I've two WebSockets getting data asynchronously, every time I get some message from the sockets I execute some code in CompareData.
The problem is that CompareData should be executed synchronously, or (better) only if it is not already running
This is my code:
function CompareData(data) {
console.log('data ', data);
AsyncFunction();
};
ws1 = new WebSocket(WS1_URL);
ws2 = new WebSocket(WS2_URL);
ws1.on('message', (data) => {
CompareData(data);
});
ws2.on('message', (data) => {
CompareData(data);
});
Can you help me, please? I'm very new to NodeJs
Node.js is single threaded. So you don't really get true concurrency issues occurring in Node programs as you might in other languages. In your example, there can only be at most one WebSocket callback for CompareData occurring at any given time.
You should not make synchronous call in node.js but you can make those call sequential. See below example might be helpful.
var messages = [];
var inProgress = false;
function CompareData(data) {
return new Promise((resolve, reject) => {
// do some stuff and resolve
setTimeout(() => {
resolve(data);
}, 1000);
});
};
const start = async () => {
if (!inProgress) {
if (messages.length !== 0) {
inProgress = true;
try {
const data = await CompareData(messages.shift());
console.log(data);
} catch (error) {
console.log(error);
}
inProgress = false;
await start();
}else{
console.log('Process Done');
}
}
}
const handler = (data) => {
messages.push(data);
start();
}
handler(1);
handler(2);
handler(3);
handler(4);
// ws1 = new WebSocket(WS1_URL);
// ws2 = new WebSocket(WS2_URL);
// ws1.on('message', handler);
// ws2.on('message', handler);
You should use some mutex in order to avoid that two async operations of compareData are executed at the same time, like node-mutex or mutexify.
My suggestions are:
First of all, you need to know when CompareData is finished. Reorganize your code to use promises or callbacks. If you're using third-party async functions, I'm almost sure they provide some feedback on completion - This is a must have in async world
Add inProgress = false flag somewhere to serve for you as simple lock. As someone posted, JS is single-threaded and you're guaranteed that your code won't get interrupted in the middle of operation. Thanks to that you can use really simple locks instead of complicated os-based mutexes known from multithreaded
langs.
In ws.on(...) check if inProgress is set. If not, lock it and run CompareData
In CompareData completion callback or on promise resolution set inProgress back to false, so you're no longer ignoring incoming data.
If you can simply discard the data, there is no need to complicate this scenario with extra queues, mutexes, etc.
If you need to serve it all, then queue incoming data and serve next piece after completion callback is fired.
This is basically what Rahul's suggests, but he uses features that are not established in current version of standard, so don't use it if you're not transpiling your code.
I am adding user validation an data modification page on a node.js application.
In a synchronous universe, in a single function I would:
Lookup the original record in the database
Lookup the user in LDAP to see if they are the owner or admin
Do the logic and write the record.
In an asynchronous universe that won't work. To solve it I've built a series of hand-off functions:
router.post('/writeRecord', jsonParser, function(req, res) {
post = req.post;
var smdb = new AWS.DynamoDB.DocumentClient();
var params = { ... }
smdb.query(params, function(err,data){
if( err == null ) writeRecordStep2(post,data);
}
});
function writeRecord2( ru, post, data ){
var conn = new LDAP();
conn.search(
'ou=groups,o=amazon.com',
{ ... },
function(err,resp){
if( err == null ){
writeRecordStep3( ru, post, data, ldap1 )
}
}
}
function writeRecord3( ru, post, data ){
var conn = new LDAP();
conn.search(
'ou=groups,o=amazon.com',
{ ... },
function(err,resp){
if( err == null ){
writeRecordStep4( ru, post, data, ldap1, ldap2 )
}
}
}
function writeRecordStep4( ru, post, data, ldap1, ldap2 ){
// Do stuff with collected data
}
Additionally, because the LDAP and Dynamo logic are in their own source documents, these functions are scattered tragically around the code.
This strikes me as inefficient, as well as inelegant. I'm eager to find a more natural asynchronous pattern to achieve the same result.
Any promise library should sort your issue out. My preferred choice is bluebird. In summary they help you in performing blocking operations.
If you haven't heard about bluebird then just use it. It converts all function of a module and return promise which is then-able. Simply put, it promisifies all functions.
Here is the mechanism:
Module1.someFunction() \\do your job and finally pass the return object to next call
.then() \\Use that object which is return from the first call, do your job and return the updated value
.then() \\same goes on
.catch() \\do your job when any error occurs.
Hope you understand. Here is an example:
var readFile = Promise.promisify(require("fs").readFile);
readFile("myfile.js",
"utf8").then(function(contents) {
return eval(contents);
}).then(function(result) {
console.log("The result of evaluating
myfile.js", result);
}).catch(SyntaxError, function(e) {
console.log("File had syntax error", e);
//Catch any other error
}).catch(function(e) {
console.log("Error reading file", e);
});
I could not tell from your pseudo-code exactly which async operations depend upon results from with other ones and knowing that is key to the most efficient way to code a series of asynchronous operations. If two operations do not depend upon one another, they can run in parallel which generally gets to an end result faster. I also can't tell exactly what data needs to be passed on to later parts of the async requests (too much pseudo-code and not enough real code to show us what you're really attempting to do).
So, without that level of detail, I'll show you two ways to approach this. The first runs each operation sequentially. Run the first async operation, when it's done, run the next one and accumulates all the results into an object that is passed along to the next link in the chain. This is general purpose since all async operations have access to all the prior results.
This makes use of promises built into the AWS.DynamboDB interface and makes our own promise for conn.search() (though if I knew more about that interface, it may already have a promise interface).
Here's the sequential version:
// promisify the search method
const util = require('util');
LDAP.prototype.searchAsync = util.promisify(LDAP.prototype.search);
// utility function that does a search and adds the result to the object passed in
// returns a promise that resolves to the object
function ldapSearch(data, key) {
var conn = new LDAP();
return conn.searchAsync('ou=groups,o=amazon.com', { ... }).then(results => {
// put our results onto the passed in object
data[key] = results;
// resolve with the original object (so we can collect data here in a promise chain)
return data;
});
}
router.post('/writeRecord', jsonParser, function(req, res) {
let post = req.post;
let smdb = new AWS.DynamoDB.DocumentClient();
let params = { ... }
// The latest AWS interface gets a promise with the .promise() method
smdb.query(params).promise().then(dbresult => {
return ldapSearch({post, dbresult}, "ldap1");
}).then(result => {
// result.dbresult
// result.ldap1
return ldapSearch(result, "ldap2")
}).then(result => {
// result.dbresult
// result.ldap1
// result.ldap2
// doSomething with all the collected data here
}).catch(err => {
console.log(err);
res.status(500).send("Internal Error");
});
});
And, here's a parallel version that runs all three async operations at once and then waits for all three of the to be done and then has all the results at once:
// if the three async operations you show can be done in parallel
// first promisify things
const util = require('util');
LDAP.prototype.searchAsync = util.promisify(LDAP.prototype.search);
function ldapSearch(params) {
var conn = new LDAP();
return conn.searchAsync('ou=groups,o=amazon.com', { ... });
}
router.post('/writeRecord', jsonParser, function(req, res) {
let post = req.post;
let smdb = new AWS.DynamoDB.DocumentClient();
let params = { ... }
Promise.all([
ldapSearch(...),
ldapSearch(...),
smdb.query(params).promise()
]).then(([ldap1Result, ldap2Result, queryResult]) => {
// process ldap1Result, ldap2Result and queryResult here
}).catch(err => {
console.log(err);
res.status(500).send("Internal Error");
});
});
Keep in mind that due to the pseudo-code nature of the code in your question, this is also pseudo-code where implementation details (exactly what parameters you're searching for, what response you're sending, etc...) have to be filled in. This should be illustrative of promise chaining to serialize operations and the use of Promise.all() for parallelizing operations and promisifying a method that didn't have promises built in.
I have restructured my code to promises, and built a wonderful long flat promise chain, consisting of multiple .then() callbacks. In the end I want to return some composite value, and need to access multiple intermediate promise results. However the resolution values from the middle of the sequence are not in scope in the last callback, how do I access them?
function getExample() {
return promiseA(…).then(function(resultA) {
// Some processing
return promiseB(…);
}).then(function(resultB) {
// More processing
return // How do I gain access to resultA here?
});
}
Break the chain
When you need to access the intermediate values in your chain, you should split your chain apart in those single pieces that you need. Instead of attaching one callback and somehow trying to use its parameter multiple times, attach multiple callbacks to the same promise - wherever you need the result value. Don't forget, a promise just represents (proxies) a future value! Next to deriving one promise from the other in a linear chain, use the promise combinators that are given to you by your library to build the result value.
This will result in a very straightforward control flow, clear composition of functionalities and therefore easy modularisation.
function getExample() {
var a = promiseA(…);
var b = a.then(function(resultA) {
// some processing
return promiseB(…);
});
return Promise.all([a, b]).then(function([resultA, resultB]) {
// more processing
return // something using both resultA and resultB
});
}
Instead of the parameter destructuring in the callback after Promise.all that only became available with ES6, in ES5 the then call would be replaced by a nifty helper method that was provided by many promise libraries (Q, Bluebird, when, …): .spread(function(resultA, resultB) { ….
Bluebird also features a dedicated join function to replace that Promise.all+spread combination with a simpler (and more efficient) construct:
…
return Promise.join(a, b, function(resultA, resultB) { … });
ECMAScript Harmony
Of course, this problem was recognized by the language designers as well. They did a lot of work and the async functions proposal finally made it into
ECMAScript 8
You don't need a single then invocation or callback function anymore, as in an asynchronous function (that returns a promise when being called) you can simply wait for promises to resolve directly. It also features arbitrary control structures like conditions, loops and try-catch-clauses, but for the sake of convenience we don't need them here:
async function getExample() {
var resultA = await promiseA(…);
// some processing
var resultB = await promiseB(…);
// more processing
return // something using both resultA and resultB
}
ECMAScript 6
While we were waiting for ES8, we already did use a very similar kind of syntax. ES6 came with generator functions, which allow breaking the execution apart in pieces at arbitrarily placed yield keywords. Those slices can be run after each other, independently, even asynchronously - and that's just what we do when we want to wait for a promise resolution before running the next step.
There are dedicated libraries (like co or task.js), but also many promise libraries have helper functions (Q, Bluebird, when, …) that do this async step-by-step execution for you when you give them a generator function that yields promises.
var getExample = Promise.coroutine(function* () {
// ^^^^^^^^^^^^^^^^^ Bluebird syntax
var resultA = yield promiseA(…);
// some processing
var resultB = yield promiseB(…);
// more processing
return // something using both resultA and resultB
});
This did work in Node.js since version 4.0, also a few browsers (or their dev editions) did support generator syntax relatively early.
ECMAScript 5
However, if you want/need to be backward-compatible you cannot use those without a transpiler. Both generator functions and async functions are supported by the current tooling, see for example the documentation of Babel on generators and async functions.
And then, there are also many other compile-to-JS languages
that are dedicated to easing asynchronous programming. They usually use a syntax similar to await, (e.g. Iced CoffeeScript), but there are also others that feature a Haskell-like do-notation (e.g. LatteJs, monadic, PureScript or LispyScript).
Synchronous inspection
Assigning promises-for-later-needed-values to variables and then getting their value via synchronous inspection. The example uses bluebird's .value() method but many libraries provide similar method.
function getExample() {
var a = promiseA(…);
return a.then(function() {
// some processing
return promiseB(…);
}).then(function(resultB) {
// a is guaranteed to be fulfilled here so we can just retrieve its
// value synchronously
var aValue = a.value();
});
}
This can be used for as many values as you like:
function getExample() {
var a = promiseA(…);
var b = a.then(function() {
return promiseB(…)
});
var c = b.then(function() {
return promiseC(…);
});
var d = c.then(function() {
return promiseD(…);
});
return d.then(function() {
return a.value() + b.value() + c.value() + d.value();
});
}
Nesting (and) closures
Using closures for maintaining the scope of variables (in our case, the success callback function parameters) is the natural JavaScript solution. With promises, we can arbitrarily nest and flatten .then() callbacks - they are semantically equivalent, except for the scope of the inner one.
function getExample() {
return promiseA(…).then(function(resultA) {
// some processing
return promiseB(…).then(function(resultB) {
// more processing
return // something using both resultA and resultB;
});
});
}
Of course, this is building an indentation pyramid. If indentation is getting too large, you still can apply the old tools to counter the pyramid of doom: modularize, use extra named functions, and flatten the promise chain as soon as you don't need a variable any more.
In theory, you can always avoid more than two levels of nesting (by making all closures explicit), in practise use as many as are reasonable.
function getExample() {
// preprocessing
return promiseA(…).then(makeAhandler(…));
}
function makeAhandler(…)
return function(resultA) {
// some processing
return promiseB(…).then(makeBhandler(resultA, …));
};
}
function makeBhandler(resultA, …) {
return function(resultB) {
// more processing
return // anything that uses the variables in scope
};
}
You can also use helper functions for this kind of partial application, like _.partial from Underscore/lodash or the native .bind() method, to further decrease indentation:
function getExample() {
// preprocessing
return promiseA(…).then(handlerA);
}
function handlerA(resultA) {
// some processing
return promiseB(…).then(handlerB.bind(null, resultA));
}
function handlerB(resultA, resultB) {
// more processing
return // anything that uses resultA and resultB
}
Explicit pass-through
Similar to nesting the callbacks, this technique relies on closures. Yet, the chain stays flat - instead of passing only the latest result, some state object is passed for every step. These state objects accumulate the results of the previous actions, handing down all values that will be needed later again plus the result of the current task.
function getExample() {
return promiseA(…).then(function(resultA) {
// some processing
return promiseB(…).then(b => [resultA, b]); // function(b) { return [resultA, b] }
}).then(function([resultA, resultB]) {
// more processing
return // something using both resultA and resultB
});
}
Here, that little arrow b => [resultA, b] is the function that closes over resultA, and passes an array of both results to the next step. Which uses parameter destructuring syntax to break it up in single variables again.
Before destructuring became available with ES6, a nifty helper method called .spread() was provided by many promise libraries (Q, Bluebird, when, …). It takes a function with multiple parameters - one for each array element - to be used as .spread(function(resultA, resultB) { ….
Of course, that closure needed here can be further simplified by some helper functions, e.g.
function addTo(x) {
// imagine complex `arguments` fiddling or anything that helps usability
// but you get the idea with this simple one:
return res => [x, res];
}
…
return promiseB(…).then(addTo(resultA));
Alternatively, you can employ Promise.all to produce the promise for the array:
function getExample() {
return promiseA(…).then(function(resultA) {
// some processing
return Promise.all([resultA, promiseB(…)]); // resultA will implicitly be wrapped
// as if passed to Promise.resolve()
}).then(function([resultA, resultB]) {
// more processing
return // something using both resultA and resultB
});
}
And you might not only use arrays, but arbitrarily complex objects. For example, with _.extend or Object.assign in a different helper function:
function augment(obj, name) {
return function (res) { var r = Object.assign({}, obj); r[name] = res; return r; };
}
function getExample() {
return promiseA(…).then(function(resultA) {
// some processing
return promiseB(…).then(augment({resultA}, "resultB"));
}).then(function(obj) {
// more processing
return // something using both obj.resultA and obj.resultB
});
}
While this pattern guarantees a flat chain and explicit state objects can improve clarity, it will become tedious for a long chain. Especially when you need the state only sporadically, you still have to pass it through every step. With this fixed interface, the single callbacks in the chain are rather tightly coupled and inflexible to change. It makes factoring out single steps harder, and callbacks cannot be supplied directly from other modules - they always need to be wrapped in boilerplate code that cares about the state. Abstract helper functions like the above can ease the pain a bit, but it will always be present.
Mutable contextual state
The trivial (but inelegant and rather errorprone) solution is to just use higher-scope variables (to which all callbacks in the chain have access) and write result values to them when you get them:
function getExample() {
var resultA;
return promiseA(…).then(function(_resultA) {
resultA = _resultA;
// some processing
return promiseB(…);
}).then(function(resultB) {
// more processing
return // something using both resultA and resultB
});
}
Instead of many variables one might also use an (initially empty) object, on which the results are stored as dynamically created properties.
This solution has several drawbacks:
Mutable state is ugly, and global variables are evil.
This pattern doesn't work across function boundaries, modularising the functions is harder as their declarations must not leave the shared scope
The scope of the variables does not prevent to access them before they are initialized. This is especially likely for complex promise constructions (loops, branching, excptions) where race conditions might happen. Passing state explicitly, a declarative design that promises encourage, forces a cleaner coding style which can prevent this.
One must choose the scope for those shared variables correctly. It needs to be local to the executed function to prevent race conditions between multiple parallel invocations, as would be the case if, for example, state was stored on an instance.
The Bluebird library encourages the use of an object that is passed along, using their bind() method to assign a context object to a promise chain. It will be accessible from each callback function via the otherwise unusable this keyword. While object properties are more prone to undetected typos than variables, the pattern is quite clever:
function getExample() {
return promiseA(…)
.bind({}) // Bluebird only!
.then(function(resultA) {
this.resultA = resultA;
// some processing
return promiseB(…);
}).then(function(resultB) {
// more processing
return // something using both this.resultA and resultB
}).bind(); // don't forget to unbind the object if you don't want the
// caller to access it
}
This approach can be easily simulated in promise libraries that do not support .bind (although in a somewhat more verbose way and cannot be used in an expression):
function getExample() {
var ctx = {};
return promiseA(…)
.then(function(resultA) {
this.resultA = resultA;
// some processing
return promiseB(…);
}.bind(ctx)).then(function(resultB) {
// more processing
return // something using both this.resultA and resultB
}.bind(ctx));
}
A less harsh spin on "Mutable contextual state"
Using a locally scoped object to collect the intermediate results in a promise chain is a reasonable approach to the question you posed. Consider the following snippet:
function getExample(){
//locally scoped
const results = {};
return promiseA(paramsA).then(function(resultA){
results.a = resultA;
return promiseB(paramsB);
}).then(function(resultB){
results.b = resultB;
return promiseC(paramsC);
}).then(function(resultC){
//Resolve with composite of all promises
return Promise.resolve(results.a + results.b + resultC);
}).catch(function(error){
return Promise.reject(error);
});
}
Global variables are bad, so this solution uses a locally scoped variable which causes no harm. It is only accessible within the function.
Mutable state is ugly, but this does not mutate state in an ugly manner. The ugly mutable state traditionally refers to modifying the state of function arguments or global variables, but this approach simply modifies the state of a locally scoped variable that exists for the sole purpose of aggregating promise results...a variable that will die a simple death once the promise resolves.
Intermediate promises are not prevented from accessing the state of the results object, but this does not introduce some scary scenario where one of the promises in the chain will go rogue and sabotage your results. The responsibility of setting the values in each step of the promise is confined to this function and the overall result will either be correct or incorrect...it will not be some bug that will crop up years later in production (unless you intend it to!)
This does not introduce a race condition scenario that would arise from parallel invocation because a new instance of the results variable is created for every invocation of the getExample function.
Example is available on jsfiddle
Node 7.4 now supports async/await calls with the harmony flag.
Try this:
async function getExample(){
let response = await returnPromise();
let response2 = await returnPromise2();
console.log(response, response2)
}
getExample()
and run the file with:
node --harmony-async-await getExample.js
Simple as can be!
Another answer, using babel-node version <6
Using async - await
npm install -g babel#5.6.14
example.js:
async function getExample(){
let response = await returnPromise();
let response2 = await returnPromise2();
console.log(response, response2)
}
getExample()
Then, run babel-node example.js and voila!
This days, I also hava meet some questions like you. At last, I find a good solution with the quesition, it's simple and good to read. I hope this can help you.
According to how-to-chain-javascript-promises
ok, let's look at the code:
const firstPromise = () => {
return new Promise((resolve, reject) => {
setTimeout(() => {
console.log('first promise is completed');
resolve({data: '123'});
}, 2000);
});
};
const secondPromise = (someStuff) => {
return new Promise((resolve, reject) => {
setTimeout(() => {
console.log('second promise is completed');
resolve({newData: `${someStuff.data} some more data`});
}, 2000);
});
};
const thirdPromise = (someStuff) => {
return new Promise((resolve, reject) => {
setTimeout(() => {
console.log('third promise is completed');
resolve({result: someStuff});
}, 2000);
});
};
firstPromise()
.then(secondPromise)
.then(thirdPromise)
.then(data => {
console.log(data);
});
I am not going to use this pattern in my own code since I'm not a big fan of using global variables. However, in a pinch it will work.
User is a promisified Mongoose model.
var globalVar = '';
User.findAsync({}).then(function(users){
globalVar = users;
}).then(function(){
console.log(globalVar);
});
Another answer, using sequential executor nsynjs:
function getExample(){
var response1 = returnPromise1().data;
// promise1 is resolved at this point, '.data' has the result from resolve(result)
var response2 = returnPromise2().data;
// promise2 is resolved at this point, '.data' has the result from resolve(result)
console.log(response, response2);
}
nynjs.run(getExample,{},function(){
console.log('all done');
})
Update: added working example
function synchronousCode() {
var urls=[
"https://ajax.googleapis.com/ajax/libs/jquery/1.7.0/jquery.min.js",
"https://ajax.googleapis.com/ajax/libs/jquery/1.8.0/jquery.min.js",
"https://ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js"
];
for(var i=0; i<urls.length; i++) {
var len=window.fetch(urls[i]).data.text().data.length;
// ^ ^
// | +- 2-nd promise result
// | assigned to 'data'
// |
// +-- 1-st promise result assigned to 'data'
//
console.log('URL #'+i+' : '+urls[i]+", length: "+len);
}
}
nsynjs.run(synchronousCode,{},function(){
console.log('all done');
})
<script src="https://rawgit.com/amaksr/nsynjs/master/nsynjs.js"></script>
When using bluebird, you can use .bind method to share variables in promise chain:
somethingAsync().bind({})
.spread(function (aValue, bValue) {
this.aValue = aValue;
this.bValue = bValue;
return somethingElseAsync(aValue, bValue);
})
.then(function (cValue) {
return this.aValue + this.bValue + cValue;
});
please check this link for further information:
http://bluebirdjs.com/docs/api/promise.bind.html
function getExample() {
var retA, retB;
return promiseA(…).then(function(resultA) {
retA = resultA;
// Some processing
return promiseB(…);
}).then(function(resultB) {
// More processing
//retA is value of promiseA
return // How do I gain access to resultA here?
});
}
easy way :D
I think you can use hash of RSVP.
Something like as below :
const mainPromise = () => {
const promise1 = new Promise((resolve, reject) => {
setTimeout(() => {
console.log('first promise is completed');
resolve({data: '123'});
}, 2000);
});
const promise2 = new Promise((resolve, reject) => {
setTimeout(() => {
console.log('second promise is completed');
resolve({data: '456'});
}, 2000);
});
return new RSVP.hash({
prom1: promise1,
prom2: promise2
});
};
mainPromise()
.then(data => {
console.log(data.prom1);
console.log(data.prom2);
});
Solution:
You can put intermediate values in scope in any later 'then' function explicitly, by using 'bind'. It is a nice solution that doesn't require changing how Promises work, and only requires a line or two of code to propagate the values just like errors are already propagated.
Here is a complete example:
// Get info asynchronously from a server
function pGetServerInfo()
{
// then value: "server info"
} // pGetServerInfo
// Write into a file asynchronously
function pWriteFile(path,string)
{
// no then value
} // pWriteFile
// The heart of the solution: Write formatted info into a log file asynchronously,
// using the pGetServerInfo and pWriteFile operations
function pLogInfo(localInfo)
{
var scope={localInfo:localInfo}; // Create an explicit scope object
var thenFunc=p2.bind(scope); // Create a temporary function with this scope
return (pGetServerInfo().then(thenFunc)); // Do the next 'then' in the chain
} // pLogInfo
// Scope of this 'then' function is {localInfo:localInfo}
function p2(serverInfo)
{
// Do the final 'then' in the chain: Writes "local info, server info"
return pWriteFile('log',this.localInfo+','+serverInfo);
} // p2
This solution can be invoked as follows:
pLogInfo("local info").then().catch(err);
(Note: a more complex and complete version of this solution has been tested, but not this example version, so it could have a bug.)
What I learn about promises is to use it only as return values avoid referencing them if possible. async/await syntax is particularly practical for that. Today all latest browsers and node support it: https://caniuse.com/#feat=async-functions , is a simple behavior and the code is like reading synchronous code, forget about callbacks...
In cases I do need to reference a promises is when creation and resolution happen at independent/not-related places. So instead an artificial association and probably an event listener just to resolve the "distant" promise, I prefer to expose the promise as a Deferred, which the following code implements it in valid es5
/**
* Promise like object that allows to resolve it promise from outside code. Example:
*
```
class Api {
fooReady = new Deferred<Data>()
private knower() {
inOtherMoment(data=>{
this.fooReady.resolve(data)
})
}
}
```
*/
var Deferred = /** #class */ (function () {
function Deferred(callback) {
var instance = this;
this.resolve = null;
this.reject = null;
this.status = 'pending';
this.promise = new Promise(function (resolve, reject) {
instance.resolve = function () { this.status = 'resolved'; resolve.apply(this, arguments); };
instance.reject = function () { this.status = 'rejected'; reject.apply(this, arguments); };
});
if (typeof callback === 'function') {
callback.call(this, this.resolve, this.reject);
}
}
Deferred.prototype.then = function (resolve) {
return this.promise.then(resolve);
};
Deferred.prototype.catch = function (r) {
return this.promise.catch(r);
};
return Deferred;
}());
transpiled form a typescript project of mine:
https://github.com/cancerberoSgx/misc-utils-of-mine/blob/2927c2477839f7b36247d054e7e50abe8a41358b/misc-utils-of-mine-generic/src/promise.ts#L31
For more complex cases I often use these guy small promise utilities without dependencies tested and typed. p-map has been useful several times. I think he covered most use cases:
https://github.com/sindresorhus?utf8=%E2%9C%93&tab=repositories&q=promise&type=source&language=
I'm using Mongoose with Node.js and have the following code that will call the callback after all the save() calls has finished. However, I feel that this is a very dirty way of doing it and would like to see the proper way to get this done.
function setup(callback) {
// Clear the DB and load fixtures
Account.remove({}, addFixtureData);
function addFixtureData() {
// Load the fixtures
fs.readFile('./fixtures/account.json', 'utf8', function(err, data) {
if (err) { throw err; }
var jsonData = JSON.parse(data);
var count = 0;
jsonData.forEach(function(json) {
count++;
var account = new Account(json);
account.save(function(err) {
if (err) { throw err; }
if (--count == 0 && callback) callback();
});
});
});
}
}
You can clean up the code a bit by using a library like async or Step.
Also, I've written a small module that handles loading fixtures for you, so you just do:
var fixtures = require('./mongoose-fixtures');
fixtures.load('./fixtures/account.json', function(err) {
//Fixtures loaded, you're ready to go
};
Github:
https://github.com/powmedia/mongoose-fixtures
It will also load a directory of fixture files, or objects.
I did a talk about common asyncronous patterns (serial and parallel) and ways to solve them:
https://github.com/masylum/i-love-async
I hope its useful.
I've recently created simpler abstraction called wait.for to call async functions in sync mode (based on Fibers). It's at an early stage but works. It is at:
https://github.com/luciotato/waitfor
Using wait.for, you can call any standard nodejs async function, as if it were a sync function, without blocking node's event loop. You can code sequentially when you need it.
using wait.for your code will be:
//in a fiber
function setup(callback) {
// Clear the DB and load fixtures
wait.for(Account.remove,{});
// Load the fixtures
var data = wait.for(fs.readFile,'./fixtures/account.json', 'utf8');
var jsonData = JSON.parse(data);
jsonData.forEach(function(json) {
var account = new Account(json);
wait.forMethod(account,'save');
}
callback();
}
That's actually the proper way of doing it, more or less. What you're doing there is a parallel loop. You can abstract it into it's own "async parallel foreach" function if you want (and many do), but that's really the only way of doing a parallel loop.
Depending on what you intended, one thing that could be done differently is the error handling. Because you're throwing, if there's a single error, that callback will never get executed (count won't be decremented). So it might be better to do:
account.save(function(err) {
if (err) return callback(err);
if (!--count) callback();
});
And handle the error in the callback. It's better node-convention-wise.
I would also change another thing to save you the trouble of incrementing count on every iteration:
var jsonData = JSON.parse(data)
, count = jsonData.length;
jsonData.forEach(function(json) {
var account = new Account(json);
account.save(function(err) {
if (err) return callback(err);
if (!--count) callback();
});
});
If you are already using underscore.js anywhere in your project, you can leverage the after method. You need to know how many async calls will be out there in advance, but aside from that it's a pretty elegant solution.