I am working with this while loop and it is not working. I decided to use the Google Chrome debugger and I saw that the code inside is not being executed.
All the time it checks the condition, starts the first line of the code inside, and goes back again to check the condition.
It is a NodeJS server and I am using the Spotify API.
app.get('/process', ensureAuthenticated, function (req, res) {
let init_array = text.split(" ");
let modtext = init_array;
while (init_array.length != 0) {
spotifyApi.searchTracks(modtext.join(" "))
.then(function (data) {
try {
console.log(data.body.tracks.items[0].name);
for (let i = 0; i < modtext.length; i++) {
init_array.shift();
}
modtext = init_array;
} catch (err) {
console.log("No song");
modtext.pop();
}
});
}
res.redirect('/');
});
This question is best understood by understanding how node.js uses an event loop. At its core, node.js runs your Javascript in a single thread and it uses an event loop in order to manage the completion of things outside that single thread such as timers, network operations, file operations, etc...
Let's first start with a very simple while() loop:
let done = false;
setTimeout(() => {
done = true;
}, 100);
while(!done) {
// do whatever you want here
}
console.log("done with while loop"); // never gets called
At first blush, you would think that the while loop would run for 100ms and then done would be set to true and the while loop would exit. That is not what happens. In fact, this is an infinite while loop. It runs and runs and runs and the variable done is never set to true. The console.log() at the end never runs.
It has this issue because setTimeout() is an asynchronous operation and it communicates its completion through the event loop. But, as we described above, node.js runs its Javascript as single threaded and only gets the next event from the event loop when that single thread finishes what it's doing. But, the while can't finish what it's doing until done gets set to true, but done can't get set to true until the while loop finishes. It's a stand-off and the while loop just runs forever.
So, in a nutshell, while any sort of loop is running, NO asynchronous operation ever gets its result processed (unless it's using await inside the loop which is something different). Asynchronous operations (or anything that uses the event loop) has to wait until the current running Javascript is done and then the interpreter can go back to the event loop.
Your while loop has the exact same issue. spotifyApi.searchTracks() is an asynchronous operation that returns a promise and all promises communicate their results via the event queue. So, you have the same standoff. Your .then() handler can't get called until the while loop finishes, but your while loop can't finish until the .then() handler gets called. Your while loop will just loop infinitely until you exhaust some system resource and your .then() handlers never get a chance to execute.
Since you haven't included code in your request handler that actually produces some result or action (all it appears to do is just modify some local variables), it's not obvious what exactly you're trying to accomplish and thus how to better write this code.
You appear to have N searches to do and you're logging something in each search. You could do them all in parallel and just use Promise.all() to track when they are all done (no while loop at all). Or, you can sequence them so you run one, get its result, then run another. Your question doesn't give us enough info to know what the best option would be.
Here's one possible solution:
Sequence the operations using async/await
Here the request handler is declared async so we can use await inside the while loop. That will suspend the while loop and allow other events to process while waiting for the promise to resolve.
app.get('/process', ensureAuthenticated, async function (req, res) {
let init_array = text.split(" ");
let modtext = init_array;
while (init_array.length != 0) {
try {
let data = await spotifyApi.searchTracks(modtext.join(" "));
console.log(data.body.tracks.items[0].name);
for (let i = 0; i < modtext.length; i++) {
init_array.shift();
}
modtext = init_array;
} catch (err) {
console.log("No song");
modtext.pop();
}
}
res.redirect('/');
});
The reason you're only seeing one line execute is because it's asynchronous. When you call an asynchronous function, it returns immediately and continues to do its work in the background. Once it's done, it calls another function (a "callback") and passes the results of the function to that. That's why your code has to go inside of a call to then() rather than just being on the next line of code.
In this case, spotifyApi.searchTracks() returns a Promise. Once the Spotify API has completed the search, the function in then() will run.
Lets use async/await to solve this problem, I have no clue what data you get in a text but I think it is good example to understand a concept of asynchronous processing.
app.get('/process', ensureAuthenticated, async function (req, res, next) {
try {
const modtext = text.split(" ");
const promises = modtext.map(item => spotify.searchTracks(item));
const response = await Promise.all(promises);
response.forEach(data => {
// if you use lodash, simply use a get function `get(data, 'body.tracks.items[0].name')` => no need to check existence of inner attributes
// or use something like this
if (data && data.body && data.body.track && data.body.tracks.items && data.body.tracks.items[0]) {
console.log(data.body.tracks.items[0].name);
} else {
console.log('No song');
}
});
res.redirect('/');
} catch(err) {
next(err)
}
});
For me much simpler and cleaner code, again, i dont know what is structure of your text attribute and logic behind it, so maybe you will have to make some changes.
Related
I am new to node js and trying to understand how to make async calls.
I am trying to wrap a function into a promise in order to make it async. For simplicity, the function is just a loop that takes time :
var p = new Promise(resolve => {
for (let i=0;i<999999999;i++){}
resolve('foo')
});
p.then(function (value) { console.log(value);});
console.log('bar');
I am expecting to see :
bar // printed immediately
foo // printed after the loop finishes
Instead, they are both printed after the loop completion.
How to make the loop block run asynchronously?
Ps. Apologies for the indentation/formatting, I had to write this from a phone.
Thanks
You seem to assume that "asynchronous code" involves concurrency, in the sense that code will run at the same time in another thread. That is not the case. Unless you start another thread by purpose, JS itself will run in a single thread. Therefore no matter what you do with promises: Either the loop runs first and then the logs run or the logs run first and then the loop runs.
You could also achieve concurrent execution through multitasking: If you stop the loop inbetween, other code can run in the meantime:
(async function() {
while(true) {
console.log("loop");
await Promise.resolve();
}
})();
console.log("in the meantime");
But there's nothing asynchronous about your promise. Creating a promise starts the execution of the function, and JS always runs to completion.
Typically, a promise is used to initiate something asynchronous, like an API call, or even a plain setTimeout, that runs outside JS's thread. Your code, however, will iterate through the empty loop bajillion times, and only after that any following lines will be run.
Replace the empty line with a timeout, and it will become async:
var p = new Promise(resolve => {
setTimeout(() => resolve("foo"), 2000);
});
p.then(function(value) {
console.log(value);
});
console.log('bar');
I have the following code that uses fetch. From what I understand, the callback function will not be invoked until the promise is fulfilled. Because of that, I was expecting the callback functions to be executed in the middle of processing other things (such as the for loop). However, it is not doing what I expect. My code is as follows:
console.log("Before fetch")
fetch('https://example.com/data')
.then(function(response){
console.log("In first then")
return response.json()
})
.then(function(json){
console.log("In second then")
console.log(json)
})
.catch(function(error){
console.log("An error has occured")
console.log(error)
})
console.log("After fetch")
for(let i = 0; i < 1000000; i++){
if (i % 10000 == 0)
console.log(i)
}
console.log("The End")
Rather than the callback being immediately run when the promise is fulfilled, it seems to wait until all the rest of my code is processed before the callback function is activated. Why is this?
The output of my code looks like this:
Before fetch
After fetch
0
10000
.
.
.
970000
980000
990000
The End
In first then
In second then
However, I was expecting the last two lines to appear somewhere prior to this point. What is going on here and how can I change my code so that it reflects when the promise is actually fulfilled?
The key here is that the for loop you're running afterwards is a long, synchronous block of code. That is the reason why synchronous APIs are deprecated / not recommended in JavaScript, as they block all asynchronous callbacks from executing until completion. JavaScript is not multithreaded, and it does not have concepts like interrupts in C, so if the thread is executing a large loop, nothing else will have the chance to run until that loop is finished.
In Node.js, the child_process API allows you to run daemon processes, and the Web Worker API for browsers allows concurrent processes to run in parallel, both of these using serialized event-based messaging to communicate between threads, but aside from that, everything in the above paragraph applies universally to JavaScript.
In general, a possible solution to breaking up long synchronous processes like the one you have there is batching. Using promises, you could rewrite the for loop like this:
(async () => {
for(let i = 0; i < 100000; i++){
if (i % 10000 == 0) {
console.log(i);
// release control for minimum of 4 ms
await new Promise(resolve => { setTimeout(resolve, 0); });
}
}
})().then(() => {
console.log("The End");
});
setTimeout(() => { console.log('Can interrupt loop'); }, 1);
Reason for 4ms minimum: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/setTimeout#Reasons_for_delays_longer_than_specified
No matter how fast is your promise fulfilled. Callbacks added to Event Loop and will invoke after all synchronous tasks are finished. In this example synchronous task is for loop. You can try event with setTimeout with 0ms, it will also work after loop. Remember JS is single-threaded and doesn't support parallel tasks.
Referance:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop
There's no guarantee when the callback will execute. The for-loop requires very little processing time, so it's possible that the JS engine just decided to wait until it's over to complete the callback functions. There's no certain way of forcing a specific order of functions either unless you chain them as callbacks.
Take the following loop:
for(var i=0; i<100; ++i){
let result = await some_slow_async_function();
do_something_with_result();
}
Does await block the loop? Or does the i continue to be incremented while awaiting?
Is the order of do_something_with_result() guaranteed sequential with regard to i? Or does it depend on how fast the awaited function is for each i?
Does await block the loop? Or does the i continue to be incremented while awaiting?
"Block" is not the right word, but yes, i does not continue to be incremented while awaiting. Instead the execution jumps back to where the async function was called, providing a promise as return value, continuing the rest of the code that follows after the function call, until the code stack has been emptied. Then when the awaiting is over, the state of the function is restored, and execution continues within that function. Whenever that function returns (completes), the corresponding promise -- that was returned earlier on -- is resolved.
Is the order of do_something_with_result() guaranteed sequential with regard to i? Or does it depend on how fast the awaited function is for each i?
The order is guaranteed. The code following the await is also guaranteed to execute only after the call stack has been emptied, i.e. at least on or after the next microtask can execute.
See how the output is in this snippet. Note especially where it says "after calling test":
async function test() {
for (let i = 0; i < 2; i++) {
console.log('Before await for ', i);
let result = await Promise.resolve(i);
console.log('After await. Value is ', result);
}
}
test().then(_ => console.log('After test() resolved'));
console.log('After calling test');
As #realbart says, it does block the loop, which then will make the calls sequential.
If you want to trigger a ton of awaitable operations and then handle them all together, you could do something like this:
const promisesToAwait = [];
for (let i = 0; i < 100; i++) {
promisesToAwait.push(fetchDataForId(i));
}
const responses = await Promise.all(promisesToAwait);
You can test async/await inside a "FOR LOOP" like this:
(async () => {
for (let i = 0; i < 100; i++) {
await delay();
console.log(i);
}
})();
function delay() {
return new Promise((resolve, reject) => {
setTimeout(resolve, 100);
});
}
async functions return a Promise, which is an object that will eventually "resolve" to a value, or "reject" with an error. The await keyword means to wait until this value (or error) has been finalized.
So from the perspective of the running function, it blocks waiting for the result of the slow async function. The javascript engine, on the other hand, sees that this function is blocked waiting for the result, so it will go check the event loop (ie. new mouse clicks, or connection requests, etc.) to see if there are any other things it can work on until the results are returned.
Note however, that if the slow async function is slow because it is computing lots of stuff in your javascript code, the javascript engine won't have lots of resources to do other stuff (and by doing other stuff would likely make the slow async function even slower). Where the benefit of async functions really shine is for I/O intensive operations like querying a database or transmitting a large file where the javascript engine is well and truly waiting on something else (ie. database, filesystem, etc.).
The following two bits of code are functionally equivalent:
let result = await some_slow_async_function();
and
let promise = some_slow_async_function(); // start the slow async function
// you could do other stuff here while the slow async function is running
let result = await promise; // wait for the final value from the slow async function
In the second example above the slow async function is called without the await keyword, so it will start execution of the function and return a promise. Then you can do other things (if you have other things to do). Then the await keyword is used to block until the promise actually "resolves". So from the perspective of the for loop it will run synchronous.
So:
yes, the await keyword has the effect of blocking the running function until the async function either "resolves" with a value or "rejects" with an error, but it does not block the javascript engine, which can still do other things if it has other things to do while awaiting
yes, the execution of the loop will be sequential
There is an awesome tutorial about all this at http://javascript.info/async.
No Event loop isn't blocked, see example below
function sayHelloAfterSomeTime (ms) {
return new Promise((resolve, reject) => {
if (typeof ms !== 'number') return reject('ms must be a number')
setTimeout(() => {
console.log('Hello after '+ ms / 1000 + ' second(s)')
resolve()
}, ms)
})
}
async function awaitGo (ms) {
await sayHelloAfterSomeTime(ms).catch(e => console.log(e))
console.log('after awaiting for saying Hello, i can do another things ...')
}
function notAwaitGo (ms) {
sayHelloAfterSomeTime(ms).catch(e => console.log(e))
console.log('i dont wait for saying Hello ...')
}
awaitGo(1000)
notAwaitGo(1000)
console.log('coucou i am event loop and i am not blocked ...')
Here is my test solution about this interesting question:
import crypto from "crypto";
function diyCrypto() {
return new Promise((resolve, reject) => {
crypto.pbkdf2('secret', 'salt', 2000000, 64, 'sha512', (err, res) => {
if (err) {
reject(err)
return
}
resolve(res.toString("base64"))
})
})
}
setTimeout(async () => {
console.log("before await...")
const a = await diyCrypto();
console.log("after await...", a)
}, 0);
setInterval(() => {
console.log("test....")
}, 200);
Inside the setTimeout's callback the await blocks the execution. But the setInterval is keep runnning, so the Event Loop is running as usual.
Let me clarify a bit because some answers here have some wrong information about how Promise execution works, specifically when related to the event loop.
In the case of the example, await will block the loop. do_something_with_result() will not be called until await finishes it's scheduled job.
https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Asynchronous/Async_await#handling_asyncawait_slowdown
As for the other points, Promise "jobs" run before the next event loop cycle, as microtasks. When you call Promise.then() or the resolve() function inside new Promise((resolve) => {}), you creating a Job. Both await and async are wrapper, of sorts, for Promise, that will both create a Job. Microtasks are meant to run before the next event loop cycle. That means adding a Promise Job means more work before it can move on to the next event loop cycle.
Here's an example how you can lock up your event loop because your promises (Jobs) take too long.
let tick = 0;
let time = performance.now();
setTimeout(() => console.log('Hi from timeout'), 0);
const tock = () => console.log(tick++);
const longTask = async () => {
console.log('begin task');
for(let i = 0; i < 1_000_000_000; i++) {
Math.sqrt(i);
}
console.log('done task');
}
requestAnimationFrame(()=> console.log('next frame after', performance.now() - time, 'ms'));
async function run() {
await tock();
await tock();
await longTask(); // Will stall your UI
await tock(); // Will execute even though it's already dropped frames
await tock(); // This will execute too
}
run();
// Promise.resolve().then(tock).then(tock).then(longTask).then(tock).then(tock);
In this sample, 5 total promises are created. 2 calls for tock, 1 for longTask and then 2 calls for tock. All 5 will run before the next event loop.
The execution would be:
Start JS execution
Execute normal script
Run 5 scheduled Promise jobs
End JS execution
Event Loop Cycle Start
Request Animation Frame fire
Timeout fire
The last line commented line is scheduling without async/await and results in the same.
Basically, you will stall the next event loop cycle unless you tell your JS execution where it can suspend. Your Promise jobs will continue to run in the current event loop run until it finishes its call stack. When you call something external, (like fetch), then it's likely using letting the call stack end and has a callback that will resolve the pending Promise. Like this:
function waitForClick() {
return new Promise((resolve) => {
// Use an event as a callback;
button.onclick = () => resolve();
// Let the call stack finish by implicitly not returning anything, or explicitly returning `undefined` (same thing).
// return undefined;
})
}
If you have a long job job that want to complete, either use a Web Worker to run it without pausing, or insert some pauses with something like setTimeout() or setImmediate().
Reshaping the longTask function, you can do something like this:
const longTask = async () => {
console.log('begin task');
for(let i = 0; i < 1_000_000_000; i++)
if (i && i % (10_000_000) === 0) {
await new Promise((r) => setTimeout(r,0));
}
Math.sqrt(i);
console.log('done task');
}
Basically, instead of doing 1 billion records in one shot, you only do 10 million and then wait until the next event (setTimeout) to run the next one. The bad here is it's slower because of how much you hand back to the event loop. Instead, you can use requestIdleCallback() which is better, but still not as good as multi-threading via Web Workers.
But be aware that just slapping on await or Promise.resolve().then() around a function won't help with the event loop. Both will wait until the function returns with either a Promise or a value before letting up for the event loop. You can mostly test by checking to see if the function you're calling returns an unresolved Promise immediately.
Does await block the loop? Or does the i continue to be incremented while awaiting?
No, await won't block the looping. Yes, i continues to be incremented while looping.
Is the order of do_something_with_result() guaranteed sequential with regard to i? Or does it depend on how fast the awaited function is for each i?
Order of do_something_with_result() is guaranteed sequentially but not with regards to i. It depends on how fast the awaited function runs.
All calls to some_slow_async_function() are batched, i.e., if do_something_with_result() was a console then we will see it printed the number of times the loop runs. And then sequentially, after this, all the await calls will be executed.
To better understand you can run below code snippet:
async function someFunction(){
for (let i=0;i<5;i++){
await callAPI();
console.log('After', i, 'th API call');
}
console.log("All API got executed");
}
function callAPI(){
setTimeout(()=>{
console.log("I was called at: "+new Date().getTime())}, 1000);
}
someFunction();
One can clearly see how line console.log('After', i, 'th API call'); gets printed first for entire stretch of the for loop and then at the end when all code is executed we get results from callAPI().
So if lines after await were dependent on result obtained from await calls then they will not work as expected.
To conclude, await in for-loop does not ensure successful operation on result obtained from await calls which might take some time to finish.
In node, if one uses neo-async library with waterfall, one can achieve this.
I'm a bit new to Node.js. I've run into a problem where I want to prevent a callback from running while it is already being executed. For example:
items.forEach(function(item) {
doLongTask(item, function handler(result) {
// If items.length > 1, this will get executed multiple times.
});
});
How do I make the other invocations of handler wait for the first one to finish before going ahead? I'm thinking something along the lines of a queue, but I'm a newbie to Node.js so I'm not exactly sure what to do. Ideas?
There are already libraries which take care of that, the most used being async.
You will be interested in the async.eachSeries() function.
As for an actual example...
const async = require('async')
async.eachSeries(
items,
(item, next) => {
// Do stuff with item, and when you are done, call next
// ...
next()
},
err => {
// either there was an error in one of the handlers and
// execution was stopped, or all items have been processed
}
)
As for how the library does this, you are better of having a look at the source code.
It should be noted that this only ever makes sense if your item handler ever performs an asynchronous operation, like interfacing with the filesystem or with internet etc. There exists no operation in Node.js that would cause a piece of JS code to be executed in parallel to another JS code within the same process. So, if all you do is some calculations, you don't need to worry about this at all.
How to prevent two callbacks from running simultaneously?
They won't run simultaneously unless they're asynchronous, because Node runs JavaScript on a single thread. Asynchronous operations can overlap, but the JavaScript thread will only ever be doing one thing at a time.
So presumably doLongTask is asynchronous. You can't use forEach for what you'd like to do, but it's still not hard: You just keep track of where you are in the list, and wait to start processing the next until the previous one completes:
var n = 0;
processItem();
function processItem() {
if (n < items.length) {
doLongTask(items[n], function handler(result) {
++n;
processItem();
});
}
}
I have a node application that is not a web application - it completes a series of asynchronous tasks before returning 1. Immediately before returning, the results of the program are printed to the console.
How do I make sure all the asynchronous work is completed before returning? I was able to achieve something similar to this in a web application by making sure all tasks we completed before calling res.end(), but I haven't any equivalent for a final 'event' to call before letting a script return.
See below for my (broken) function currently, attempting to wait until callStack is empty. I just discovered that this is a kind of nonsensical approach because node waits for processHub to complete before entering any of the asynchronous functions called in processObjWithRef.
function processHub(hubFileContents){
var callStack = [];
var myNewObj = {};
processObjWithRef(samplePayload, myNewObj, callStack);
while(callStack.length>0){
//do nothing
}
return 1
}
Note: I have tried many times previously to achieve this kind of behavior with libraries like async (see my related question at How can I make this call to request in nodejs synchronous?) so please take the answer and comments there into account before suggesting any answers based on 'just use asynch'.
You cannot wait for an asynchronous event before returning--that's the definition of asynchronous! Trying to force Node into this programming style will only cause you pain. A naive example would be to check periodically to see if callstack is empty.
var callstack = [...];
function processHub(contents) {
doSomethingAsync(..., callstack);
}
// check every second to see if callstack is empty
var interval = setInterval(function() {
if (callstack.length == 0) {
clearInterval(interval);
doSomething()
}
}, 1000);
Instead, the usual way to do async stuff in Node is to implement a callback to your function.
function processHub(hubFileContents, callback){
var callStack = [];
var myNewObj = {};
processObjWithRef(samplePayload, myNewObj, callStack, function() {
if (callStack.length == 0) {
callback(some_results);
}
});
}
If you really want to return something, check out promises; they are guaranteed to emit an event either immediately or at some point in the future when they are resolved.
function processHub(hubFileContents){
var callStack = [];
var myNewObj = {};
var promise = new Promise();
// assuming processObjWithRef takes a callback
processObjWithRef(samplePayload, myNewObj, callStack, function() {
if (callStack.length == 0) {
promise.resolve(some_results);
}
});
return promise;
}
processHubPromise = processHub(...);
processHubPromise.then(function(result) {
// do something with 'result' when complete
});
The problem is with your design of the function. You want to return a synchronous result from a list of tasks that are executed asynchronously.
You should implement your function with an extra parameter that will be the callback where you would put the result (in this case, 1) for some consumer to do something with it.
Also you need to have a callback parameter in your inner function, otherwise you won't know when it ends. If this last thing is not possible, then you should do some kind of polling (using setInterval perhaps) to test when the callStack array is populated.
Remember, in Javascript you should never ever do a busy wait. That will lock your program entirely as it runs on a single process.
deasync is desinged to address your problem exactly. Just replace
while(callStack.length>0){
//do nothing
}
with
require('deasync').loopWhile(function(){return callStack.length>0;});
The problem is that node.js is single-threaded, which means that if one function runs, nothing else runs (event-loop) until that function has returned. So you can not block a function to make it return after async stuff is done.
You could, for example, set up a counter variable that counts started async tasks and decrement that counter using a callback function (that gets called after the task has finished) from your async code.
Node.js runs on A SINGLE threaded event loop and leverages asynchronous calls for doing various things, like I/O operations.
if you need to wait for a number of asynchronous operations to finish before executing additional code
you can try using Async -
Node.js Async Tutorial
You'll need to start designing and thinking asynchronously, which can take a little while to get used to at first. This is a simple example of how you would tackle something like "returning" after a function call.
function doStuff(param, cb) {
//do something
var newData = param;
//"return"
cb(newData);
}
doStuff({some:data}, function(myNewData) {
//you're done with doStuff in here
});
There's also a lot of helpful utility functions in the async library available on npm.