Currently I use promises in the store actions but want to convert it into async/await. This is an example of the store action with promises:
fetchActiveWorkspace (context, workspaceID) {
if (workspaceID) {
return this.$axios.get(`#api-v01/workspaces/workspace/${workspaceID}`)
.then(response => {
context.commit('setActiveWorkspace', response.data)
})
.catch(err => {
throw err
})
} else {
return Promise.resolve(true)
}
},
This fetchActiveWorkspace action is resolved in components because it returns promise. How can I convert this code snippet into a async/await structure and use it in components?
This is how I would try to translate it; take into account that as I have no access to the original code in full context, I cannot try it first-hand to make sure it works; but still, this is how you can use async/await with promises.
// 1. Mark the function as `async` (otherwise you cannot use `await` inside of it)
async fetchActiveWorkspace(context, workspaceID) {
if (workspaceID) {
// 2. Call the promise-returning function with `await` to wait for result before moving on.
// Capture the response in a varible (it used to go as argument for `then`)
let response = await this.$axios.get(`#api-v01/workspaces/workspace/${workspaceID}`);
context.commit('setActiveWorkspace', response.data);
}
// 3. I don't think this is necessary, as actions are not meant to return values and instead should be for asynchronous mutations.
else {
return true;
}
}
You can surround the function's body with try/catch in case you want to capture and handle exceptions. I didn't add it in order to keep things simple and because your promise-based code will just capture and re-throw the exception, without doing anything else.
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.
So I am working on couple of cases in my app where I need the following to happen
When event triggered, do the following
List item
check if the data with that context is already cached, serve cached
if no cache, debounce 500ms
check if other http calls are running (for the same context) and kill them
make http call
On success cache and update/replace model data
Pretty much standard when it comes to typeahead functionality
I would like to use observables with this... in the way, I can cancel them if previous calls are running
any good tutorials on that? I was looking around, couldn't find anything remotely up to date
OK, to give you some clue what I did now:
onChartSelection(chart: any){
let date1:any, date2:any;
try{
date1 = Math.round(chart.xAxis[0].min);
date2 = Math.round(chart.xAxis[0].max);
let data = this.tableService.getCachedChartData(this.currentTable, date1, date2);
if(data){
this.table.data = data;
}else{
if(this.chartTableRes){
this.chartTableRes.unsubscribe();
}
this.chartTableRes = this.tableService.getChartTable(this.currentTable, date1, date2)
.subscribe(
data => {
console.log(data);
this.table.data = data;
this.chartTableRes = null;
},
error => {
console.log(error);
}
);
}
}catch(e){
throw e;
}
}
Missing debounce here
-- I ended up implementing lodash's debounce
import {debounce} from 'lodash';
...
onChartSelectionDebaunced: Function;
constructor(...){
...
this.onChartSelectionDebaunced = debounce(this.onChartSelection, 200);
}
For debaunce you can use Underscore.js. The function will look this way:
onChartSelection: Function = _.debounce((chart: any) => {
...
});
Regarding the cancelation of Observable, it is better to use Observable method share. In your case you should change the method getChartTable in your tableService by adding .share() to your Observable that you return.
This way there will be only one call done to the server even if you subscribe to it multiple times (without this every new subscription will invoke new call).
Take a look at: What is the correct way to share the result of an Angular 2 Http network call in RxJs 5?
Task: get data from server with $.post, process them by method .success(), after that call some function.
var t;
$.when($.post("get_json.php", function(res) {
t = res;
}, 'json')).done(function() {
console.log(t);
});
Do I understand correctly that the Deferred method .done() is executed after .success is done (ie t = res)?
But why "console.log(t)" shows "undefined"?
Is .done() fires after request, but before .success()?
Passing a "success" callback to $.post() is an alternative to the (preferred) chaining of .done(...). Do one or the other, not both, then you don't need to worry about the execution order.
Also, unless you have a decent caching strategy for async data, you shouldn't be setting t as an outer var.
$.post("get_json.php", ...).done(function(t) {
console.log(t);
//do awesome things with t here
});
Caching would be something like this :
var asyncCache = {};
...
function get_t() {
return (asyncCache.t) ? $.when(asyncCache.t) : $.post("get_json.php", ...).done(function(t) {
asyncCache.t = t;
});
}
...
get_t().done(function(t) {
console.log(t);
//do awesome things with t here
}
$.when() is ONLY needed when you have multiple promises and you want to wait for all of them to complete. You simply don't need it at all for your single ajax call. You can just do it like this:
$.post("get_json.php").done(function(t) {
// use the results of the ajax call here
console.log(t);
});
In addition, your code example was using BOTH a success callback function AND a .done() handler. Pick one of the other, not both as they are different ways of getting a callback when the ajax call is done. I'd suggest the promise implementation above because it's more flexible. But, you could also use just the success handler:
$.post("get_json.php", function(t) {
// use the results of the ajax call here
console.log(t);
}, 'json');
Note, when you have an asynchronous operation like this, you need to consume the results of the ajax call in the success callback (or the promise callback) or call some function from there and pass it the data. You do not want to put the data into a global variable or a variable in a higher scope because other code will simply have no way of knowing when the data is ready and when it is not. Put your action in the callback.
If you have more than one ajax call that you want to wait for, then you can do it like this:
$.when($.post("get_json.php"), $.post("get_json2.php")).done(function(r1, r2) {
// use the results of the ajax call here
console.log(r1[0]); // results from first ajax call
console.log(r2[0]); // results from second ajax call
});
I need to search the CouchDB based on several criteria entered in a form. Name, an array of Tags and so on. I would then need various views to index on these fields. Ultimately, all the results will be collated in data.js and provided to mustache.html. Say there are 3 views - docsByName, docsByTags, docsById.
What I don't know is, how to query all these views in query.js. Can this be done and how ?
Or should the approach be of that to write one view that makes multiple emits for each search somehow ?
Thank you.
From what you say I assume you are using Evently, so I will quote from Evently primer:
The async function is the main star, which in this case makes an Ajax request (but it can do anything it wants). Another important thing to note is that the first argument to the async function is a callback which you use to tell Evently when you are done with your asynchronous action. [...] Whatever you pass to the callback function then becomes the first item passed to the data function.
In short: put your Ajax requests in async.js.
As a side note: Evently is only one of the possible choices to write a couchapp and it is not clear if it is maintained. However it works and it is easy to rearrange the code to not use it.
EDIT: here is a sample async function (cut&paste from an old program):
function(cb, e) {
var app = $$(this).app
;
app.db.openDoc('SOMEDOCID', {
error: function(code, error, reason) {
alert("Error("+code+" "+error+"): "+reason);
}
, success: function(doc) {
app.view('SOMEVIEWNAME', {
include_docs: true
, error: function(code, error, reason) {
alert("Error("+code+" "+error+"): "+reason);
}
, success: function(resp) {
resp.doc = doc;
cb(resp);
}
});
}
});
}