We Keep Coding

Passing parameters to a promise

I am trying to write a promise in such a way that I can pass some parameters. Inside, this promise will call a Fetch. This is the code I wrote so far:
const myFunction = (parA, parB, parC) => {
return new Promise ((resolve, reject) => {
url = ... // calculated based on the parameters passed above;
fetch(url)
.then(response => {
var object = response.json();
resolve(object);// the idea is that 'object' is returned by the outmost promise
console.log('Check');// So far OK, this is correctly printed to the console
})
.catch(err => {
console.log('Error: ' + err);
reject(err);
});
// resolve('1') // Used for test only. With this, everything works, but "object" here is undefined -- I guess it gets executed too early, before the completion of the Fetch
});
and this is where the promise is called
myFunction(a, b, c).then(res => {
console.log('OK');// just testing
console.log(res);// just testing
});
What happens is that the Fetch resolves OK, but my overall promise doesn't. The last two console.log instructions are never executed.
In short, my problem is: how can I resolve my promise returning the result from the Fetch? It's a promise inside a promise, I guess I could chain them with .then, but I also want to be able to pass parameters.
I also guess I could rewrite the code avoiding this promise chaining, but since I'm also doing it as a way of learning, I prefer to try to figure out first how to solve this problem with this structure.
p.s.: at the moment I am stuck with the promises, cannot use async/await because I am with an old version of node.js and cannot update it

As #VLAZ already mentioned, it's not necessary to create your own promise here, since fetch() itself already returns a promise.
Therefore, you could try this:
const myFunction = (parA, parB, parC) => {
const url = ... // calculated based on the parameters passed above;
return fetch(url)
.then(response => response.json())
.then(object => {
console.log('Check', object);
return object;
})
.catch(err => {
console.log('Error: ' + err);
});
};

How do I get data from pending resolved promise without async/await?

I have abstraction:
function fetchDataFromAPI() {
const url = `https://api...`
return fetch(url).then(response => response.json())
}
I want to use it in my other piece of code like:
if(something){
const data = fetchDataFromAPI()
return data
}
if I console.log data what I get is resolved pending promise
Promise {<pending>}
__proto__: Promise
[[PromiseStatus]]: "resolved"
[[PromiseValue]]: Object
How do I get that Object in data instead of Promise?

You can not. Here is why:
Promise is a language construct that makes JavaScript engine to continue to execute the code without waiting the return of inner function, also known as the executor function. A promise always run inside the event loop.
var p = new Promise(function(resolve, reject) {
setTimeout(function() {
resolve('foo');
}, 300);
});
console.log(p);
Basically a promise is a glorified syntactic sugar for a callback. We will see how but first lets have a more realistic code:
function someApiCall(){
return new Promise(function(resolve, reject){
setTimeout(()=>{
resolve('Hello');
})
})
}
let data = someApiCall();
console.log(data);
This is a so-called asynchronous code, when JavaScript engine executes it, someApiCall immediately returns a result, in this case pending promise:
> Promise {<pending>}
If you pay attention to the executor, you will see we needed to pass resolve and reject arguments aka callbacks. Yes, they are callbacks required by the language construct. When either of them called, promise will change its state and hence be settled. We don't call it resolved because resolving implies successful execution but a function also can error out.
How do we get the data? Well we need more callbacks, which will be called by the executor function once the promise is settled:
var p = new Promise(function(resolve, reject) {
setTimeout(function() {
resolve('foo');
}, 300);
});
p.then((result) => {
console.log(result); // foo
}).catch((err) => {
console.log(err);
});
Why we need to pass separate callbacks? Because one will be fed to the resolve, and the other to the reject. Then callback will be called by the resolve function, the catch callback by the reject function.
Javascript engine will execute these callbacks later on its leisure, for a regular function it means when the event loop is cleared, for timeout when the time is up.
Now to answer your question, how do we get data out from a promise. Well we can't.
If you look closely, you will see we don't really get the data out but keep feeding callbacks. There is no getting data out, but passing callbacks in.
p.then((result) => {
console.log(result);
}).catch((err) => {
console.log(err);
});
Some say use await:
async function() {
let result = await p;
}
But there is a catch. We have to or wrap it in async function. Always. Why? Because Async/await is another level of abstraction or syntactic sugar, whichever you prefer, on top of promise api. That is why we can not use await directly but always wrap it in async statement.
To sum up, when we use promise or async/await we need to follow certain convention and write terse code with closely knitted callbacks. Either javascript engine or transpilers like babeljs or typescript converts these code to regular javascript to be run.
I can understand your confusion because people keep saying getting data out when talking about promises, but we don't get any data out but pass callback to be executed when the data is ready.
Hope everything is clear now.

No, you cannot without using promises or async/await etc because calling a REST API is an asynchronous operation and is non blocking.
When you make a call to a REST API, the code shouldn't wait until the API returns a value because it may take a lot of time, making program non-responsive, thus by design making a network request is categorized as an asynchronous operation.

To avoid async/await, you'll need to use another .then:
if (something) {
return fetchDataFromAPI()
.then((data) => data /* you can console.log(data) here */)
}

Async / await vs then which is the best for performance?

I have a simple code in JavaScript that execute a request in an API and return the response, simple. But in this case I will have thousands of requests. So, which one of the code options will perform better, and why. Also which one is recommended as good pratices these days?
First options is using the .then to resolve the promises and the seccond one is using async / await.
In my tests the two options had very similar results without significant differences, but I'm not sure in scale.
// Using then
doSomething(payload) {
const url = 'https://link-here/consultas';
return this.axios.get(url, {
params: {
token: payload.token,
chave: payload.chave,
},
}).then(resp => resp.data);
}
// Using Async / await
async doSomething(payload) {
const url = 'https://link-here/consultas';
const resp = await this.axios.get(url, {
params: {
token: payload.token,
chave: payload.chave,
},
});
return resp.data;
}
Any explanation will be of great value.

From a performance point of view, await is just an internal version of .then() (doing basically the same thing). The reason to choose one over the other doesn't really have to do with performance, but has to do with desired coding style or coding convenience. Certainly, the interpreter has a few more opportunities to optimize things internally with await, but its unlikely that should be how you decide which to use. If all else was equal, I would choose await for the reason cited above. But, I'd first choose which made the code simpler to write and understand and maintain and test.
Used properly, await can often save you a bunch of lines of code making your code simpler to read, test and maintain. That's why it was invented.
There's no meaningful difference between the two versions of your code. Both achieve the same result when the axios call is successful or has an error.
Where await could make more of a convenience difference is if you had multiple successive asynchronous calls that needed to be serialized. Then, rather than bracketing them each inside a .then() handler to chain them properly, you could just use await and have simpler looking code.
A common mistake with both await and .then() is to forget proper error handling. If your error handling desire in this function is to just return the rejected promise, then both of your versions do that identically. But, if you have multiple async calls in a row and you want to do anything more complex than just returning the first rejection, then the error handling techniques for await and .then()/.catch() are quite different and which seems simpler will depend upon the situation.

There should be some corrections in this thread. await and .then are going to give very different results, and should be used for different reasons.
await will WAIT for something, and then continue to the next line. It's also the simpler of the two because it behaves mechanically more like synchronous behavior. You do step #1, wait, and then continue.
console.log("Runs first.");
await SomeFunction();
console.log("Runs last.");
.then splits off from the original call and starts operating within its own scope, and will update at a time the original scope cannot predict. If we can put semantics aside for a moment, it's "more asynchronous," because it leaves the old scope and branches off into a new one.
console.log("Runs first.");
SomeFunction().then((value) => {console.log("Runs last (probably). Didn't use await on SomeFunction().")})
console.log("Runs second (probably).");

As more explanation to #user280209 answer let's consider the following function which returns promise and compare its execution with .then() and async await.
function call(timeout) {
return new Promise((resolve, reject) => {
setTimeout(() => {
console.log(`This call took ${timeout} seconds`);
resolve(true);
}, timeout * 1000);
});
}
With .then()
(async () => {
call(5).then((r) => {
console.log(r);
});
await call(2); //This will print result first
await call(1);
})();
When running the above call the logs will be
This call took 2 seconds
This call took 1 seconds
This call took 5 seconds
true
As we can see .then() didn't pause the execution of its below line until it completes.
With async/wait
(async () => {
await call(5); //This will print result first
await call(2);
await call(1);
})();
When run the above function logs will be
This call took 5 seconds
This call took 2 seconds
This call took 1 seconds
So I think if your promise's result won't be used in the following lines, .then() may be better.

For those saying await blocks the code until the async call returns you are missing the point. "await" is syntactic sugar for a promise.then(). It is effectively wrapping the rest of your function in the then block of a promise it is creating for you. There is no real "blocking" or "waiting".
run();
async function run() {
console.log('running');
makePromises();
console.log('exiting right away!');
}
async function makePromises() {
console.log('make promise 1');
const myPromise = promiseMe(1)
.then(msg => {
console.log(`What i want to run after the promise is resolved ${msg}`)
})
console.log('make promise 2')
const msg = await promiseMe(2);
console.log(`What i want to run after the promise is resolved via await ${msg}`)
}
function promiseMe(num: number): Promise<string> {
return new Promise((resolve, reject) => {
console.log(`promise`)
resolve(`hello promise ${num}`);
})
}
The await line in makePromises does not block anything and the output is:
running
make promise 1
promise
make promise 2
promise
exiting right away!
What i want to run after the promise is resolved hello promise 1
What i want to run after the promise is resolved via await hello promise 2

Actually.
Await/Async can perform more efficiently as Promise.then() loses the scope in which it was called after execution, you are attaching a callback to the callback stack.
What it causes is: The system now has to store a reference to where the .then() was called. In case of error it has to correctly point to where the error happens, otherwise, without the scope (as the system resumed execution after called the Promise, waiting to comeback to the .then() later) it isn't able to point to where the error happened.
Async/Await you suspend the exection of the method where it is being called thus preserving reference.

If we just consider performance(time taken) then it actually depends on whether your operations are serial or parallel. If your tasks are serial then there will be no difference between await and .then. But if your tasks are parallel then .then will take less time. Consider the following example
let time = Date.now();
// first task takes 1.5 secs
async function firstTask () {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve(1);
},1500)
})
}
// second task takes 2 secs
async function secondTask () {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve(2);
},2000)
})
}
// using await
async function call(){
const d1 = await firstTask();
const d2 = await secondTask();
console.log(Date.now()-time, d1+d2)
}
call()
// using .then
async function call2(){
let d1=null,d2=null;
firstTask().then(data => {
d1=data;
if(d2){
console.log(Date.now()-time, d1+d2);
}
})
secondTask().then(data => {
d2=data;
if(d1){
console.log(Date.now()-time, d1+d2);
}
})
}
call2()
Here are the two tasks, first takes 1.5 secs and second takes 2 secs. Call function uses await where as call2 function uses .then . The output is as follows
From call2 2012 3
From call 3506 3
I hope it helps.

As far as I understand .then() and await are not the same thing. An async function won't proceed with the next command until the promise is resolved/rejected since it's basically an implementation of generators. On the contrast, in the case of .then(), the execution of the function will proceed with the next command and the resolve/reject callback will be executed "when there's time" aka when the current event loop (not entirely sure about that part) will be completed.
tldr; on a single promise await and .then() behave similarly but when one promise needs another one to be resolved first then the two of them behave entirely different

Many answer have been provided to this question already. However, to point out key information in the answers above and from my understanding, note below point:
only use await when not handling error return
if no crucial need for error handling use await instead
use .then .catch if returned error message or data is crucial for debugging / or proper error handling instead of try catch for await
Choose any prefer method from code sample below
const getData = (params = {name: 'john', email: 'ex#gmail.com'}) => {
return axios.post(url, params);
}
// anywhere you want to get the return data
// using await
const setData = async () => {
const data = await getData();
}
// to handle error with await
const setData = async () => {
try {
const data = await getData();
}
catch(err) {
console.log(err.message);
}
}
// using .then .catch
const setData = () => {
var data;
getData().then((res) => {
data = res.data; console.log(data)
}).catch((err) => {
console.log(err.message);
});
}

What is a nice way to override the value a promise resolves to? [duplicate]

This question already has answers here:
How do I access previous promise results in a .then() chain?
(17 answers)
Closed 5 years ago.
I am writing a Node.js script to populate an SQL database with a test dataset.
In the promise chain illustrated in the code snippet below (the real code is a bit more hairy), the function insertData() requires the db object to be passed through from the previous stage. However, asynchronous calls inside dropAndCreateTables() on the previous stage use db object but do not return it. I came up with a solution that wraps promises inside dropAndCreateTables() into another promise object that resolves to a db object. However:
I heard that using Promise() constructor in non-library code is an antipattern and may lead to subtle and hard-to-diagnose mistakes
I heard that nesting then()-chains is also an antipattern
It does not allow me to ignore errors from the promiseDrop (for example, I don't care if tables don't exist on drop)
It is ugly
Questions:
Is there a simpler, nicer and more socially accepted way to override the return value of a promise? (in this case, created with Promise.all())
Is there a way to restructure my code in a way that this problem does not occur? (That is, I don't exclude the possibility of "XY problem" here)
Code:
const dropAndCreateTables = (db, startClean) => {
if(startClean) {
const sqlDrop = fs.readFileSync('drop.sql').toString()
const promiseDrop = db.raw(sqlDrop)
const sqlCreate = fs.readFileSync('create.sql').toString()
const promiseCreate = db.raw(sqlCreate)
/********* Problems here? ************************************/
return new Promise((resolve, reject) => { // Ew?
Promise.all([promiseDrop, promiseCreate])
.then(() => {
resolve(db) // Override the returned value
})
.catch(reject)
})
}
return Promise.resolve(db)
}
initDB({ debug: false })
.then((db) => {
return dropAndCreateTables(db, START_CLEAN) // Without my hack this does not return `db`
})
.then((db) => {
return insertData(db, DO_UPSERT) // This needs the `db` object
})
.then(() => {
console.info(`\n${timestamp()} done`)
})
.catch(handleError)

(Some fairly important notes midway and later in the answer, please do read all the way to the end.)
Is there a simpler, nicer and more socially accepted way to override the return value of a promise? (in this case, created with Promise.all())
Yes, you simply return a value from the then handler, and return the promise then returns:
return Promise.all([promiseDrop, promiseCreate])
.then(() => db);
then (and catch) create promise chains. Each link in the chain can transform the result. then and catch return a new promise that will be fulfilled or rejected based on what happens in their callback:
If their callback throws, the promise rejects with the error thrown
If their callback returns a non-thenable value (e.g., promise), the promise is fulfilled with that value
If their callback returns a thenable value, the promise is resolved to that thenable — it waits for the other promise to settle, then settles the same way
(If the term "thenable" isn't familiar, or you're not clear on the distinction between "fulfill" and "resolve," I go into promise terminology in this post on my blog.)
I heard that using Promise() constructor in non-library code is an antipattern and may lead to subtle and hard-to-diagnose mistakes
The distinction isn't library code vs. non-library code, it's between code that doesn't already have a promise to work with and code that does. If you already have a promise to work with, you almost never want to use new Promise. More: What is the explicit promise construction antipattern and how do I avoid it?
I heard that nesting then()-chains is also an antipattern
You almost never need to nest then chains, because again each link in the chain already has the means of tranforming the result passing through it. So:
// Unnecessary nesting
doSomething()
.then(a => {
return doSomethingElse(a * 2)
.then(b => b * 3);
})
.catch(e => { /*...handle error...*/ });
can be more idiomatically and simply written:
doSomething()
.then(a => doSomethingElse(a * 2))
.then(b => b * 3);
.catch(e => { /*...handle error...*/ });
Is there a way to restructure my code in a way that this problem does not occur? (That is, I don't exclude the possibility of "XY problem" here)
Not an X/Y per se, but you have a problem in that code: There's no guarantee the drop will happen before the create! So instead of starting both and letting them run in parallel and watching for the results with Promise.all, ensure those operations happen in sequence:
// Fairly minimal changes
const dropAndCreateTables = (db, startClean) => {
if(startClean) {
const sqlDrop = fs.readFileSync('drop.sql').toString()
return db.raw(sqlDrop)
.then(() => {
const sqlCreate = fs.readFileSync('create.sql').toString()
return db.raw(sqlCreate);
})
.then(() => db);
}
return Promise.resolve(db)
}
But, I wouldn't use sync file I/O. Instead
const promisify = require("utils").promisify;
const readWithPromise = promisify(fs.readFile);
and then
const dropAndCreateTables = (db, startClean) => {
if(startClean) {
const getDrop = readWithPromise('drop.sql'); // Start this first
const getCreate = readWithPromise('create.sql'); // Then start this
return getDrop
.then(dropSql => db.raw(dropSql)) // Got the drop SQL, run it
.then(() => getCreate) // Make sure we have the create SQl
.then(createSql => db.raw(createSql)) // Run it
.then(() => db);
}
return Promise.resolve(db)
}
Note how we avoid ever busy-waiting on I/O, and we can overlap the DB's drop operation with reading the create SQL.

You don't need to call the Promise constructor when returning another promise, you can just write it like:
return Promise.all([promiseDrop, promiseCreate])
.then(() => db)
.catch(error => {
// handle the error or rethrow it
})

You might omit resolving db from dropAndCreateTables like this:
.then((db) => {
return dropAndCreateTables(db, START_CLEAN).then(Promise.resolve(db));
})

You should not let dropAndCreateTables return a db promise, there is no real usecase for it. So:
return Promise.all([promiseDrop, promiseCreate]);
is enough. Now the chaining part:
initDB({ debug: false }).then(async (db) => {
await dropAndCreateTables(db, START_CLEAN);
await insertData(db, DO_UPSERT);
console.info(`\n${timestamp()} done`)
}).catch(handleError)

promise and function callback [duplicate]

I've been developing JavaScript for a few years and I don't understand the fuss about promises at all.
It seems like all I do is change:
api(function(result){
api2(function(result2){
api3(function(result3){
// do work
});
});
});
Which I could use a library like async for anyway, with something like:
api().then(function(result){
api2().then(function(result2){
api3().then(function(result3){
// do work
});
});
});
Which is more code and less readable. I didn't gain anything here, it's not suddenly magically 'flat' either. Not to mention having to convert things to promises.
So, what's the big fuss about promises here?

Promises are not callbacks. A promise represents the future result of an asynchronous operation. Of course, writing them the way you do, you get little benefit. But if you write them the way they are meant to be used, you can write asynchronous code in a way that resembles synchronous code and is much more easy to follow:
api().then(function(result){
return api2();
}).then(function(result2){
return api3();
}).then(function(result3){
// do work
});
Certainly, not much less code, but much more readable.
But this is not the end. Let's discover the true benefits: What if you wanted to check for any error in any of the steps? It would be hell to do it with callbacks, but with promises, is a piece of cake:
api().then(function(result){
return api2();
}).then(function(result2){
return api3();
}).then(function(result3){
// do work
}).catch(function(error) {
//handle any error that may occur before this point
});
Pretty much the same as a try { ... } catch block.
Even better:
api().then(function(result){
return api2();
}).then(function(result2){
return api3();
}).then(function(result3){
// do work
}).catch(function(error) {
//handle any error that may occur before this point
}).then(function() {
//do something whether there was an error or not
//like hiding an spinner if you were performing an AJAX request.
});
And even better: What if those 3 calls to api, api2, api3 could run simultaneously (e.g. if they were AJAX calls) but you needed to wait for the three? Without promises, you should have to create some sort of counter. With promises, using the ES6 notation, is another piece of cake and pretty neat:
Promise.all([api(), api2(), api3()]).then(function(result) {
//do work. result is an array contains the values of the three fulfilled promises.
}).catch(function(error) {
//handle the error. At least one of the promises rejected.
});
Hope you see Promises in a new light now.

Yes, Promises are asynchronous callbacks. They can't do anything that callbacks can't do, and you face the same problems with asynchrony as with plain callbacks.
However, Promises are more than just callbacks. They are a very mighty abstraction, allow cleaner and better, functional code with less error-prone boilerplate.
So what's the main idea?
Promises are objects representing the result of a single (asynchronous) computation. They resolve to that result only once. There's a few things what this means:
Promises implement an observer pattern:
You don't need to know the callbacks that will use the value before the task completes.
Instead of expecting callbacks as arguments to your functions, you can easily return a Promise object
The promise will store the value, and you can transparently add a callback whenever you want. It will be called when the result is available. "Transparency" implies that when you have a promise and add a callback to it, it doesn't make a difference to your code whether the result has arrived yet - the API and contracts are the same, simplifying caching/memoisation a lot.
You can add multiple callbacks easily
Promises are chainable (monadic, if you want):
If you need to transform the value that a promise represents, you map a transform function over the promise and get back a new promise that represents the transformed result. You cannot synchronously get the value to use it somehow, but you can easily lift the transformation in the promise context. No boilerplate callbacks.
If you want to chain two asynchronous tasks, you can use the .then() method. It will take a callback to be called with the first result, and returns a promise for the result of the promise that the callback returns.
Sounds complicated? Time for a code example.
var p1 = api1(); // returning a promise
var p3 = p1.then(function(api1Result) {
var p2 = api2(); // returning a promise
return p2; // The result of p2 …
}); // … becomes the result of p3
// So it does not make a difference whether you write
api1().then(function(api1Result) {
return api2().then(console.log)
})
// or the flattened version
api1().then(function(api1Result) {
return api2();
}).then(console.log)
Flattening does not come magically, but you can easily do it. For your heavily nested example, the (near) equivalent would be
api1().then(api2).then(api3).then(/* do-work-callback */);
If seeing the code of these methods helps understanding, here's a most basic promise lib in a few lines.
What's the big fuss about promises?
The Promise abstraction allows much better composability of functions. For example, next to then for chaining, the all function creates a promise for the combined result of multiple parallel-waiting promises.
Last but not least Promises come with integrated error handling. The result of the computation might be that either the promise is fulfilled with a value, or it is rejected with a reason. All the composition functions handle this automatically and propagate errors in promise chains, so that you don't need to care about it explicitly everywhere - in contrast to a plain-callback implementation. In the end, you can add a dedicated error callback for all occurred exceptions.
Not to mention having to convert things to promises.
That's quite trivial actually with good promise libraries, see How do I convert an existing callback API to promises?

In addition to the already established answers, with ES6 arrow functions Promises turn from a modestly shining small blue dwarf straight into a red giant. That is about to collapse into a supernova:
api().then(result => api2()).then(result2 => api3()).then(result3 => console.log(result3))
As oligofren pointed out, without arguments between api calls you don't need the anonymous wrapper functions at all:
api().then(api2).then(api3).then(r3 => console.log(r3))
And finally, if you want to reach a supermassive black hole level, Promises can be awaited:
async function callApis() {
let api1Result = await api();
let api2Result = await api2(api1Result);
let api3Result = await api3(api2Result);
return api3Result;
}

In addition to the awesome answers above, 2 more points may be added:
1. Semantic difference:
Promises may be already resolved upon creation. This means they guarantee conditions rather than events. If they are resolved already, the resolved function passed to it is still called.
Conversely, callbacks handle events. So, if the event you are interested in has happened before the callback has been registered, the callback is not called.
2. Inversion of control
Callbacks involve inversion of control. When you register a callback function with any API, the Javascript runtime stores the callback function and calls it from the event loop once it is ready to be run.
Refer The Javascript Event loop for an explanation.
With Promises, control resides with the calling program. The .then() method may be called at any time if we store the promise object.

In addition to the other answers, the ES2015 syntax blends seamlessly with promises, reducing even more boilerplate code:
// Sequentially:
api1()
.then(r1 => api2(r1))
.then(r2 => api3(r2))
.then(r3 => {
// Done
});
// Parallel:
Promise.all([
api1(),
api2(),
api3()
]).then(([r1, r2, r3]) => {
// Done
});

Promises are not callbacks, both are programming idioms that facilitate async programming. Using an async/await-style of programming using coroutines or generators that return promises could be considered a 3rd such idiom. A comparison of these idioms across different programming languages (including Javascript) is here: https://github.com/KjellSchubert/promise-future-task

No, Not at all.
Callbacks are simply Functions In JavaScript which are to be called and then executed after the execution of another function has finished. So how it happens?
Actually, In JavaScript, functions are itself considered as objects and hence as all other objects, even functions can be sent as arguments to other functions. The most common and generic use case one can think of is setTimeout() function in JavaScript.
Promises are nothing but a much more improvised approach of handling and structuring asynchronous code in comparison to doing the same with callbacks.
The Promise receives two Callbacks in constructor function: resolve and reject. These callbacks inside promises provide us with fine-grained control over error handling and success cases. The resolve callback is used when the execution of promise performed successfully and the reject callback is used to handle the error cases.

No promises are just wrapper on callbacks
example
You can use javascript native promises with node js
my cloud 9 code link : https://ide.c9.io/adx2803/native-promises-in-node
/**
* Created by dixit-lab on 20/6/16.
*/
var express = require('express');
var request = require('request'); //Simplified HTTP request client.
var app = express();
function promisify(url) {
return new Promise(function (resolve, reject) {
request.get(url, function (error, response, body) {
if (!error && response.statusCode == 200) {
resolve(body);
}
else {
reject(error);
}
})
});
}
//get all the albums of a user who have posted post 100
app.get('/listAlbums', function (req, res) {
//get the post with post id 100
promisify('http://jsonplaceholder.typicode.com/posts/100').then(function (result) {
var obj = JSON.parse(result);
return promisify('http://jsonplaceholder.typicode.com/users/' + obj.userId + '/albums')
})
.catch(function (e) {
console.log(e);
})
.then(function (result) {
res.end(result);
}
)
})
var server = app.listen(8081, function () {
var host = server.address().address
var port = server.address().port
console.log("Example app listening at http://%s:%s", host, port)
})
//run webservice on browser : http://localhost:8081/listAlbums

JavaScript Promises actually use callback functions to determine what to do after a Promise has been resolved or rejected, therefore both are not fundamentally different. The main idea behind Promises is to take callbacks - especially nested callbacks where you want to perform a sort of actions, but it would be more readable.

Promises overview:
In JS we can wrap asynchronous operations (e.g database calls, AJAX calls) in promises. Usually we want to run some additional logic on the retrieved data. JS promises have handler functions which process the result of the asynchronous operations. The handler functions can even have other asynchronous operations within them which could rely on the value of the previous asynchronous operations.
A promise always has of the 3 following states:
pending: starting state of every promise, neither fulfilled nor rejected.
fulfilled: The operation completed successfully.
rejected: The operation failed.
A pending promise can be resolved/fullfilled or rejected with a value. Then the following handler methods which take callbacks as arguments are called:
Promise.prototype.then() : When the promise is resolved the callback argument of this function will be called.
Promise.prototype.catch() : When the promise is rejected the callback argument of this function will be called.
Although the above methods skill get callback arguments they are far superior than using
only callbacks here is an example that will clarify a lot:
Example
function createProm(resolveVal, rejectVal) {
return new Promise((resolve, reject) => {
setTimeout(() => {
if (Math.random() > 0.5) {
console.log("Resolved");
resolve(resolveVal);
} else {
console.log("Rejected");
reject(rejectVal);
}
}, 1000);
});
}
createProm(1, 2)
.then((resVal) => {
console.log(resVal);
return resVal + 1;
})
.then((resVal) => {
console.log(resVal);
return resVal + 2;
})
.catch((rejectVal) => {
console.log(rejectVal);
return rejectVal + 1;
})
.then((resVal) => {
console.log(resVal);
})
.finally(() => {
console.log("Promise done");
});
The createProm function creates a promises which is resolved or rejected based on a random Nr after 1 second
If the promise is resolved the first then method is called and the resolved value is passed in as an argument of the callback
If the promise is rejected the first catch method is called and the rejected value is passed in as an argument
The catch and then methods return promises that's why we can chain them. They wrap any returned value in Promise.resolve and any thrown value (using the throw keyword) in Promise.reject. So any value returned is transformed into a promise and on this promise we can again call a handler function.
Promise chains give us more fine tuned control and better overview than nested callbacks. For example the catch method handles all the errors which have occurred before the catch handler.

Promises allows programmers to write simpler and far more readable code than by using callbacks.
In a program, there are steps want to do in series.
function f() {
step_a();
step_b();
step_c();
...
}
There's usually information carried between each step.
function f() {
const a = step_a( );
const b = step_b( a );
const c = step_c( b );
...
}
Some of these steps can take a (relatively) long time, so sometimes you want to do them in parallel with other things. One way to do that is using threads. Another is asynchronous programming. (Both approaches has pros and cons, which won't be discussed here.) Here, we're talking about asynchronous programming.
The simple way to achieve the above when using asynchronous programming would be to provide a callback which is called once a step is complete.
// step_* calls the provided function with the returned value once complete.
function f() {
step_a(
function( a )
step_b(
function( b )
step_c(
...
)
},
)
},
)
}
That's quite hard to read. Promises offer a way to flatten the code.
// step_* returns a promise.
function f() {
step_a()
.then( step_b )
.then( step_c )
...
}
The object returned is called a promise because it represents the future result (i.e. promised result) of the function (which could be a value or an exception).
As much as promises help, it's still a bit complicated to use promises. This is where async and await come in. In a function declared as async, await can be used in lieu of then.
// step_* returns a promise.
async function f()
const a = await step_a( );
const b = await step_b( a );
const c = await step_c( b );
...
}
This is undeniably much much more readable than using callbacks.