Develop Reference

Using JS Promise as an interrupt

I wanted to directly call a function (like interrupt handler) when a certain condition is met. I didn't want to using "polling" for that as it increases time complexity.
count = 1
p = new Promise((resolve, reject)=>{
if(count == 2){
resolve("hello")
}
});
p.then((msg)=>{
console.log(msg)
})
console.log("1 now");
count = 2;
I expected console.log(msg) to run when count=2 but this is not the case. It turned out that the promise is still "pending". What is the reason this happens? And how do I implement my question.

You can use a Proxy to listen variable changes.
const count = new Proxy({
value: 0
}, {
set(target, prop, val) {
// set value
target[prop] = val;
console.log(`count is ${val}`);
// stop condition
if (val == 2) {
console.log(`count is 2(trigger stop condition)`);
}
}
});
// wait 2 seconds and change count.value to 1
setTimeout(() => count.value = 1, 2000);
// wait 2 seconds and change count.value to 2
// it triggers the stop condition
setTimeout(() => count.value = 2, 2000);
console.log("Waiting for changes ...");
reference: Listen to js variable change

Proxy is one of the solutions for this. But I post another approach for your case.
You can define a custom class or object, and work with that class. Also you register your custom listener for it, and do whatever.
This is a sample of my code. Maybe it will give you some ideas for your solution.
class MyObject {
constructor(value, func) {
this._value = value;
this._listener = func;
}
get value() {
return this._value;
}
set value(newValue) {
this._listener(newValue);
this._value = newValue;
}
}
function customListener(changedValue) {
console.log(`New Value Detected: ${changedValue}`);
}
const count = new MyObject(1, customListener);
count.value = 2;

The issue you're having is that the code inside the Promise resolves synchronously. It seems like you are assuming Promises are by default async, but that is a common async misconception. So, the code
if(count == 2){
resolve("hello")
}
resolves synchronously (that is, right after you declare count to be 1) so the Promise will never be resolved. If you want to asynchronously check for a condition without using libraries, you can use setInterval:
function checkForCondition(count, time){
return new Promise(resolve => {
const interval = setInterval(() => {
if (count == 2){
resolve("The count is 2!");
}
} , time);
});
}
If you call this function, the callback inside setInterval will be placed on the event loop every x ms, where x is equal to the time parameter.

Listen to js variable change

Let's say there is a code in place 2
var place2IsReady = true;
In place 1 I need to implement the logic below :
Once place2IsReady value was changed (to true) then display alert('ready!');
Notes:
place2IsReady variable is not available in the scope of place 1.
the code from place 1 gets executed before place 2 gets executed (or there is a race condition).
Solution 1
I believe I can use window.place2IsReady instead and use setTimeout/setInterval in place 1 until I get window.place2IsReady === true.
Any better options? Using Listeners? On the variable change?
P.S. I need to track only first possible change of place2IsReady.
Is there a better way? Thank you.

You can create a listener for the variable change using setTimeout, something like:
let place2IsReady = false;
setReadyListener();
// testing wait 2 seconds to set place2IsReady to true
// so: an alert should occur after 2 seconds
setTimeout(() => place2IsReady = true, 2000);
function setReadyListener() {
const readyListener = () => {
if (place2IsReady) {
return alert("Ready!");
}
return setTimeout(readyListener, 250);
};
readyListener();
}
A more generic listener 'factory' could be:
let place2IsReady = false;
let fromObj = {
place2IsReady: "busy",
done() { this.place2IsReady = "done"; },
};
const listen = changeListenerFactory();
listen(
() => place2IsReady,
() => console.log("place2IsReady") );
listen(
() => fromObj.place2IsReady === "done",
() => console.log("formObj.place2IsReady done!") );
console.log("Listening...");
// test change variables with listeners
setTimeout(() => place2IsReady = true, 1000);
setTimeout(() => fromObj.done(), 3000);
function changeListenerFactory() {
const readyListener = (condition, callback, delay) => {
if (!condition || typeof condition !== "function") { return true; }
if (condition()) {
return callback();
}
setTimeout(() => readyListener(condition, callback, delay), delay);
};
return (condition, callback = () => {}, delay = 250) =>
readyListener(condition, callback, delay);
}
Or maybe using a Proxy (with a set trap) works for you
const readyState = new Proxy({ ready: false }, {
set (target, prop, val) {
console.log(`readyState.ready changed from ${target[prop]} to ${val}`);
target[prop] = val;
}
});
console.log("Waiting for changes ...");
setTimeout(() => readyState.ready = true, 2000);

Assuming you can replace place2IsReady with an object:
place2IsReady = {
state: false,
set ready(value) {
this.state = value
state && place_1_call()
},
get ready() {
return state
}
}
place_1_call = () => {
alert('ready')
}
place2IsReady.ready = true

Calling a function as many times as possible in a given time interval

I am trying to call the function test() as many times as possible in a given time interval.
Here the function should be running for 15 seconds.
function test(): void; // Only type def
function run() {
return new Promise(resolve => {
setTimeout(() => {
resolve();
}, 15000); // 15 seconds
while (true) {
test();
}
});
}
run()
.then(() => {
console.log('Ended');
});
However, the function doesn't stop running, and the Ended console.log does not appear. (Promise not resolved obviously). Is there a way to achieve this in Javascript ?
I was wondering, I could probably use console timers and put the condition in the while statement ? (But is that the best way ?)

The reason why your function does not stop executing is because resolving a promise does not stop script executing. What you want is to store a flag somewhere in your run() method, so that you can flip the flag once the promise is intended to be resolved.
See proof-of-concept below: I've shortened the period to 1.5s and added a dummy test() method just for illustration purpose:
let i = 0;
function test() {
console.log(`test: ${i++}`);
}
function run() {
return new Promise(resolve => {
let shouldInvoke = true;
setTimeout(() => {
shouldInvoke = false;
resolve();
}, 1500); // 15 seconds
const timer = setInterval(() => {
if (shouldInvoke)
test();
else
window.clearInterval(timer);
}, 0);
});
}
run()
.then(() => {
console.log('Ended');
});

Loop on a promise indefinitely until rejection

I have a function that does some async work and returns a Promise, and I want to execute this function indefinitely until the promise is rejected.
Something like the following :
doSomethingAsync().
.then(doSomethingAsync)
.then(doSomethingAsync)
.then(doSomethingAsync)
// ... until rejection
I made a little CodePen so I can test potential solutions : http://codepen.io/JesmoDrazik/pen/pbAovZ?editors=0011
I found several potential answers but nothing seems to work for my case.
If anyone has a solution, I'd be glad, because I just can't find anything !
Thanks.

You can do
(function loop(){
doSomethingAsync().then(loop);
})();
But looking at your pen it's not clear where the rejection should come from. If you want to stop repeating an operation when the user clicks a button, you can change a state in the button handling and then do
(function loop(){
doSomethingAsync().then(function(){
if (!stopped) loop();
});
})();

Made a modification to your codepen
var FAKE_COUNT = 0;
function doSomething() {
return new Promise((resolve, reject) => {
setTimeout(() => {
console.log('doing something async: ' + FAKE_COUNT)
if (FAKE_COUNT < 10) {
resolve();
} else {
reject();
}
FAKE_COUNT ++;
}, 1000);
});
}
const button = document.querySelector('.js-button');
button.addEventListener('click', () => {
// maybe we can do something here ?
})
function test() {
doSomething().then(test).catch(() => {
console.log("Rejected")
});
}
test();
FAKE_COUNT just becomes the flag, so if you want to stop the promise with a click instead of a count you could just make it a bool and check that when executing the async task

Depending on where you want the logic and rejection to take place, but assuming you want the promise itself to be self executing (otherwise the asynchronous execution itself could loop) it could return itself as a new promise:
function doSomething() {
return new Promise((resolve, reject) => {
setTimeout(() => {
console.log('doing something async');
resolve();
}, 1000);
}).then(()=>doSomething());
}
For the rejection part, the easiest way would be to introduce a flag, (although that would make the promise a little less self containing):
var stop = false;
function doSomething() {
return new Promise((resolve, reject) => {
if(stop)
reject();
else{
setTimeout(() => {
console.log('doing something async');
resolve(); //(check for stop could be done here as well)
}, 1000);
}
}).then(()=>doSomething());
}
const button = document.querySelector('.js-button');
button.addEventListener('click', () => {
stop = true;
});
doSomething();

Make function wait until element exists

I'm trying to add a canvas over another canvas – how can I make this function wait to start until the first canvas is created?
function PaintObject(brush) {
this.started = false;
// get handle of the main canvas, as a DOM object, not as a jQuery Object. Context is unfortunately not yet
// available in jquery canvas wrapper object.
var mainCanvas = $("#" + brush).get(0);
// Check if everything is ok
if (!mainCanvas) {alert("canvas undefined, does not seem to be supported by your browser");}
if (!mainCanvas.getContext) {alert('Error: canvas.getContext() undefined !');}
// Get the context for drawing in the canvas
var mainContext = mainCanvas.getContext('2d');
if (!mainContext) {alert("could not get the context for the main canvas");}
this.getMainCanvas = function () {
return mainCanvas;
}
this.getMainContext = function () {
return mainContext;
}
// Prepare a second canvas on top of the previous one, kind of second "layer" that we will use
// in order to draw elastic objects like a line, a rectangle or an ellipse we adjust using the mouse
// and that follows mouse movements
var frontCanvas = document.createElement('canvas');
frontCanvas.id = 'canvasFront';
// Add the temporary canvas as a second child of the mainCanvas parent.
mainCanvas.parentNode.appendChild(frontCanvas);
if (!frontCanvas) {
alert("frontCanvas null");
}
if (!frontCanvas.getContext) {
alert('Error: no frontCanvas.getContext!');
}
var frontContext = frontCanvas.getContext('2d');
if (!frontContext) {
alert("no TempContext null");
}
this.getFrontCanvas = function () {
return frontCanvas;
}
this.getFrontContext = function () {
return frontContext;
}

If you have access to the code that creates the canvas - simply call the function right there after the canvas is created.
If you have no access to that code (eg. If it is a 3rd party code such as google maps) then what you could do is test for the existence in an interval:
var checkExist = setInterval(function() {
if ($('#the-canvas').length) {
console.log("Exists!");
clearInterval(checkExist);
}
}, 100); // check every 100ms
But note - many times 3rd party code has an option to activate your code (by callback or event triggering) when it finishes to load. That may be where you can put your function. The interval solution is really a bad solution and should be used only if nothing else works.

Depending on which browser you need to support, there's the option of MutationObserver.
EDIT: All major browsers support MutationObserver now.
Something along the lines of this should do the trick:
// callback executed when canvas was found
function handleCanvas(canvas) { ... }
// set up the mutation observer
var observer = new MutationObserver(function (mutations, me) {
// `mutations` is an array of mutations that occurred
// `me` is the MutationObserver instance
var canvas = document.getElementById('my-canvas');
if (canvas) {
handleCanvas(canvas);
me.disconnect(); // stop observing
return;
}
});
// start observing
observer.observe(document, {
childList: true,
subtree: true
});
N.B. I haven't tested this code myself, but that's the general idea.
You can easily extend this to only search the part of the DOM that changed. For that, use the mutations argument, it's an array of MutationRecord objects.

This will only work with modern browsers but I find it easier to just use a then so please test first but:
ES5
function rafAsync() {
return new Promise(resolve => {
requestAnimationFrame(resolve); //faster than set time out
});
}
function checkElement(selector) {
if (document.querySelector(selector) === null) {
return rafAsync().then(() => checkElement(selector));
} else {
return Promise.resolve(true);
}
}
ES6
async function checkElement(selector) {
const querySelector = null;
while (querySelector === null) {
await rafAsync();
querySelector = document.querySelector(selector);
}
return querySelector;
}
Usage
checkElement('body') //use whichever selector you want
.then((element) => {
console.info(element);
//Do whatever you want now the element is there
});

A more modern approach to waiting for elements:
while(!document.querySelector(".my-selector")) {
await new Promise(r => setTimeout(r, 500));
}
// now the element is loaded
Note that this code would need to be wrapped in an async function.

Here's a minor improvement over Jamie Hutber's answer
const checkElement = async selector => {
while ( document.querySelector(selector) === null) {
await new Promise( resolve => requestAnimationFrame(resolve) )
}
return document.querySelector(selector);
};
To use:
checkElement('.myElement').then((selector) => {
console.log(selector);
});

If you want a generic solution using MutationObserver you can use this function
// MIT Licensed
// Author: jwilson8767
/**
* Waits for an element satisfying selector to exist, then resolves promise with the element.
* Useful for resolving race conditions.
*
* #param selector
* #returns {Promise}
*/
export function elementReady(selector) {
return new Promise((resolve, reject) => {
const el = document.querySelector(selector);
if (el) {resolve(el);}
new MutationObserver((mutationRecords, observer) => {
// Query for elements matching the specified selector
Array.from(document.querySelectorAll(selector)).forEach((element) => {
resolve(element);
//Once we have resolved we don't need the observer anymore.
observer.disconnect();
});
})
.observe(document.documentElement, {
childList: true,
subtree: true
});
});
}
Source: https://gist.github.com/jwilson8767/db379026efcbd932f64382db4b02853e
Example how to use it
elementReady('#someWidget').then((someWidget)=>{someWidget.remove();});
Note: MutationObserver has a great browser support; https://caniuse.com/#feat=mutationobserver
Et voilà ! :)

Is better to relay in requestAnimationFrame than in a setTimeout. this is my solution in es6 modules and using Promises.
es6, modules and promises:
// onElementReady.js
const onElementReady = $element => (
new Promise((resolve) => {
const waitForElement = () => {
if ($element) {
resolve($element);
} else {
window.requestAnimationFrame(waitForElement);
}
};
waitForElement();
})
);
export default onElementReady;
// in your app
import onElementReady from './onElementReady';
const $someElement = document.querySelector('.some-className');
onElementReady($someElement)
.then(() => {
// your element is ready
}
plain js and promises:
var onElementReady = function($element) {
return new Promise((resolve) => {
var waitForElement = function() {
if ($element) {
resolve($element);
} else {
window.requestAnimationFrame(waitForElement);
}
};
waitForElement();
})
};
var $someElement = document.querySelector('.some-className');
onElementReady($someElement)
.then(() => {
// your element is ready
});

Here is a solution using observables.
waitForElementToAppear(elementId) {
return Observable.create(function(observer) {
var el_ref;
var f = () => {
el_ref = document.getElementById(elementId);
if (el_ref) {
observer.next(el_ref);
observer.complete();
return;
}
window.requestAnimationFrame(f);
};
f();
});
}
Now you can write
waitForElementToAppear(elementId).subscribe(el_ref => doSomethingWith(el_ref);

You can check if the dom already exists by setting a timeout until it is already rendered in the dom.
var panelMainWrapper = document.getElementById('panelMainWrapper');
setTimeout(function waitPanelMainWrapper() {
if (document.body.contains(panelMainWrapper)) {
$("#panelMainWrapper").html(data).fadeIn("fast");
} else {
setTimeout(waitPanelMainWrapper, 10);
}
}, 10);

Another variation of Iftah
var counter = 10;
var checkExist = setInterval(function() {
console.log(counter);
counter--
if ($('#the-canvas').length || counter === 0) {
console.log("by bye!");
clearInterval(checkExist);
}
}, 200);
Just in case the element is never shown, so we don't check infinitely.

A pure promise based JavaScript approach, you can tell for many milliseconds to wait.
const waitElementFor = function(query, ms = 3000) { // 3000 === 3 seconds
return new Promise((resolve) => {
var waited = 0;
var el = null;
var wi = setInterval(function() {
el = document.querySelector(query);
if (waited >= ms || el) {
clearInterval(wi);
if(el) {
resolve(el);
} else {
resolve(null);
}
}
waited += 10;
}, 10);
});
}
To use the function, simply use the following code in an asynchronous function.
var element = await waitElementFor('#elementID');
Snippet:
const waitElementFor = function(query, ms = 3000) { // 3000 === 3 seconds
return new Promise((resolve) => {
var waited = 0;
var el = null;
var wi = setInterval(function() {
el = document.querySelector(query);
if (waited >= ms || el) {
clearInterval(wi);
if(el) {
resolve(el);
} else {
resolve(null);
}
}
waited += 10;
}, 10);
});
}
async function snippetTestAyncFunction(){
var element = await waitElementFor('#elementID');
console.log(element);
}
snippetTestAyncFunction();

Maybe I'm a little bit late :), but here is a nice and brief solution by chrisjhoughton, which allows to perform a callback function when the wait is over.
https://gist.github.com/chrisjhoughton/7890303
var waitForEl = function(selector, callback) {
if (jQuery(selector).length) {
callback();
} else {
setTimeout(function() {
waitForEl(selector, callback);
}, 100);
}
};
waitForEl(selector, function() {
// work the magic
});
If you need to pass parameters to a callback function, you can use it this way:
waitForEl("#" + elDomId, () => callbackFunction(param1, param2));
But be careful! This solution by default can fall into a trap of an infinite loop.
Several improvements of the topicstarter's suggestion are also provided in The GitHub thread.
Enjoy!

This is for those of you who are running code in the Chrome console and not just hard-coded into the html.
user993683 above offered code that will work in your console code. His/her code is as follows:
while(!document.querySelector(".my-selector")) {
await new Promise(r => setTimeout(r, 500));
}
// now the element is loaded
He/she added that it "needs to be inside an async function." And if you are using code in Chrome's console then in fact you DON'T need to wrap it in a function. It will work just as written. You only need to place it in your code at the place right before you try to access the element to make sure it exists.
The only caveat is that it won't work on elements that are only sometimes present under other circumstances. Otherwise it will loop indefinitely if the element never downloads and you'll have to close the browser to stop the wait. Only use it for elements which you are certain will be present.
My company's form page has a dozen or more fields to fill out for each case number. And I have hundreds of case numbers in the script array every day. The elements do not all load simultaneously when changing the iFrame SRC and "onload" does not work in Chrome console scripts. So this method is a god-send to me and it saves me at least 45 minutes every day over the old generic async wait 10 seconds here or 30 seconds there due to fluctuating load times.
The only change I made is "getElementById" instead of the general "querySelector" because all of the elements I need have ID's.
while(!document.getElementById("myFrame").contentWindow.document.getElementById('someDocID')) {
await new Promise(r => setTimeout(r, 500));
}
// After completing the wait above it is now safe to access the element
document.getElementById("myFrame").contentWindow.document.getElementById('someDocID'
).innerText = "Smith, John R";
// and now click the submit button then change the SRC to a fresh form, and use
//*emphasized text* the code again to wait for it to fully load
I apologize to the monitors, but I added this as an answer because after several months of research on console scripts and waiting for elements to load, user993683's remark about a function finally made me realize that console scripts do not require a function for this code. My goal here is only to save other consoler script users the same learning curve that I went through.

Just use setTimeOut with recursion:
waitUntilElementIsPresent(callback: () => void): void {
if (!this.methodToCheckIfElementIsPresent()) {
setTimeout(() => this.waitUntilElementIsPresent(callback), 500);
return;
}
callback();
}
Usage:
this.waitUntilElementIsPresent(() => console.log('Element is present!'));
You can limit amount of attempts, so an error will be thrown when the element is not present after the limit:
waitUntilElementIsPresent(callback: () => void, attempt: number = 0): void {
const maxAttempts = 10;
if (!this.methodToCheckIfElementIsPresent()) {
attempt++;
setTimeout(() => this.waitUntilElementIsPresent(callback, attempt), 500);
return;
} else if (attempt >= maxAttempts) {
return;
}
callback();
}