Related
I want to create a task that only builds the entire project if something changed. For that, I am comparing hashes (irrelevant to the question).
const buildIfChanged = async () => {
const hash = await getHash();
const newHash = await getNewHash();
if (hash !== newHash) {
console.log("START");
const task = series(build, cleanup)();
console.log("END", task);
}
};
In this example, task is undefined, so I cannot add a .on("end", ...) and resolve the promise after that. I also cannot await it.
The problem is, because I am not waiting for it to complete, the buildIfChanged task completes before build even has a chance to run.
Is there any way to do this with modern gulpfiles?
I have found a solution on how to do this in modern gulpfiles. The function call of parallel and series actually take a done function as a parameter.
So to solve this, you can do:
const buildIfChanged = async () => {
return new Promise(async (res, rej) => {
try {
const hash = await getHash();
const newHash = await getNewHash();
if (hash !== newHash) {
console.log("START");
const task = series(build, cleanup)(() => res()); // add the done function here
console.log("END", task);
}
} catch(e) {
rej(e);
}
})
};
I'm with Node.js and TypeScript and I'm using async/await.
This is my test case:
async function doSomethingInSeries() {
const res1 = await callApi();
const res2 = await persistInDB(res1);
const res3 = await doHeavyComputation(res1);
return 'simle';
}
I'd like to set a timeout for the overall function. I.e. if res1 takes 2 seconds, res2 takes 0.5 seconds, res3 takes 5 seconds I'd like to have a timeout that after 3 seconds let me throw an error.
With a normal setTimeout call is a problem because the scope is lost:
async function doSomethingInSeries() {
const timerId = setTimeout(function() {
throw new Error('timeout');
});
const res1 = await callApi();
const res2 = await persistInDB(res1);
const res3 = await doHeavyComputation(res1);
clearTimeout(timerId);
return 'simle';
}
And I cannot catch it with normal Promise.catch:
doSomethingInSeries().catch(function(err) {
// errors in res1, res2, res3 will be catched here
// but the setTimeout thing is not!!
});
Any ideas on how to resolve?
You can use Promise.race to make a timeout:
Promise.race([
doSomethingInSeries(),
new Promise((_, reject) => setTimeout(() => reject(new Error('timeout')), 11.5e3))
]).catch(function(err) {
// errors in res1, res2, res3 and the timeout will be caught here
})
You cannot use setTimeout without wrapping it in a promise.
Ok I found this way:
async function _doSomethingInSeries() {
const res1 = await callApi();
const res2 = await persistInDB(res1);
const res3 = await doHeavyComputation(res1);
return 'simle';
}
async function doSomethingInSeries(): Promise<any> {
let timeoutId;
const delay = new Promise(function(resolve, reject){
timeoutId = setTimeout(function(){
reject(new Error('timeout'));
}, 1000);
});
// overall timeout
return Promise.race([delay, _doSomethingInSeries()])
.then( (res) => {
clearTimeout(timeoutId);
return res;
});
}
Anyone errors?
The things that smells a bit to me is that using Promises as asynchronicity strategy will send us to allocate too many object that some other strategy needs but this is off-topic.
Problem with #Bergi answer that doSomethingInSeries continues executing even if you already rejected the promise. It is much better to cancel it.
LATEST ANSWER
You can try use AbortController for that. Check the old answer to see how to use it - api is similar.
Keep in mind that task is not cancelled immediately, so continuation (awaiting, then or catch) is not called exactly after timeout.
To guarantee that you can combine this and #Bergi approach.
OLD ANSWER
This is how it should look like:
async const doSomethingInSeries = (cancellationToken) => {
cancellationToken.throwIfCancelled();
const res1 = await callApi();
cancellationToken.throwIfCancelled();
const res2 = await persistInDB(res1);
cancellationToken.throwIfCancelled();
const res3 = await doHeavyComputation(res1);
cancellationToken.throwIfCancelled();
return 'simle';
}
Here is simple implementation:
const makeCancellationToken = (tag) => {
let cancelled = false;
return {
isCancelled: () => cancelled,
cancel: () => {
cancelled = true;
},
throwIfCancelled: () => {
if (cancelled) {
const error = new Error(`${tag ?? 'Task'} cancelled`);
error.cancelled = true;
throw error;
}
}
}
}
And finally usage:
const cancellationToken = makeCancellationToken('doSomething')
setTimeout(cancellationToken.cancel, 5000);
try {
await doSomethingInSeries(cancellationToken);
} catch (error) {
if (error.cancelled) {
// handle cancellation
}
}
Keep in mind that task is not cancelled immediately, so continuation (awaiting, then or catch) is not called exactly after 5 secs.
To guarantee that you can combine this and #Bergi approach.
I have javascript function like this:
function myFunction(number) {
var x=number;
...
... more initializations
//here need to wait until flag==true
while(flag==false)
{}
...
... do something
}
The problem is that the javascript is stuck in the while and stuck my program. so my question is how can I wait in the middle of the function until flag is true without "busy-wait"?
Javascript is single threaded, hence the page blocking behaviour. You can use the deferred/promise approach suggested by others. The most basic way would be to use window.setTimeout. E.g.
function checkFlag() {
if(flag === false) {
window.setTimeout(checkFlag, 100); /* this checks the flag every 100 milliseconds*/
} else {
/* do something*/
}
}
checkFlag();
Here is a good tutorial with further explanation: Tutorial
EDIT
As others pointed out, the best way would be to re-structure your code to use callbacks. However, this answer should give you an idea how you can 'simulate' an asynchronous behaviour with window.setTimeout.
You created an infinite loop where the flag value that terminates the loop can never be changed by code outside this loop because no code outside the loop ever gets to run - thus this will never work.
Because javascript in a browser is single threaded (except for webworkers which aren't involved here) and one thread of javascript execution runs to completion before another can run, your statement:
// this won't work!
while(flag==false) {}
will simply run forever (or until the browser complains about a non-responsive javascript loop), the page will appear to be hung and no other javascript will ever get a chance to run, thus the flag's value can never be changed by code outside this loop.
For a little more explanation, Javascript is an event driven language. That means that it runs a piece of Javascript until it returns control back to the interpreter. Then, only when it returns back to the interpreter, Javascript gets the next event from the event queue and runs it.
All things like timers and network events run through the event queue. So, when a timer fires or a network request arrives, it does not ever "interrupt" the currently running Javascript. Instead, an event gets put in the Javascript event queue and then, when the currently running Javascript finishes, the next event is pulled from the event queue and it gets its turn to run.
So, when you do an infinite loop such as while(flag==false) {}, the currently running Javascript never finishes and thus the next event is never pulled from the event queue and thus the value of flag never gets changed. They key here is that Javascript is not interrupt driven. When a timer fires, it does not interrupt the currently running Javascript, run some other Javascript and then let the currently running Javascript continue. It just gets put in the event queue waiting until the currently running Javascript is done to get its turn to run.
What you need to do is rethink how your code works and find a different way to trigger whatever code you want to run when the flag value changes. Javascript is designed as an event-driven language. So, what you need to do is figure out what events you can register an interest in so you can either listen for the event that might cause the flag to change and you can examine the flag on that event or you can trigger your own event from whatever code might change the flag or you can implement a callback function that whatever code changes that flag can call your callback whenever the piece of code responsible for changing the flag value would change it's value to true, it just calls the callback function and thus your code that wants to run when the flag gets set to true will get to run at the right time. This is much, much more efficient than trying to use some sort of timer to constantly check the flag value.
function codeThatMightChangeFlag(callback) {
// do a bunch of stuff
if (condition happens to change flag value) {
// call the callback to notify other code
callback();
}
}
Solution using Promise, async\await and EventEmitter which allows to react immediate on flag change without any kind of loops at all
const EventEmitter = require('events');
const bus = new EventEmitter();
let lock = false;
async function lockable() {
if (lock) await new Promise(resolve => bus.once('unlocked', resolve));
....
lock = true;
...some logic....
lock = false;
bus.emit('unlocked');
}
EventEmitter is builtin in node. In browser you shall need to include it by your own, for example using this package: https://www.npmjs.com/package/eventemitter3
ES6 with Async / Await ,
let meaningOfLife = false;
async function waitForMeaningOfLife(){
while (true){
if (meaningOfLife) { console.log(42); return };
await null; // prevents app from hanging
}
}
waitForMeaningOfLife();
setTimeout(()=>meaningOfLife=true,420)
Modern solution using Promise
myFunction() in the original question can be modified as follows
async function myFunction(number) {
var x=number;
...
... more initializations
await until(_ => flag == true);
...
... do something
}
where until() is this utility function
function until(conditionFunction) {
const poll = resolve => {
if(conditionFunction()) resolve();
else setTimeout(_ => poll(resolve), 400);
}
return new Promise(poll);
}
Some references to async/await and arrow functions are in a similar post:
https://stackoverflow.com/a/52652681/209794
function waitFor(condition, callback) {
if(!condition()) {
console.log('waiting');
window.setTimeout(waitFor.bind(null, condition, callback), 100); /* this checks the flag every 100 milliseconds*/
} else {
console.log('done');
callback();
}
}
Use:
waitFor(() => window.waitForMe, () => console.log('got you'))
I solved this issue by implementing the method below.
const waitUntil = (condition, checkInterval=100) => {
return new Promise(resolve => {
let interval = setInterval(() => {
if (!condition()) return;
clearInterval(interval);
resolve();
}, checkInterval)
})
}
Now, whenever you want to wait until a certain condition is met you can call it like this.
await waitUntil(() => /* your condition */)
const waitUntil = (condition, checkInterval=100) => {
return new Promise(resolve => {
let interval = setInterval(() => {
if (!condition()) return;
clearInterval(interval);
resolve();
}, checkInterval)
})
}
async function start() {
let flag = false;
console.log('wait 5 sec');
setTimeout(()=> {flag=true}, 5000); // set flag=true after 5 seconds
await waitUntil(() => flag==true ); // wait
console.log('do something when flag is true...');
}
start();
With Ecma Script 2017 You can use async-await and while together to do that
And while will not crash or lock the program even variable never be true
//First define some delay function which is called from async function
function __delay__(timer) {
return new Promise(resolve => {
timer = timer || 2000;
setTimeout(function () {
resolve();
}, timer);
});
};
//Then Declare Some Variable Global or In Scope
//Depends on you
var flag = false;
//And define what ever you want with async fuction
async function some() {
while (!flag)
await __delay__(1000);
//...code here because when Variable = true this function will
};
For iterating over ($.each) objects and executing a longish-running operation (containing nested ajax sync calls) on each object:
I first set a custom done=false property on each.
Then, in a recursive function, set each done=true and continued using setTimeout. (It's an operation meant to stop all other UI, show a progress bar and block all other use so I forgave myself for the sync calls.)
function start()
{
GlobalProducts = getproductsfromsomewhere();
$.each(GlobalProducts, function(index, product) {
product["done"] = false;
});
DoProducts();
}
function DoProducts()
{
var doneProducts = Enumerable.From(GlobalProducts).Where("$.done == true").ToArray(); //linqjs
//update progress bar here
var nextProduct = Enumerable.From(GlobalProducts).Where("$.done == false").First();
if (nextProduct) {
nextProduct.done = true;
Me.UploadProduct(nextProduct.id); //does the long-running work
setTimeout(Me.UpdateProducts, 500)
}
}
If you are allowed to use: async/await on your code, you can try this one:
const waitFor = async (condFunc: () => boolean) => {
return new Promise((resolve) => {
if (condFunc()) {
resolve();
}
else {
setTimeout(async () => {
await waitFor(condFunc);
resolve();
}, 100);
}
});
};
const myFunc = async () => {
await waitFor(() => (window as any).goahead === true);
console.log('hello world');
};
myFunc();
Demo here:
https://stackblitz.com/edit/typescript-bgtnhj?file=index.ts
On the console, just copy/paste: goahead = true.
TMCDR; (= "too much code... didn't read")
Simplest in terms of readability of the calling code and conciseness of the implementing code:
const until = (predFn) => {
const poll = (done) => (predFn() ? done() : setTimeout(() => poll(done), 500));
return new Promise(poll);
};
Example calling code:
await until(() => { myBankBalance > 1000000 });
More detailed example:
https://replit.com/#SteveChambers1/Javascript-until-function?v=1
The cleanest solution (improvement of #tdxius solution) based on controlled time interval loop, promise and timeout to reject the promise and clear intervals in case condition isn't met in a given time
const waitUntil = (condition) => {
return new Promise((resolve, reject) => {
const interval = setInterval(() => {
if (!condition()) {
return;
}
clearInterval(interval);
resolve();
}, 100);
setTimeout(() => {
clearInterval(interval);
reject('your error msg');
}, 5000);
});
};
Now, whenever you want to wait until a certain condition is met, you can call it like this.
waitUntil(CONDITION_FUNCTION)
.then(() => DO_SOMETHING)
.catch((YOUR_ERROR_MSG) => console.warn(YOUR_ERROR_MSG))
using non blocking javascript with EventTarget API
In my example, i need to wait for a callback before to use it. I have no idea when this callback is set. It can be before of after i need to execute it. And i can need to call it several time (everything async)
// bus to pass event
const bus = new EventTarget();
// it's magic
const waitForCallback = new Promise((resolve, reject) => {
bus.addEventListener("initialized", (event) => {
resolve(event.detail);
});
});
// LET'S TEST IT !
// launch before callback has been set
waitForCallback.then((callback) => {
console.log(callback("world"));
});
// async init
setTimeout(() => {
const callback = (param) => { return `hello ${param.toString()}`; }
bus.dispatchEvent(new CustomEvent("initialized", {detail: callback}));
}, 500);
// launch after callback has been set
setTimeout(() => {
waitForCallback.then((callback) => {
console.log(callback("my little pony"));
});
}, 1000);
Did anyone ever think of just doing this?
function resolveAfter2Seconds() {
return new Promise(resolve => {
setTimeout(() => {
resolve();
}, 2000);
});
}
function myFunction(number) {
var x=number;
...
... more initializations
//here need to wait until flag==true
while(flag==false)
{
await resolveAfter2Seconds();
}
...
... do something
}
I tried to used #Kiran approach like follow:
checkFlag: function() {
var currentObject = this;
if(flag == false) {
setTimeout(currentObject.checkFlag, 100);
} else {
/* do something*/
}
}
(framework that I am using force me to define functions this way).
But without success because when execution come inside checkFlag function second time, this is not my object it is Window.
So, I finished with code below
checkFlag: function() {
var worker = setInterval (function(){
if(flag == true){
/* do something*/
clearInterval (worker);
}
},100);
}
there is a node package delay very easy to use
const delay = require('delay');
(async () => {
bar();
await delay(100);
// Executed 100 milliseconds later
baz();
})();
I took an approach along the lines of the callback solutions here, but tried to make it a bit more generic. The idea is you add functions that you need to execute after something changes to a queue. When the thing happens, you then loop through the queue, call the functions and empty the queue.
Add function to queue:
let _queue = [];
const _addToQueue = (funcToQ) => {
_queue.push(funcToQ);
}
Execute and flush the queue:
const _runQueue = () => {
if (!_queue || !_queue.length) {
return;
}
_queue.forEach(queuedFunc => {
queuedFunc();
});
_queue = [];
}
And when you invoke _addToQueue you'll want to wrap the callback:
_addToQueue(() => methodYouWantToCallLater(<pass any args here like you normally would>));
When you've met the condition, call _runQueue()
This was useful for me because I had several things that needed to wait on the same condition. And it decouples the detection of the condition from whatever needs to be executed when that condition is hit.
Try avoid while loop as it could be blocking your code, use async and promises.
Just wrote this library:
https://www.npmjs.com/package/utilzed
There is a function waitForTrue
import utilzed from 'utilzed'
const checkCondition = async () => {
// anything that you are polling for to be expecting to be true
const response = await callSomeExternalApi();
return response.success;
}
// this will waitForTrue checkCondition to be true
// checkCondition will be called every 100ms
const success = await utilzed.waitForTrue(100, checkCondition, 1000);
if (success) {
// Meaning checkCondition function returns true before 1000 ms
return;
}
// meaning after 1000ms the checkCondition returns false still
// handle unsuccessful "poll for true"
Similar to Lightbeard's answer, I use the following approach
function sleep(ms) {
return new Promise(resolve => setTimeout(resolve, ms))
}
async function until(fn) {
while (!fn()) {
await sleep(0)
}
}
async function myFunction(number) {
let x = number
...
... more initialization
await until(() => flag == true)
...
... do something
}
//function a(callback){
setTimeout(function() {
console.log('Hi I am order 1');
}, 3000);
// callback();
//}
//function b(callback){
setTimeout(function() {
console.log('Hi I am order 2');
}, 2000);
// callback();
//}
//function c(callback){
setTimeout(function() {
console.log('Hi I am order 3');
}, 1000);
// callback();
//}
/*function d(callback){
a(function(){
b(function(){
c(callback);
});
});
}
d();*/
async function funa(){
var pr1=new Promise((res,rej)=>{
setTimeout(()=>res("Hi4 I am order 1"),3000)
})
var pr2=new Promise((res,rej)=>{
setTimeout(()=>res("Hi4 I am order 2"),2000)
})
var pr3=new Promise((res,rej)=>{
setTimeout(()=>res("Hi4 I am order 3"),1000)
})
var res1 = await pr1;
var res2 = await pr2;
var res3 = await pr3;
console.log(res1,res2,res3);
console.log(res1);
console.log(res2);
console.log(res3);
}
funa();
async function f1(){
await new Promise(r=>setTimeout(r,3000))
.then(()=>console.log('Hi3 I am order 1'))
return 1;
}
async function f2(){
await new Promise(r=>setTimeout(r,2000))
.then(()=>console.log('Hi3 I am order 2'))
return 2;
}
async function f3(){
await new Promise(r=>setTimeout(r,1000))
.then(()=>console.log('Hi3 I am order 3'))
return 3;
}
async function finaloutput2(arr){
return await Promise.all([f3(),f2(),f1()]);
}
//f1().then(f2().then(f3()));
//f3().then(f2().then(f1()));
//finaloutput2();
//var pr1=new Promise(f3)
async function f(){
console.log("makesure");
var pr=new Promise((res,rej)=>{
setTimeout(function() {
console.log('Hi2 I am order 1');
}, 3000);
});
var result=await pr;
console.log(result);
}
// f();
async function g(){
console.log("makesure");
var pr=new Promise((res,rej)=>{
setTimeout(function() {
console.log('Hi2 I am order 2');
}, 2000);
});
var result=await pr;
console.log(result);
}
// g();
async function h(){
console.log("makesure");
var pr=new Promise((res,rej)=>{
setTimeout(function() {
console.log('Hi2 I am order 3');
}, 1000);
});
var result=await pr;
console.log(result);
}
async function finaloutput(arr){
return await Promise.all([f(),g(),h()]);
}
//finaloutput();
//h();
In my example, I log a new counter value every second:
var promises_arr = [];
var new_cntr_val = 0;
// fill array with promises
for (let seconds = 1; seconds < 10; seconds++) {
new_cntr_val = new_cntr_val + 5; // count to 50
promises_arr.push(new Promise(function (resolve, reject) {
// create two timeouts: one to work and one to resolve the promise
setTimeout(function(cntr) {
console.log(cntr);
}, seconds * 1000, new_cntr_val); // feed setTimeout the counter parameter
setTimeout(resolve, seconds * 1000);
}));
}
// wait for promises to finish
Promise.all(promises_arr).then(function (values) {
console.log("all promises have returned");
});
Modern and simple solution
async function waitUntil(condition, time = 100) {
while (!condition()) {
await new Promise((resolve) => setTimeout(resolve, time));
}
}
Usage
async function foo() {
await waitUntil(() => flag === true);
console.log('condition is met!');
}
Inspired by jfriend00, this worked for me
const seconds = new Date();
// wait 5 seconds for flag to become true
const waitTime = 5
const extraSeconds = seconds.setSeconds(seconds.getSeconds() + waitTime);
while (Date.now() < extraSeconds) {
// break when flag is false
if (flag === false) break;
}
How would you wait for a Promise to resolve/reject, for a maximum execution time ? The code below is obviously wrong, it's just to explain what I'm trying to achieve. I'm clueless.
await doSomething();
if ( executionTime > maxExecutionTime ) {
doSomethingElse();
}
This is not for a bluebird promise.
You can use Promise.race() which will immediately resolve/reject when the first promise in its iterable resolves or rejects. E.g.
const longTask = () => new Promise(resolve =>
setTimeout(() => resolve("Long task complete."), 300))
const timeout = (cb, interval) => () =>
new Promise(resolve => setTimeout(() => cb(resolve), interval))
const onTimeout = timeout(resolve =>
resolve("The 'maybeLongTask' ran too long!"), 200)
Promise.race([longTask, onTimeout].map(f => f())).then(console.log)
The only issue is you can't really cancel the 'longTask' just because of its long execution. In theory, you'd either set some flag (to tell it not to continue onto the next stage of its pipeline), or design your application with the consequences of the promise in mind.
See what happens when you swap the 200 and 300 intervals.
Edit: Per spsaucier's comment, I've delayed the execution of each promise until the Promise.line line.
The code below will give you some idea:
function doSomething(maxExecutionTime) {
return new Promise(resolve => {
setTimeout(() => resolve(true), 2000); // this setTimeout simulates your async action which sould not exced maxExecutionTime
setTimeout(() => resolve(false), maxExecutionTime);
});
}
async function someFunc(maxExecutionTime) {
var exced = await doSomething(maxExecutionTime);
if (exced) {
console.log("Doesn't exced max time");
} else {
console.log("Exced max time");
}
}
someFunc(1000);
someFunc(3000);
In ECMA6 You can do something like this:
let promise = new Promise((resolve, reject) => {
let wait = setTimeout(() => {
clearTimeout(wait);
resolve('Timed out after 200 ms..');
}, 200)
})
As noahnu suggested, you can use Promise.race. You can wrap it in a function that takes a promise.
With some syntax sugar you can use thisOrThat which takes logic, a function that takes 2 functions, first and second. You can apply logic here as to try the first function and when to try the second (in our case if the first doesn't resolve within a certain time then try the second).
thisOrThat then takes an argument first that is a function returning a promise (in our case doSomething.
thisOrThat returns an object that has an or property that takes a function returning a promise. That parameter is called second and is passed to logic as second (in our case it is doSomethingElse).
var timeoutPromise =
time =>
promiseFn =>
Promise.race([
promiseFn(),
new Promise(
(_,r)=>
setTimeout(
_=>r("timed out")
,time
)
)
]);
var thisOrThat =
logic =>
first => ({
or:second=>
logic(first)(second)
});
var within75ms = thisOrThat
(first=>second=>
timeoutPromise(75)(first)
.catch(_=>second())
);
var within25ms = thisOrThat
(first=>second=>
timeoutPromise(25)(first)
.catch(_=>second())
);
var doSomething = () =>
console.log("do something called")||
new Promise(r=>setTimeout(x=>r("something"),50));
var doSomethingElse = () =>
console.log("do something else called") ||
new Promise(r=>setTimeout(x=>r("something else"),50));
async function someFunc() {
const doesNotTimeOut =
await within75ms(doSomething).or(doSomethingElse);
console.log("within 75ms resolved to:",doesNotTimeOut);
const doesTimeOut =
await within25ms(doSomething).or(doSomethingElse)
console.log("within 25ms resolved to:",doesTimeOut);
};
someFunc();
I have used Promise.race
// Try to get config from mongo
async function getProjectConfigThreshold(projectName) {
function onTimeoutResolveDefaultThreshold() {
return new Promise(async (resolve) => {
setTimeout(() => {
resolve(DEFAULT_THRESHOLD);
}, 2000)
});
}
async function getThresholdFromProjectConfig() {
const projectConfig = await getProjectsConfig(projectName);
const threshold = projectConfig.threshold || DEFAULT_THRESHOLD;
return threshold;
}
return await Promise.race([getThresholdFromProjectConfig(), onTimeoutResolveDefaultThreshold()]);
}
I have javascript function like this:
function myFunction(number) {
var x=number;
...
... more initializations
//here need to wait until flag==true
while(flag==false)
{}
...
... do something
}
The problem is that the javascript is stuck in the while and stuck my program. so my question is how can I wait in the middle of the function until flag is true without "busy-wait"?
Javascript is single threaded, hence the page blocking behaviour. You can use the deferred/promise approach suggested by others. The most basic way would be to use window.setTimeout. E.g.
function checkFlag() {
if(flag === false) {
window.setTimeout(checkFlag, 100); /* this checks the flag every 100 milliseconds*/
} else {
/* do something*/
}
}
checkFlag();
Here is a good tutorial with further explanation: Tutorial
EDIT
As others pointed out, the best way would be to re-structure your code to use callbacks. However, this answer should give you an idea how you can 'simulate' an asynchronous behaviour with window.setTimeout.
You created an infinite loop where the flag value that terminates the loop can never be changed by code outside this loop because no code outside the loop ever gets to run - thus this will never work.
Because javascript in a browser is single threaded (except for webworkers which aren't involved here) and one thread of javascript execution runs to completion before another can run, your statement:
// this won't work!
while(flag==false) {}
will simply run forever (or until the browser complains about a non-responsive javascript loop), the page will appear to be hung and no other javascript will ever get a chance to run, thus the flag's value can never be changed by code outside this loop.
For a little more explanation, Javascript is an event driven language. That means that it runs a piece of Javascript until it returns control back to the interpreter. Then, only when it returns back to the interpreter, Javascript gets the next event from the event queue and runs it.
All things like timers and network events run through the event queue. So, when a timer fires or a network request arrives, it does not ever "interrupt" the currently running Javascript. Instead, an event gets put in the Javascript event queue and then, when the currently running Javascript finishes, the next event is pulled from the event queue and it gets its turn to run.
So, when you do an infinite loop such as while(flag==false) {}, the currently running Javascript never finishes and thus the next event is never pulled from the event queue and thus the value of flag never gets changed. They key here is that Javascript is not interrupt driven. When a timer fires, it does not interrupt the currently running Javascript, run some other Javascript and then let the currently running Javascript continue. It just gets put in the event queue waiting until the currently running Javascript is done to get its turn to run.
What you need to do is rethink how your code works and find a different way to trigger whatever code you want to run when the flag value changes. Javascript is designed as an event-driven language. So, what you need to do is figure out what events you can register an interest in so you can either listen for the event that might cause the flag to change and you can examine the flag on that event or you can trigger your own event from whatever code might change the flag or you can implement a callback function that whatever code changes that flag can call your callback whenever the piece of code responsible for changing the flag value would change it's value to true, it just calls the callback function and thus your code that wants to run when the flag gets set to true will get to run at the right time. This is much, much more efficient than trying to use some sort of timer to constantly check the flag value.
function codeThatMightChangeFlag(callback) {
// do a bunch of stuff
if (condition happens to change flag value) {
// call the callback to notify other code
callback();
}
}
Solution using Promise, async\await and EventEmitter which allows to react immediate on flag change without any kind of loops at all
const EventEmitter = require('events');
const bus = new EventEmitter();
let lock = false;
async function lockable() {
if (lock) await new Promise(resolve => bus.once('unlocked', resolve));
....
lock = true;
...some logic....
lock = false;
bus.emit('unlocked');
}
EventEmitter is builtin in node. In browser you shall need to include it by your own, for example using this package: https://www.npmjs.com/package/eventemitter3
ES6 with Async / Await ,
let meaningOfLife = false;
async function waitForMeaningOfLife(){
while (true){
if (meaningOfLife) { console.log(42); return };
await null; // prevents app from hanging
}
}
waitForMeaningOfLife();
setTimeout(()=>meaningOfLife=true,420)
Modern solution using Promise
myFunction() in the original question can be modified as follows
async function myFunction(number) {
var x=number;
...
... more initializations
await until(_ => flag == true);
...
... do something
}
where until() is this utility function
function until(conditionFunction) {
const poll = resolve => {
if(conditionFunction()) resolve();
else setTimeout(_ => poll(resolve), 400);
}
return new Promise(poll);
}
Some references to async/await and arrow functions are in a similar post:
https://stackoverflow.com/a/52652681/209794
function waitFor(condition, callback) {
if(!condition()) {
console.log('waiting');
window.setTimeout(waitFor.bind(null, condition, callback), 100); /* this checks the flag every 100 milliseconds*/
} else {
console.log('done');
callback();
}
}
Use:
waitFor(() => window.waitForMe, () => console.log('got you'))
I solved this issue by implementing the method below.
const waitUntil = (condition, checkInterval=100) => {
return new Promise(resolve => {
let interval = setInterval(() => {
if (!condition()) return;
clearInterval(interval);
resolve();
}, checkInterval)
})
}
Now, whenever you want to wait until a certain condition is met you can call it like this.
await waitUntil(() => /* your condition */)
const waitUntil = (condition, checkInterval=100) => {
return new Promise(resolve => {
let interval = setInterval(() => {
if (!condition()) return;
clearInterval(interval);
resolve();
}, checkInterval)
})
}
async function start() {
let flag = false;
console.log('wait 5 sec');
setTimeout(()=> {flag=true}, 5000); // set flag=true after 5 seconds
await waitUntil(() => flag==true ); // wait
console.log('do something when flag is true...');
}
start();
With Ecma Script 2017 You can use async-await and while together to do that
And while will not crash or lock the program even variable never be true
//First define some delay function which is called from async function
function __delay__(timer) {
return new Promise(resolve => {
timer = timer || 2000;
setTimeout(function () {
resolve();
}, timer);
});
};
//Then Declare Some Variable Global or In Scope
//Depends on you
var flag = false;
//And define what ever you want with async fuction
async function some() {
while (!flag)
await __delay__(1000);
//...code here because when Variable = true this function will
};
For iterating over ($.each) objects and executing a longish-running operation (containing nested ajax sync calls) on each object:
I first set a custom done=false property on each.
Then, in a recursive function, set each done=true and continued using setTimeout. (It's an operation meant to stop all other UI, show a progress bar and block all other use so I forgave myself for the sync calls.)
function start()
{
GlobalProducts = getproductsfromsomewhere();
$.each(GlobalProducts, function(index, product) {
product["done"] = false;
});
DoProducts();
}
function DoProducts()
{
var doneProducts = Enumerable.From(GlobalProducts).Where("$.done == true").ToArray(); //linqjs
//update progress bar here
var nextProduct = Enumerable.From(GlobalProducts).Where("$.done == false").First();
if (nextProduct) {
nextProduct.done = true;
Me.UploadProduct(nextProduct.id); //does the long-running work
setTimeout(Me.UpdateProducts, 500)
}
}
If you are allowed to use: async/await on your code, you can try this one:
const waitFor = async (condFunc: () => boolean) => {
return new Promise((resolve) => {
if (condFunc()) {
resolve();
}
else {
setTimeout(async () => {
await waitFor(condFunc);
resolve();
}, 100);
}
});
};
const myFunc = async () => {
await waitFor(() => (window as any).goahead === true);
console.log('hello world');
};
myFunc();
Demo here:
https://stackblitz.com/edit/typescript-bgtnhj?file=index.ts
On the console, just copy/paste: goahead = true.
TMCDR; (= "too much code... didn't read")
Simplest in terms of readability of the calling code and conciseness of the implementing code:
const until = (predFn) => {
const poll = (done) => (predFn() ? done() : setTimeout(() => poll(done), 500));
return new Promise(poll);
};
Example calling code:
await until(() => { myBankBalance > 1000000 });
More detailed example:
https://replit.com/#SteveChambers1/Javascript-until-function?v=1
The cleanest solution (improvement of #tdxius solution) based on controlled time interval loop, promise and timeout to reject the promise and clear intervals in case condition isn't met in a given time
const waitUntil = (condition) => {
return new Promise((resolve, reject) => {
const interval = setInterval(() => {
if (!condition()) {
return;
}
clearInterval(interval);
resolve();
}, 100);
setTimeout(() => {
clearInterval(interval);
reject('your error msg');
}, 5000);
});
};
Now, whenever you want to wait until a certain condition is met, you can call it like this.
waitUntil(CONDITION_FUNCTION)
.then(() => DO_SOMETHING)
.catch((YOUR_ERROR_MSG) => console.warn(YOUR_ERROR_MSG))
using non blocking javascript with EventTarget API
In my example, i need to wait for a callback before to use it. I have no idea when this callback is set. It can be before of after i need to execute it. And i can need to call it several time (everything async)
// bus to pass event
const bus = new EventTarget();
// it's magic
const waitForCallback = new Promise((resolve, reject) => {
bus.addEventListener("initialized", (event) => {
resolve(event.detail);
});
});
// LET'S TEST IT !
// launch before callback has been set
waitForCallback.then((callback) => {
console.log(callback("world"));
});
// async init
setTimeout(() => {
const callback = (param) => { return `hello ${param.toString()}`; }
bus.dispatchEvent(new CustomEvent("initialized", {detail: callback}));
}, 500);
// launch after callback has been set
setTimeout(() => {
waitForCallback.then((callback) => {
console.log(callback("my little pony"));
});
}, 1000);
Did anyone ever think of just doing this?
function resolveAfter2Seconds() {
return new Promise(resolve => {
setTimeout(() => {
resolve();
}, 2000);
});
}
function myFunction(number) {
var x=number;
...
... more initializations
//here need to wait until flag==true
while(flag==false)
{
await resolveAfter2Seconds();
}
...
... do something
}
I tried to used #Kiran approach like follow:
checkFlag: function() {
var currentObject = this;
if(flag == false) {
setTimeout(currentObject.checkFlag, 100);
} else {
/* do something*/
}
}
(framework that I am using force me to define functions this way).
But without success because when execution come inside checkFlag function second time, this is not my object it is Window.
So, I finished with code below
checkFlag: function() {
var worker = setInterval (function(){
if(flag == true){
/* do something*/
clearInterval (worker);
}
},100);
}
there is a node package delay very easy to use
const delay = require('delay');
(async () => {
bar();
await delay(100);
// Executed 100 milliseconds later
baz();
})();
I took an approach along the lines of the callback solutions here, but tried to make it a bit more generic. The idea is you add functions that you need to execute after something changes to a queue. When the thing happens, you then loop through the queue, call the functions and empty the queue.
Add function to queue:
let _queue = [];
const _addToQueue = (funcToQ) => {
_queue.push(funcToQ);
}
Execute and flush the queue:
const _runQueue = () => {
if (!_queue || !_queue.length) {
return;
}
_queue.forEach(queuedFunc => {
queuedFunc();
});
_queue = [];
}
And when you invoke _addToQueue you'll want to wrap the callback:
_addToQueue(() => methodYouWantToCallLater(<pass any args here like you normally would>));
When you've met the condition, call _runQueue()
This was useful for me because I had several things that needed to wait on the same condition. And it decouples the detection of the condition from whatever needs to be executed when that condition is hit.
Try avoid while loop as it could be blocking your code, use async and promises.
Just wrote this library:
https://www.npmjs.com/package/utilzed
There is a function waitForTrue
import utilzed from 'utilzed'
const checkCondition = async () => {
// anything that you are polling for to be expecting to be true
const response = await callSomeExternalApi();
return response.success;
}
// this will waitForTrue checkCondition to be true
// checkCondition will be called every 100ms
const success = await utilzed.waitForTrue(100, checkCondition, 1000);
if (success) {
// Meaning checkCondition function returns true before 1000 ms
return;
}
// meaning after 1000ms the checkCondition returns false still
// handle unsuccessful "poll for true"
Similar to Lightbeard's answer, I use the following approach
function sleep(ms) {
return new Promise(resolve => setTimeout(resolve, ms))
}
async function until(fn) {
while (!fn()) {
await sleep(0)
}
}
async function myFunction(number) {
let x = number
...
... more initialization
await until(() => flag == true)
...
... do something
}
//function a(callback){
setTimeout(function() {
console.log('Hi I am order 1');
}, 3000);
// callback();
//}
//function b(callback){
setTimeout(function() {
console.log('Hi I am order 2');
}, 2000);
// callback();
//}
//function c(callback){
setTimeout(function() {
console.log('Hi I am order 3');
}, 1000);
// callback();
//}
/*function d(callback){
a(function(){
b(function(){
c(callback);
});
});
}
d();*/
async function funa(){
var pr1=new Promise((res,rej)=>{
setTimeout(()=>res("Hi4 I am order 1"),3000)
})
var pr2=new Promise((res,rej)=>{
setTimeout(()=>res("Hi4 I am order 2"),2000)
})
var pr3=new Promise((res,rej)=>{
setTimeout(()=>res("Hi4 I am order 3"),1000)
})
var res1 = await pr1;
var res2 = await pr2;
var res3 = await pr3;
console.log(res1,res2,res3);
console.log(res1);
console.log(res2);
console.log(res3);
}
funa();
async function f1(){
await new Promise(r=>setTimeout(r,3000))
.then(()=>console.log('Hi3 I am order 1'))
return 1;
}
async function f2(){
await new Promise(r=>setTimeout(r,2000))
.then(()=>console.log('Hi3 I am order 2'))
return 2;
}
async function f3(){
await new Promise(r=>setTimeout(r,1000))
.then(()=>console.log('Hi3 I am order 3'))
return 3;
}
async function finaloutput2(arr){
return await Promise.all([f3(),f2(),f1()]);
}
//f1().then(f2().then(f3()));
//f3().then(f2().then(f1()));
//finaloutput2();
//var pr1=new Promise(f3)
async function f(){
console.log("makesure");
var pr=new Promise((res,rej)=>{
setTimeout(function() {
console.log('Hi2 I am order 1');
}, 3000);
});
var result=await pr;
console.log(result);
}
// f();
async function g(){
console.log("makesure");
var pr=new Promise((res,rej)=>{
setTimeout(function() {
console.log('Hi2 I am order 2');
}, 2000);
});
var result=await pr;
console.log(result);
}
// g();
async function h(){
console.log("makesure");
var pr=new Promise((res,rej)=>{
setTimeout(function() {
console.log('Hi2 I am order 3');
}, 1000);
});
var result=await pr;
console.log(result);
}
async function finaloutput(arr){
return await Promise.all([f(),g(),h()]);
}
//finaloutput();
//h();
In my example, I log a new counter value every second:
var promises_arr = [];
var new_cntr_val = 0;
// fill array with promises
for (let seconds = 1; seconds < 10; seconds++) {
new_cntr_val = new_cntr_val + 5; // count to 50
promises_arr.push(new Promise(function (resolve, reject) {
// create two timeouts: one to work and one to resolve the promise
setTimeout(function(cntr) {
console.log(cntr);
}, seconds * 1000, new_cntr_val); // feed setTimeout the counter parameter
setTimeout(resolve, seconds * 1000);
}));
}
// wait for promises to finish
Promise.all(promises_arr).then(function (values) {
console.log("all promises have returned");
});
Modern and simple solution
async function waitUntil(condition, time = 100) {
while (!condition()) {
await new Promise((resolve) => setTimeout(resolve, time));
}
}
Usage
async function foo() {
await waitUntil(() => flag === true);
console.log('condition is met!');
}
Inspired by jfriend00, this worked for me
const seconds = new Date();
// wait 5 seconds for flag to become true
const waitTime = 5
const extraSeconds = seconds.setSeconds(seconds.getSeconds() + waitTime);
while (Date.now() < extraSeconds) {
// break when flag is false
if (flag === false) break;
}