Building a node.js CLI application. Users should choose some tasks to run and based on that, tasks should work and then spinners (using ora package) should show success and stop spin.
The issue here is spinner succeed while tasks are still going on. Which means it doesn't wait.
Tried using typical Async/Await as to have an async function and await each function under condition. Didn't work.
Tried using promise.all. Didn't work.
Tried using waterfall. Same.
Here's the code of the task runner, I create an array of functions and pass it to waterfall (Async-waterfall package) or promise.all() method.
const runner = async () => {
let tasks = [];
spinner.start('Running tasks');
if (syncOptions.includes('taskOne')) {
tasks.push(taskOne);
}
if (syncOptions.includes('taskTwo')) {
tasks.push(taskTwo);
}
if (syncOptions.includes('taskThree')) {
tasks.push(taskThree);
}
if (syncOptions.includes('taskFour')) {
tasks.push(taskFour);
}
// Option One
waterfall(tasks, () => {
spinner.succeed('Done');
});
// Option Two
Promise.all(tasks).then(() => {
spinner.succeed('Done');
});
};
Here's an example of one of the functions:
const os = require('os');
const fs = require('fs');
const homedir = os.homedir();
const outputDir = `${homedir}/output`;
const file = `${homedir}/.file`;
const targetFile = `${outputDir}/.file`;
module.exports = async () => {
await fs.writeFileSync(targetFile, fs.readFileSync(file));
};
I tried searching concepts. Talked to the best 5 people I know who can write JS properly. No clue.. What am I doing wrong ?
You don't show us all your code, but the first warning sign is that it doesn't appear you are actually running taskOne(), taskTwo(), etc...
You are pushing what look like functions into an array with code like:
tasks.push(taskFour);
And, then attempting to do:
Promise.all(tasks).then(...)
That won't do anything useful because the tasks themselves are never executed. To use Promise.all(), you need to pass it an array of promises, not an array of functions.
So, you would use:
tasks.push(taskFour());
and then:
Promise.all(tasks).then(...);
And, all this assumes that taskOne(), taskTwo(), etc... are function that return a promise that resolves/rejects when their asynchronous operation is complete.
In addition, you also need to either await Promise.all(...) or return Promise.all() so that the caller will be able to know when they are all done. Since this is the last line of your function, I'd generally just use return Promise.all(...) and this will let the caller get the resolved results from all the tasks (if those are relevant).
Also, this doesn't make much sense:
module.exports = async () => {
await fs.writeFileSync(targetFile, fs.readFileSync(file));
};
You're using two synchronous file operations. They are not asynchronous and do not use promises so there's no reason to put them in an async function or to use await with them. You're mixing two models incorrectly. If you want them to be synchronous, then you can just do this:
module.exports = () => {
fs.writeFileSync(targetFile, fs.readFileSync(file));
};
If you want them to be asynchronous and return a promise, then you can do this:
module.exports = async () => {
return fs.promises.writeFile(targetFile, await fs.promises.readFile(file));
};
Your implementation was attempting to be half and half. Pick one architecture or the other (synchronous or asynchronous) and be consistent in the implementation.
FYI, the fs module now has multiple versions of fs.copyFile() so you could also use that and let it do the copying for you. If this file was large, copyFile() would likely use less memory in doing so.
As for your use of waterfall(), it is probably not necessary here and waterfall uses a very different calling model than Promise.all() so you certainly can't use the same model with Promise.all() as you do with waterfall(). Also, waterfall() runs your functions in sequence (one after the other) and you pass it an array of functions that have their own calling convention.
So, assuming that taskOne, taskTwo, etc... are functions that return a promise that resolve/reject when their asynchronous operations are done, then you would do this:
const runner = () => {
let tasks = [];
spinner.start('Running tasks');
if (syncOptions.includes('taskOne')) {
tasks.push(taskOne());
}
if (syncOptions.includes('taskTwo')) {
tasks.push(taskTwo());
}
if (syncOptions.includes('taskThree')) {
tasks.push(taskThree());
}
if (syncOptions.includes('taskFour')) {
tasks.push(taskFour());
}
return Promise.all(tasks).then(() => {
spinner.succeed('Done');
});
};
This would run the tasks in parallel.
If you want to run the tasks in sequence (one after the other), then you would do this:
const runner = async () => {
spinner.start('Running tasks');
if (syncOptions.includes('taskOne')) {
await taskOne();
}
if (syncOptions.includes('taskTwo')) {
await taskTwo();
}
if (syncOptions.includes('taskThree')) {
await taskThree();
}
if (syncOptions.includes('taskFour')) {
await taskFour();
}
spinner.succeed('Done');
};
I want to force Node to wait for the promise to complete (either by failure or success). So far, I'm not seeing what I wanted. I'm trying to wait for two web services to complete before merging, and especially before the function ends.
I've tried these two approaches below. In both cases, Node refuses to wait.
Approach 1:
function getStrategy() {
// this is a web service that takes a few ms to run,
// but so far I haven't seen any evidence that it bothers.
}
function getConfig() {
// Both strategy and jwt are set to Promises
const strategy = getStrategy();
const jwt = getJwt();
const lodash = require('lodash');
var config = {};
try {
Promise.all([strategy,jwt])
.then(data => { config = lodash.merge(config,data)})
} catch(error) {
console.log(' Print error message');
}
return config;
}
Approach 2:
function getStrategy() {
// this is a web service that takes a few ms to run,
// but so far I haven't seen any evidence that it bothers.
}
async function getConfig() {
// Both strategy and jwt are set to Promises
const strategy = getStrategy();
const jwt = getJwt();
const lodash = require('lodash');
var config = {};
try {
var promiseResult = await Promise.all([strategy, jwt]);
const lodash = require('lodash');
config = lodash.merge(config, strategy[0]);
config = lodash.merge(config, jwt);
} catch (reason) {
console.error('------------------------------------------------------------------------------------');
console.error(reason);
console.error("in getConfig(): Could not fetch strategy or jwt");
console.error('------------------------------------------------------------------------------------');
}
return config;
}
I wish approach 2 worked, but it does not. It will not print any console.log statements after the call to Promise.all. So within that function it does wait. Except that it because I told it to "await", I have to make the function async. That allows Node to say, "oh, it's an async function, I can just go off and do something else and completely ignore the await keyword." It does this by returning to the function calling getConfig().
In the first approach, neither the "then" handler, nor any exceptions are thrown. It just impatiently leaves the function and goes back to the caller.
How do I get the thread that calls the getConfig() function to wait for the result. I mean really wait, not like, partially "await". Or, throw an exception and let me handle that. I'm finding that in Node, as soon as something because asynchronous, I have no idea how to get the await, or the then handler to work.
Updated attempt:
I separated the two service calls to individually control each service. I now have
async function getSynchronousStrategy(isVaultAvailable) {
const secretsVaultReader = require('./src/configuration/secretsVaultReader');
const secretsConfigReader = require('./src/configuration/secretsConfigReader');
const strategy = isVaultAvailable
? secretsVaultReader.getStrategy()
: secretsConfigReader.getStrategy();
await strategy;
console.log('## strategy after await=' + strategy);
return strategy; ''
}
...
const strategy = await getSynchronousStrategy(isVaultAvailable);
console.log('### strategy =' + JSON.stringify(strategy));
...
In the above case, Node sees the await strategy, but then prints
"## strategy after await=[object Promise]"
However, the second await seems to work, and it prints the desired strategy. I'm guessing the promise was eventually settled and it was able to print the result. I don't mind the time, I just want it to wait.
Obviously, it did not wait. It
I would suggest simply using await with both the function calls directly, in this manner it will wait for both the functions to return responses before proceeding further.
NodeJS 6.9.3
What I previously had went like this:
An outer function called "get_user()":
return database_queries.get_user(user_name)
.then(function(results_from_database) {
and that function then ran a database call, using Knex, and returned:
var dbquery = Multiline.stripIndent(function () {/*
SELECT
u.id as profile_id,
'user' as type_of_profile
FROM
user_profile u
WHERE name REGEXP "[[:<:]]||user_name||[[:>:]]"
*/});
dbquery = dbquery.replaceAll('||user_name||', user_name);
return DB.knex.raw(dbquery).then(function(result1) {
for(var index_of_results = 0; index_of_results < result1[0].length; index_of_results++) {
var document1 = result1[0][index_of_results];
array_of_maps_with_profile_type_and_profile_id[document1["type_of_profile"]].push(document1["profile_id"]);
}
When I did this, the database query ran, and got data, but this happened asynchronously, without the results ever being returned to the outer function. In other words, the outer function had completed long before the database queries had run.
So I tried to wrap the inner function in a Promise:
function get_user(user_name) {
return new Promise(function(resolve, reject) {
resolve ()
.then(function() {
var dbquery = Multiline.stripIndent(function () {/*
SELECT
u.id as profile_id,
'user' as type_of_profile
FROM
user_profile u
WHERE name REGEXP "[[:<:]]||user_name||[[:>:]]"
*/});
dbquery = dbquery.replaceAll('||user_name||', user_name);
return DB.knex.raw(dbquery).then(function(result1) {
for(var index_of_results = 0; index_of_results < result1[0].length; index_of_results++) {
var document1 = result1[0][index_of_results];
array_of_maps_with_profile_type_and_profile_id[document1["type_of_profile"]].push(document1["profile_id"]);
}
Now the database calls don't seem to ever be called. When they run, they appear in the logs, but now there are no database queries appearing in the logs. It would seem this inner function now returns a Promise, but the "resolve()" part of the Promise is never called.
What am I doing wrong here?
Here is a bit simpler way to write essentially the same query:
function get_user(user_name) {
const regex = `[[:<:]]${user_name}[[:>:]]`;
return DB.knex('user_profile')
.where(DB.knex.raw(`?? REGEXP ?`, ['name', regex]))
.then(res => {
// do what ever you like with returned rows here
});
}
You might want to simplify that a bit.
Take a look at these example projects.
https://github.com/joicenunes/helloapp
https://github.com/joicenunes/exercicio02
https://github.com/joicenunes/exercicio03
https://github.com/joicenunes/exercicio-04
(there are more, but you can find the rest)
Also avoid "replaceAll" and use binding variables.
Finally, since you are using node 6.x, you can use a few es6 goodies (arrow functions, multi-line strings, classes, etc), make the language work for you.
var assert = require('assert');
var parseJSON = require('json-parse-async');
var contact = new Object();
contact.firstname = "Jesper";
contact.surname = "Aaberg";
contact.phone = ["555-0100", "555-0120"];
var contact2 = new Object();
contact2.firstname = "JESPER";
contact2.surname = "AABERG";
contact2.phone = ["555-0100", "555-0120"];
contact.toJSON = function(key) {
var replacement = new Object();
for (var val in this) {
if (typeof(this[val]) === 'string')
replacement[val] = this[val].toUpperCase();
else
replacement[val] = this[val]
}
return replacement;
};
var jsonText = JSON.stringify(contact);
contact = JSON.parse(jsonText);
console.log(contact);
console.log(contact2);
assert.deepEqual(contact, contact2, 'these two objects are the same');
What are the asynchronous equivalent functions of JSON.parse, JSON.stringify and assert.deepEqual? I am trying to create a race condition and non-deterministic behavior within the following code but I have not been able lto find non-blocking, asynchronous equivalents of the functions mentioned above.
node.js does not have an actual asynchronous JSON parser built-in. If you want something that will actually do the parsing outside the main node.js Javascript thread, then you would have to find a third party module that parses the JSON outside of the Javascript thread (e.g. in a native code thread or in some other process). There are some modules in NPM that advertise themselves as asynchronous such as async-json-parser or async-json-parse or json-parse-async. You would have to verify that whichever implementation you were interested in was truly an asynchronous implementation (your Javascript continues to run while the parsing happens in the background).
But, reading the detail in your question about the problem you're trying to solve, it doesn't sound like you actually need a parser that truly happens in the background. To give you your ability to test what you're trying to test, it seems to me like you just need an indeterminate finish that allows other code to run before the parsing finishes. That can be done by wrapping the synchronous JSON.parse() in a setTimeout() with a promise that has a random delay. That will give some random amount of time for other code to run (to try to test for your race conditions). That could be done like this:
JSON.parseAsyncRandom = function(str) {
return new Promise(function(resolve, reject) {
// use a random 0-10 second delay
setTimeout(function() {
try {
resolve(JSON.parse(str));
} catch(e) {
reject(e);
}
}, Math.floor(Math.random() * 10000));
});
}
JSON.parseAsyncRandom(str).then(function(obj) {
// process obj here
}, function(err) {
// handle err here
});
Note: This is not true asynchronous execution. It's an asynchronous result (in that it arrives some random time later and other code will run before the result arrives), but true asynchronous execution happens in the background in parallel with other JS running and this isn't quite that. But, given your comment that you just want variable and asynchronous results for testing purposes, this should do that.
I've recently faced this problem myself, so I decided to create a library to handle JSON parsing in a really asynchronous way.
The idea behind it is to divide the parsing process into chunks, and then run each separately in the event loop so that other events (user interactions, etc) can still be evaluated within a few milliseconds, keeping the UI interactive.
If you are interested, the library it's called RAJI and you can find it here: https://github.com/federico-terzi/raji
After installing RAJI, you can then convert your synchronous JSON.parse calls into async raji.parse calls, such as:
const object = await parse(payload);
These calls won't block the UI
You can use 'bluebird', like this example to convert calling function to promise.
I write code below using javascript es6.
const Promise = require('bluebird')
function stringifyPromise(jsonText) {
return Promise.try(() => JSON.stringify(jsonText))
}
function parsePromise(str) {
return Promise.try(() => JSON.parse(str))
}
stringifyPromise(contact)
.then(jsonText => parsePromise(jsonText))
.then(contact => {
assert.deepEqual(contact, contact2, 'these two objects are the same')
})
})
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=