I'm going about this all wrong. In that case, I would appreciate some redirection.
Thank you for your time.
Understand what a Promise is. Its a value, you can treat it as such. In order to "read" the value, you pass a function to the Promise's then method. You don't need myBigFunction. Anything you want to run after the Promise resolves just needs to be passed to then:
var req = performRequest(composeURL());
req.then(doStuffWithResponse);
Now, I don't particularly care for this way although I do it fairly often. I prefer to have functions that take promises and invoke their then method:
var takesAPromise = function(p) {
return p.then(/* does stuff */);
};
Note that it returns the Promise of the completed task. But what I like even better is this ES6 one-liner:
let wrap = f => p => p.then(val => f.call(null, val));
Now you can wrap arbitrary functions to take Promises as input and return them as output. If Promises were a monad, this would be their bind function. Making it work seamlessly with functions of arbitrary arity is left as an exercise to the reader.
You'll always want to return a promise from your functions:
function performRequest(url) {
return got(url, {
//^^^^^^
json: true
}).then(function(response) {
return formatResponse(response.body);
}, function(error) {
throw new Error(error.response.body);
});
}
With this, you can wait for the result in your big functions using another then:
function myBigFunction() {
var url = composeUrl();
var promise = performRequest(url);
return promise.then(function(res) {
return doMoreStuffWithResponse(res);
});
}
or in short
function myBigFunction() {
return performRequest(composeUrl()).then(doMoreStuffWithResponse);
}
so that you can call it like
myBigFunction().catch(function(error) {
console.log(error.message);
});
Related
I am trying to use a promise to call a function getTweets.
Not using an AJAX call, but a simple promise 'call' from 1 javascript file to another.
The function works, but i keep getting 'undefined'.
I have read dozens of questions here on stackoverflow and have spent days
to understand promises, but still can't solve it.
var Twit = require('twit') // Imports the Twitter library
require('dotenv').config() // to get the environment vars into the app
// This is the function:
function getTweets (screen_name) {
let T = new Twit({ /* <twitter key and token here> */ });
T.get('statuses/user_timeline', { screen_name: screen_name, count: 3}, function (err, data, response) {
let myTweets = [];
for (let i in data) {
let text = data[i].text;
myTweets.push(text);
}
return myTweets;
})
}
module.exports.getTweets = getTweets;
And this is the promise that tries to get the tweets:
var promise = tweets.getTweets('dizid');
promise.then(
console.log(myTweets),
console.log(err))
// gives error: promise.then(
// ^
// TypeError: Cannot read property 'then' of undefined
Any help greatly appreciated.
Your problem is that you never return anything from your getTweets() function even though it needs to return a promise. The function calls T.get() and pass it a callback function. You return from this callback function but this doesn't do anything, it doesn't mean that this value gets returned from getTweets().
This is a pretty common mistake when it comes to working with asynchronous calls. What needs to be done is to make getTweets() return a promise that gets resolved when it should.
When working with asynchronous calls that don't implement the promise interface, you need to wrap this call with a new promise. Your getTweets() function should then look like this:
function getTweets (screen_name) {
let T = new Twit({ /* <twitter key and token here> */ });
return new Promise(function(resolve, reject) {
T.get('statuses/user_timeline', { screen_name: screen_name, count: 3}, function (err, data, response) {
if (err) {
reject(err); // Reject the promise since there was an error
} else {
let myTweets = [];
for (let i in data) {
let text = data[i].text;
myTweets.push(text);
}
resolve(myTweets); // Resolve the promise with the result
}
});
});
}
However, it seems the Twit API does support the promise interface, so instead of providing a callback function you can just use the promise created by T.get(). HMR's answer explains how to do this.
Another mistake you've made is with this code:
promise.then(
console.log(myTweets),
console.log(err))
The way you've written it, it reads "Run console.log(myTweets) and console.log(err), then invoke promise.then() with the result of the former as the first argument and the result of the latter as the second argument.
then() takes callback functions (which get invoked depending on the resolving/rejection of the promise) as arguments, so the code should look like this:
promise.then(
function(myTweets) {
console.log(myTweets);
},
function(err) {
console.log(err);
});
Async/await
If you're interested in taking things further, the modern approach to working with asynchronous code is async/await, which is syntactic sugar for promises that lets you write asynchronous code more similar to regular synchronous code.
A function marked as async will implicitly return a promise, but you write it as if you return a regular value. Using the await keyword inside an async function will implicitly wait for a promise to resolve and unwrap the resolved value. The main practical benefits of this is that you can use asynchronous calls in loops and handle errors with regular try-catch blocks. Your getTweets() function would look like this using async/await:
async function getTweets(screen_name) {
let T = new Twit({ /* <twitter key and token here> */ });
const data = await T.get('statuses/user_timeline', { screen_name: screen_name, count: 3});
// Let's also use map() instead of a for loop
let myTweets = data.map(function(item) { return item.text; });
return myTweets;
}
Since get seems to return a promise you don't need to use a callback. Get Tweets can look something like this:
// in getTweets
return T.get(
'statuses/user_timeline',
{ screen_name: screen_name, count: 3}
).then(
function (data) {
console.log("data:",JSON.stringify(data,undefined,2));
return data.map(item=>item.text);
}
)
// exports the function getTweets so that other modules can use it
module.exports.getTweets = getTweets;
If that didn't work please let us know what the output of the program is (update question).
You can call getTweets like so:
tweets.getTweets('dizid')
.then(
myTweets=>
console.log(myTweets),
err=>
console.log(err)
)
I think you forget add function like
promise.then(function(res){
//code
}
Your .then() should include a call back function.
promise.then( res => {
console.log(res);
});
edit: I'm using an ES6 syntax for arrow functions, in case you're new to that.
How to add code that will run only when both processes are completed?
normalise1();
normalise2();
function normalise1() {
return knex("ingredients")
.select("name", "id")
.map(function (ingredient) {
var normalised_name = common.normalise(ingredient.name);
knex('ingredients').where('id', ingredient.id).update({ name_normalised: normalised_name }).then();
});
};
function normalise2() {
return knex("synonyms")
.select("synon_name as name", "id")
.map(function (ingredient) {
var normalised_name = common.normalise(ingredient.name);
knex('synonyms').where('id', ingredient.id).update({ synon_name_normalised: normalised_name }).then();
});
};
I tried something like in different ways
Promise.all([normalise1(), normalise2()])
.then(() => console.log('Done'));
but it didn't work.
Basically console.log('Done') appears before all process is done. I believe that this is because of missing Promise part inside functions, but I cannot figure out exactly how.
The functions are not called when passed to Promise.all(), no Promise is returned from .map().
Call the functions and return knex() from .map() call, which may also require using Promise.all() within the function calls.
I have many asynchronous methods to execute and my program flows can change a lot depending on each method return. The logic below is one example. I could not write it in a easy-to-read way using Promises. How would you write it?
Ps: more complex flows are welcome.
Ps2: is_business is a predefined flag where we say whether we are writing a "business user" or a "person user".
begin transaction
update users
if updated
if is_business
update_business
if not updated
insert business
end if
else
delete business
end if
else
if upsert
insert user
if is_business
insert business
end if
end if
end if
commit transaction
The nice thing about promises is that they make a simple analogy between synchronous code and asynchronous code. To illustrate (using the Q library):
Synchronous:
var thisReturnsAValue = function() {
var result = mySynchronousFunction();
if(result) {
return getOneValue();
} else {
return getAnotherValue();
}
};
try {
var value = thisReturnsAValue();
console.log(value);
} catch(err) {
console.error(err);
}
Asynchronous:
var Q = require('q');
var thisReturnsAPromiseForAValue = function() {
return Q.Promise(function() {
return myAsynchronousFunction().then(function(result) {
if(result) {
// Even getOneValue() would work here, because a non-promise
// value is automatically cast to a pre-resolved promise
return getOneValueAsynchronously();
} else {
return getAnotherValueAsynchronously();
}
});
});
};
thisReturnsAPromiseForAValue().then(function(value) {
console.log(value);
}, function(err) {
console.error(err);
});
You just need to get used to the idea that return values are always accessed as arguments to then-callbacks, and that chaining promises equates to composing function calls (f(g(h(x)))) or otherwise executing functions in sequence (var x2 = h(x); var x3 = g(x2);). That's essentially it! Things get a little tricky when you introduce branches, but you can figure out what to do from these first principles. Because then-callbacks accept promises as return values, you can mutate a value you got asynchronously by returning another promise for an asynchronous operation which resolves to a new value based on the old one, and the parent promise will not resolve until the new one resolves! And, of course, you can return these promises from within if-else branches.
The other really nice thing illustrated in the example above is that promises (at least ones that are compliant with Promises/A+) handle exceptions in an equally analogous way. The first error "raised" bypasses the non-error callbacks and bubbles up to the first available error callback, much like a try-catch block.
For what it's worth, I think trying to mimic this behavior using hand-crafted Node.js-style callbacks and the async library is its own special kind of hell :).
Following these guidelines your code would become (assuming all functions are async and return promises):
beginTransaction().then(function() {
// beginTransaction() has run
return updateUsers(); // resolves the boolean value `updated`
}).then(function(updated) {
// updateUsers() has "returned" `updated`
if(updated) {
if(isBusiness) {
return updateBusiness().then(function(updated) {
if(!updated) {
return insertBusiness();
}
// It's okay if we don't return anything -- it will
// result in a promise which immediately resolves to
// `undefined`, which is a no-op, just like a missing
// else-branch
});
} else {
return deleteBusiness();
}
} else {
if(upsert) {
return insertUser().then(function() {
if(isBusiness) {
return insertBusiness();
}
});
}
}
}).then(function() {
return commitTransaction();
}).done(function() {
console.log('all done!');
}, function(err) {
console.error(err);
});
The solution is a mix of #mooiamaduck answer and #Kevin comment.
Using promises, ES6 generators and co library makes the code much clearer. I found a good example when reading a postgresql node library example (pg). In the example below pool.connect and client.query are asynchronous operations that returns Promises. We can easily add an if/else after geting result and then make more async operations keeping code looking like synchronous.
co(function * () {
var client = yield pool.connect()
try {
yield client.query('BEGIN')
var result = yield client.query('SELECT $1::text as name', ['foo'])
yield client.query('INSERT INTO something(name) VALUES($1)', [result.rows[0].name])
yield client.query('COMMIT')
client.release()
} catch(e) {
// pass truthy value to release to destroy the client
// instead of returning it to the pool
// the pool will create a new client next time
// this will also roll back the transaction within postgres
client.release(true)
}
})
I'm new on AngularJS and JavaScript.
I am getting remote information for each of the elements of an array (cars) and creating a new array (interested prospects). So I need to sync the requests. I need the responses of each request to be added in the new array in the same order of the cars.
I did it first in with a for:
for (a in cars) {
//async request
.then(function () {
//update the new array
});
}
This make all the requests but naturally didn't update the new array.
After seeking in forums, I found this great examples and explanations for returning a intermediate promise and sync all of them.
1. http://pouchdb.com/2015/05/18/we-have-a-problem-with-promises.html
2. http://stackoverflow.com/questions/25605215/return-a-promise-from-inside-a-for-loop
3. http://www.html5rocks.com/en/tutorials/es6/promises/
(#MaurizioIndenmark, #Mark Rajcok , #Michelle Tilley, #Nolan Lawson)
I couldn't use the Promise.resolve() suggested in the second reference. So I had used $q.defer() and resolve(). I guess I have to inject a dependency or something else that I missed. As shown below:
In the Controller I have:
$scope.interestedProspects = [] ;
RequestDetailsOfAsync = function ($scope) {
var deferred = $q.defer();
var id = carLists.map(function (car) {
return car.id;
}).reduce(function (previousValue, currentValue) {
return previousValue.then(function () {
TheService.AsyncRequest(currentValue).then(function (rData) {
$scope.interestedProspects.push(rData);
});
});
}, deferred.resolve());
};
In the Service I have something like:
angular.module('app', []).factory('TheService', function ($http) {
return {
AsyncRequest = function (keyID) {
var deferred = $q.defer();
var promise = authorized.get("somep.provider.api/theService.json?" + keyID).done(function (data) {
deferred.resolve(data);
}).fail(function (err) {
deferred.reject(err);
});
return deferred.promise;
}
}
}
The displayed error I got: Uncaught TypeError: previousValue.then is not a function
I made a jsfiddle reusing others available, so that it could be easier to solve this http://jsfiddle.net/alisatest/pf31g36y/3/. How to wait for AsyncRequests for each element from an array using reduce and promises
I don't know if the mistakes are:
the place where the resolve is placed in the controller function.
the way the reduce function is used
The previousValue and currentValue sometimes are seen by javascript like type of Promise initially and then as a number. In the jsfiddle I have a working example of the use of the reduce and an http request for the example.
Look at this pattern for what you want to do:
cars.reduce(function(promise, car) {
return promise.then(function(){
return TheService.AsyncRequest(car).then(function (rData) {
$scope.details.push(rData);
});
});
}, $q.when());
This will do all the asynchronous calls for every car exactly in the sequence they are in the cars array. $q.all may also be sufficient if the order the async calls are made doesn't matter.
It seems you are calling reduce on an array of ids, but assume in the passed function that you are dealing with promises.
In general, when you want to sync a set of promises, you can use $q.all
You pass an array of promises and get another promise in return that will be resolved with an array of results.
I'm still struggling with promises, but making some progress thanks to the community here.
I have a simple JS function which queries a Parse database. It's supposed to return the array of results, but obviously due to the asynchronous nature of the query (hence the promises), the function returns before the results, leaving me with an undefined array.
What do I need to do to make this function wait for the result of the promise?
Here's my code:
function resultsByName(name)
{
var Card = Parse.Object.extend("Card");
var query = new Parse.Query(Card);
query.equalTo("name", name.toString());
var resultsArray = [];
var promise = query.find({
success: function(results) {
// results is an array of Parse.Object.
console.log(results);
//resultsArray = results;
return results;
},
error: function(error) {
// error is an instance of Parse.Error.
console.log("Error");
}
});
}
Instead of returning a resultsArray you return a promise for a results array and then then that on the call site - this has the added benefit of the caller knowing the function is performing asynchronous I/O. Coding concurrency in JavaScript is based on that - you might want to read this question to get a broader idea:
function resultsByName(name)
{
var Card = Parse.Object.extend("Card");
var query = new Parse.Query(Card);
query.equalTo("name", name.toString());
var resultsArray = [];
return query.find({});
}
// later
resultsByName("Some Name").then(function(results){
// access results here by chaining to the returned promise
});
You can see more examples of using parse promises with queries in Parse's own blog post about it.
What do I need to do to make this function wait for the result of the promise?
Use async/await (NOT Part of ECMA6, but
available for Chrome, Edge, Firefox and Safari since end of 2017, see canIuse)
MDN
async function waitForPromise() {
// let result = await any Promise, like:
let result = await Promise.resolve('this is a sample promise');
}
Added due to comment:
An async function always returns a Promise, and in TypeScript it would look like:
async function waitForPromise() {
// let result = await any Promise, like:
let result: Promise<string> = await Promise.resolve('this is a sample promise');
}
You're not actually using promises here. Parse lets you use callbacks or promises; your choice.
To use promises, do the following:
query.find().then(function() {
console.log("success!");
}, function() {
console.log("error");
});
Now, to execute stuff after the promise is complete, you can just execute it inside the promise callback inside the then() call. So far this would be exactly the same as regular callbacks.
To actually make good use of promises is when you chain them, like this:
query.find().then(function() {
console.log("success!");
return new Parse.Query(Obj).get("sOmE_oBjEcT");
}, function() {
console.log("error");
}).then(function() {
console.log("success on second callback!");
}, function() {
console.log("error on second callback");
});
You don't want to make the function wait, because JavaScript is intended to be non-blocking.
Rather return the promise at the end of the function, then the calling function can use the promise to get the server response.
var promise = query.find();
return promise;
//Or return query.find();