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 know there are a lot of good examples over the web and I read a lot of them, but currently I'm stucked with resolving promises with the new functionality of generators in nodejs 0.11.x.
For e.g. I have the following function:
SolrBaseDomain.prototype.promisedQuery = function(query, callback) {
var solrClient = solr.createClient(this.configuration);
var defer = Q.defer();
solrClient.search(query, function(err,obj){
if (!err) {
if (obj.response.numFound > 0) {
defer.resolve(obj.response.docs);
} else {
defer.resolve(null);
}
} else {
defer.reject(err);
}
});
var promise = defer.promise;
return Q.async(function* (){
var result = yield promise;
return result;
});
};
I expected that every call to this method will wait until the promise is fullfilled and the return-statement gives back the result of the promise.
But currently it seems that instead the code inside "Q.async..." will not be executed or the async call arrives after the return statement of the method was executed.
It's strange, in every example I know, this is one of the recommended ways in order to wait for async calls in nodejs but currently it does not work for me.
I've tried a lot of different variations of the above example, but the result is everytime the same, I get not back a valid result.
I have nodejs installed in version 0.11.10 and the --harmony-flag is set, when the code is executede.
Can anyone point me to right direction? I'm wondering if I oversee something ... :)
Thanks for your feedback.
Best regards
Udo
I expected that every call to this method will wait until the promise is fullfilled and the return-statement gives back the result of the promise.
No. Generators will not make functions synchronous, you cannot (and don't want to) block while waiting for a result. When calling a generator function and running sequentially through the async steps that it yields, the result you will get back in the end is still asynchronous - and therefore a promise. Only inside of the generator, your code can use synchronous control flow and yield.
This means that the (then-) callback-based code
SolrBaseDomain.prototype.promisedQuery = function(query) {
var promise = Q.ninvoke(solr.createClient(this.configuration), "search", query);
return promise.then(function(obj) {
if (obj.response.numFound > 0) {
return obj.response.docs;
} else {
return null;
}
});
};
becomes
SolrBaseDomain.prototype.promisedQuery = Q.async(function* (query) {
var promise = Q.ninvoke(solr.createClient(this.configuration), "search", query);
var obj = yield promise;
// ^^^^^
if (obj.response.numFound > 0) {
return obj.response.docs;
} else {
return null;
}
});
Try this
SolrBaseDomain.prototype.promisedQuery = Q.async(function*(query) {
var solrClient = solr.createClient(this.configuration);
var obj = yield Q.ninvoke(solrClient, "search", query);
return obj.response.numFound > 0 ? obj.response.docs : null;
});
This does the same thing for promises as this does for callbacks:
SolrBaseDomain.prototype.query = function (query, callback) {
var solrClient = solr.createClient(this.configuration);
solrClient.search(query, function(err, obj) {
if (err) return callback(err);
callback(null, obj.response.numFound > 0 ? obj.response.docs : null);
});
};
Therefore if the first return a promise that resolves to undefined so will the callback version call the callback with undefined.
according to your suggestions, my code looks now like this:
...
SolrBaseDomain.prototype.query = Q.async(function* (query) {
var solrClient = solr.createClient(this.configuration);
var obj = yield Q.ninvoke(solrClient, "search", query);
return obj.response.numFound > 0 ? obj.response.docs : null;
});
...
I share the above query-function over all data access layers in order to have a central method which is querying the different indexes in an asynchronous way.
For e.g. in the domain data access layer, the code which deals with that function looks like this:
SolrHostDomain.prototype.getByName = Q.async(function* (domain) {
var queryObject = {
"domain": domain
};
var query = this.getQuery("byName", queryObject);
var docs = yield this.query(query);
var domain = null;
if (docs != null && docs.length > 0) {
domain = this.dataMapper.merge(docs[0]);
}
return domain;});
Currently I'm not sure if the generator in the "getByName"-function is necessary, but it seems to work. Dealing with promises is some unclear concept for me, since I'm new to nodejs.
So maybe, if you can help me on that topic and point me in the right direction, this would be helpfull.
The main question for me is, how can I ensure, that a synchronous method can call an asynchronous method and get back not a promise but the final result of this promise.
I've searched a long time, but I could not find a good documentation which describes the use of generator functions or promises in conjunction with synchronous calls. Even examples are focusing only of using the mechanism but not working together with synchronous function.
Best regards and many thanks for your help
Udo
Got it!!!
After a few trial and errors, I think I got it now and I have a working solutions:
Query function:
SolrBaseDomain.prototype.query = Q.async(function* (query) {
var solrClient = solr.createClient(this.configuration);
var obj = yield Q.ninvoke(solrClient, "search", query);
return obj.response.numFound > 0 ? obj.response.docs : null;
});
Calling method:
SolrHostDomain.prototype.getByName = function(domain) {
var queryObject = {
"domain": domain
};
var query = this.getQuery("byName", queryObject);
var docsPromise = this.query(query);
var _self = this;
return docsPromise.then(function(docs) {
var domain = null;
if (docs != null && docs.length > 0) {
domain = _self.dataMapper.merge(docs[0]);
}
return domain;
});
};
The solution was to understand, that the "query"-method still returns a promise instead of the concrete result even if yield is used.
So I have to add every code which is working on the result of the promise within the "then"-functions (or "done" if no other caller up in the calling hierarchy of methods will follow).
After the settlement of the promise, each code which is set within the "then"-functions will be processed.
BR
Udo
I want to promisify node-postgres' pg.connect method along with the inner connection.query method provided in the callback.
I can .promisify the latter, but I need to implement the first one manually (if I'm missing something here, please explain).
The thing is, I'm not sure if this code is correct or should be improved? The code is working, I just want to know if I'm using Bluebird as meant.
// aliases
var asPromise = Promise.promisify;
// save reference to original method
var connect = pg.connect.bind(pg);
// promisify method
pg.connect = function (data) {
var deferred = Promise.defer();
connect(data, function promisify(err, connection, release) {
if (err) return deferred.reject(err);
// promisify query factory
connection.query = asPromise(connection.query, connection);
// resolve promised connection
deferred.resolve([connection,release]);
});
return deferred.promise;
};
Throw all that horrible callback code away, then do this somewhere in your application initialization:
var pg = require("pg");
var Promise = require("bluebird");
Object.keys(pg).forEach(function(key) {
var Class = pg[key];
if (typeof Class === "function") {
Promise.promisifyAll(Class.prototype);
Promise.promisifyAll(Class);
}
})
Promise.promisifyAll(pg);
Later in anywhere you can use the pg module as if it was designed to use promises to begin with:
// Later
// Don't even need to require bluebird here
var pg = require("pg");
// Note how it's the pg API but with *Async suffix
pg.connectAsync(...).spread(function(connection, release) {
return connection.queryAsync("...")
.then(function(result) {
console.log("rows", result.rows);
})
.finally(function() {
// Creating a superfluous anonymous function cos I am
// unsure of your JS skill level
release();
});
});
By now there are a number of libraries which do this for you:
pg-promise - generic Promises/A+ for PG
postgres-bluebird
dbh-ph
pg-bluebird
Update for bluebird 3:
The pg.connectAsync(...).spread(function(connection, release) { ... }) call will not work anymore, because the API of bluebird has changed: http://bluebirdjs.com/docs/new-in-bluebird-3.html#promisification-api-changes .
The problem is that promisifyAll in bluebird 3 does not handle multiple arguments by default. This results in the .spread() call reporting a TypeError like the following:
TypeError: expecting an array or an iterable object but got [object Null]
To solve this, you can explicitly enable multiple arguments for connect / connectAsync. Do the following after all the promisifying stuff mentioned above:
...
pg.connectAsync = Promise.promisify(pg.connect, { multiArgs: true });
I suggest to modify Petka Antonov solution a bit
var Promise = require('bluebird');
var pg = require('pg');
Object.keys(pg).forEach(function (key) {
var Cls = null;
try {
Cls = pg[key];
if (typeof Cls === 'function') {
Promise.promisifyAll(Cls.prototype);
Promise.promisifyAll(Cls);
}
} catch (e) {
console.log(e);
}
});
Promise.promisifyAll(pg);
here 'pg[key] wrapped up in try-catch block because pg[key] can retrun error when attempt to access pg['native']