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How does promise work in Node.js and AWS lambda functions

I am very new to Javascript and Node.js in general . As such i am trying to get my head around the concept of asynchronous programming in Node.js and also how to use the same in AWS lambda.
I came across this blog post on Amazon blogs which explains the support for async/await in Node.js lambda functions : https://aws.amazon.com/blogs/compute/node-js-8-10-runtime-now-available-in-aws-lambda/
So far as i knew the concept of a function generating a promise is as in the below code snippet:
const func1 = () => {
console.log("Starting with execution of func1...")
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve("First promise ended after 3 secs")
}, 3000)
})
}
So in here the function explicitly generates a Promise and returns the same.
But in the above AWS blog post i see a function definition as such :
let AWS = require('aws-sdk');
let lambda = new AWS.Lambda();
exports.handler = async (event) => {
return await lambda.getAccountSettings().promise() ;
};
Now i have checked the AWS SDK for Node.js documentation (https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/Lambda.html#getAccountSettings-property) and i see that the function getAccountSettings accepts a callback as the third parameter.
I am confused with the .promise() syntax of generating promises .How does a function ensure that using that syntax it would give back a promise object . Because from the docs there is no mention that if i use .promise() it would return a promise instead. I am assuming there might be a rule of thumb here in this aspect.
Also instead of return await lambda.getAccountSettings().promise() if i just write return await lambda.getAccountSettings() what difference would it make .
Is there any documentation on this that i can refer to ?
Request you to please throw some more light on this new way of getting promise object back.
Thanks for any help in advance.

If you would like to understand why the .promise() method was made available, and why the promise was not just returned just-like-that, note how such an API evolves over time, and backwards compatibility must be maintained.
Let's build something similar, but much simplified. Let's make a function that provides 1/x for a given number, but an error object when x=0. This will be done asynchronously.
Also, we want the function to return synchronously an object that allows one to register a listener for when an error occurs, another for when there is success, and yet another when either of the two happens. This is a much simplified idea from what AWS returns: there you get a very rich Request object.
So imagine we are in 2012 and promises are not yet widely available/used in JavaScript. So we provide the following API for our asynchronous 1/x function:
// Implementation in the first version of the API (without promises):
// * returns an object with which you can register a listener
// * accepts an optional callback
function getAsyncInverse(num, callback) {
var onSuccess = [], // callbacks that are called on success
onError = [], // callbacks that are called on failure
onComplete = []; // callbacks that are called in both situations
if (callback) onComplete.push(callback);
function complete(err=null) {
var result = null;
if (num === 0) err = new Error("Division by Zero");
else result = 1/num;
// Communicate the result/error to the appropriate listeners:
if (err) for (var i = 0; i < onError.length; i++) onError[i](err);
else for (var i = 0; i < onSuccess.length; i++) onSuccess[i](result);
for (var i = 0; i < onComplete.length; i++) onComplete[i](err, result);
}
var timeoutId = setTimeout(complete, 100);
var request = {
on: function (type, callback) {
if (type === "success") onSuccess.push(callback);
else if (type === "error") onError.push(callback);
else if (type === "complete") onComplete.push(callback);
return request;
},
abort: function () {
clearTimeout(timeoutId);
complete(new Error("aborted"));
return request;
}
}
return request;
}
// How version 1 is used, by registering a listener via the returned object
var num = 2;
var request = getAsyncInverse(num); // let's not pass a callback here
request.on("success", function (result) { // ... but use the request object
console.log("The result is:", result);
}).on("error", function (err) {
console.log("There was an error:", err);
});
But then promises become more popular and users of your API are pushing for a promise API. You want to ensure backwards compatibility, and so decide to just extend the returned request-object with one additional property, a method: promise()
Here is how the above implementation would be altered to make that happen:
// Implementation in the second version of the API (with promise), but backwards-compatible
// * returns an object with which you can register a listener, or get the promise object
// * accepts an optional callback
function getAsyncInverse(num, callback) {
let onSuccess = [], // callbacks that are called on success
onError = [], // callbacks that are called on failure
onComplete = []; // callbacks that are called in both situations
if (callback) onComplete.push(callback);
let request;
// New: create a promise, and put the logic inside the promise-constructor callback
let promise = new Promise(function (resolve, reject) {
function complete(err=null) {
let result = null;
if (num === 0) err = new Error("Division by Zero");
else result = 1/num;
// Communicate the result/error to the appropriate listeners:
if (err) for (let callback of onError) callback(err);
else for (let callback of onSuccess) callback(result);
for (let callback of onComplete) callback(err, result);
// New: also call resolve/reject
if (err) reject(err);
else resolve(result);
}
let timeoutId = setTimeout(complete, 100);
request = {
on: function (type, callback) {
if (type === "success") onSuccess.push(callback);
else if (type === "error") onError.push(callback);
else if (type === "complete") onComplete.push(callback);
return request;
},
abort: function () {
clearTimeout(timeoutId);
complete(new Error("aborted"));
return request;
},
promise: function () { // <--- added feature!
return promise;
}
};
});
return request; // We return the same as in version-1, but with additional promise method
}
// How version 2 is used, by getting the new promise method
let num = 2;
let promise = getAsyncInverse(num).promise();
promise.then(function (result) {
console.log("The result is:", result);
}).catch(function (err) {
console.log("There was an error:", err);
});
As you can see, it would not be a good idea to omit the request object, and have the function return the promise. This would break existing code using your API (no backwards compatibility).

How to delay execution of a function in Javascript [duplicate]

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;
}

resolving Promises sequentially

EDIT: This is for an Electron project, with a local server spun up on the user's system. So, any concerns about what happens if multiple users attempt simultaneous access can be ignored!
My client code is generating an array of JavaScript objects, which I'm calling packets. There can be potentially infinite packets, although between 1 and 10 is the most common use-case.
I need to make API calls to a backend route, one per packet, passing a packet as an argument to each call.
However, the backend does some heavy computational work on each packet, to the point where attempting to crunch more than 2-3 packets at once crashes the server.
Is it possible to resolve Promises synchronously, such that the second Promise only fires when the first resolves, the third firing after the second, and so on?
It's my understanding that Promise.all() makes all calls simultaneously, which doesn't help me here.
(I also know that this is an anti-pattern, but for my specific set of requirements, I'm not sure what else to do)
I know this is purely abstract, but any thoughts would be appreciated!!!

Get weird with Promises
An async queue, a spin-off one of my previous answers; I've added random completion time to simulate a real environment:
class Queue {
constructor() {
this.queue = [];
}
enqueue(obj) {
return this.queue.push(obj);
}
dequeue() {
return this.queue.shift();
}
hasWork() {
return (this.queue.length > 0);
}
}
class AsyncQueue extends Queue {
constructor(job) {
super();
this.job = job;
}
process(cb) {
return this.job(this.dequeue()).then(data => {
cb(data);
if (this.hasWork())
return this.process(cb);
});
}
}
//MUST RETURN Promise
function work() {
var duration = chooseDelay();
console.log('START JOB, I.E., MAKE REQUEST (will take %s)', duration);
return t_o(duration);
}
function report() {
console.log('JOB DONE');
}
function done() {
console.log('ALL WORK DONE');
}
function t_o(delay) {
return new Promise(function (resolve, reject) {
setTimeout(function () {
resolve();
}, delay);
});
}
function chooseDelay() {
var delays = [200, 1000, 4000, 100, 50, 7000];
return delays[parseInt(Math.random() * 100) % delays.length];
}
var q = new AsyncQueue(work);
for (var packet = 0; packet < 10; ++packet)
q.enqueue(packet);
q.process(report).then(done);

As an alternative to an otherwise good answer, here's a really simple queue that works (work function shamelessly copied and expanded from other answer)
// Here is the queue "creator"
let promiseQueue = fn => {
let q = Promise.resolve();
return (...args) => q = q.then(() => fn(...args));
};
// that's it, that's the whole code for a promise queue
// here we create a queue
var q = promiseQueue(work);
// simple mock asynchronous function
function work({index, data}) {
var duration = parseInt(Math.random() * 100) + 100;
console.log('START JOB %s, I.E., MAKE REQUEST (will take %s) and should result with %s', index, duration, (index +1) * data);
return new Promise(resolve => setTimeout(resolve, duration)) // mock a delay
.then(() => ({index, result:(index + 1) * data})); // and some "processing"
}
// simulating two "chunks" of packets, generated a millisecond apart, but still, the sequence will be maintained
setTimeout(() => {
var packets = Array.from({length:10}, (_, index) => ({index, data:parseInt(Math.random() * 10000)}));
var promises = packets.map(packet => q(packet));
// the results in promise all are all the results of this batch of "packets"
Promise.all(promises).then(results => console.log(results));
}, 100);
setTimeout(() => {
var packets = Array.from({length:10}, (_, index) => ({index: index + 10, data:parseInt(Math.random() * 10000)}));
var promises = packets.map(packet => q(packet));
Promise.all(promises).then(results => console.log(results));
}, 101);

the simple function to execute promises sequentially
const sequentiallyExecuting = (promises) => {
let promise = Promise.resolve();
promises.forEach((task) => {
promise = promise.then((data) => {
return task;
})
});
return promise;
}
// pass array of promises to this function
sequentiallyExecuting(promises).then((data) =>{
console.log("all requests completed sequentially");
})

'use strict';
// job to be done
function job(params) {
return function () {
console.log('job started', params);
return new Promise(function (resolve) {
setTimeout(function () {
console.log('job finished');
resolve();
}, 1000);
})
}
}
// data to be processed sequentially
var params = [
1,
2,
3,
4,
5
];
// reduce data to Promise sequence
params.reduce(function (cum, cur) {
return cum.then(job(cur));
}, Promise.resolve());

With async/await it becomes trivial:
while (promiseArray.length > 0)
await promiseArray.shift();

Using chrome.tabs.executeScript to execute an async function

I have a function I want to execute in the page using chrome.tabs.executeScript, running from a browser action popup. The permissions are set up correctly and it works fine with a synchronous callback:
chrome.tabs.executeScript(
tab.id,
{ code: `(function() {
// Do lots of things
return true;
})()` },
r => console.log(r[0])); // Logs true
The problem is that the function I want to call goes through several callbacks, so I want to use async and await:
chrome.tabs.executeScript(
tab.id,
{ code: `(async function() {
// Do lots of things with await
return true;
})()` },
async r => {
console.log(r); // Logs array with single value [Object]
console.log(await r[0]); // Logs empty Object {}
});
The problem is that the callback result r. It should be an array of script results, so I expect r[0] to be a promise that resolves when the script finishes.
Promise syntax (using .then()) doesn't work either.
If I execute the exact same function in the page it returns a promise as expected and can be awaited.
Any idea what I'm doing wrong and is there any way around it?

The problem is that events and native objects are not directly available between the page and the extension. Essentially you get a serialised copy, something like you will if you do JSON.parse(JSON.stringify(obj)).
This means some native objects (for instance new Error or new Promise) will be emptied (become {}), events are lost and no implementation of promise can work across the boundary.
The solution is to use chrome.runtime.sendMessage to return the message in the script, and chrome.runtime.onMessage.addListener in popup.js to listen for it:
chrome.tabs.executeScript(
tab.id,
{ code: `(async function() {
// Do lots of things with await
let result = true;
chrome.runtime.sendMessage(result, function (response) {
console.log(response); // Logs 'true'
});
})()` },
async emptyPromise => {
// Create a promise that resolves when chrome.runtime.onMessage fires
const message = new Promise(resolve => {
const listener = request => {
chrome.runtime.onMessage.removeListener(listener);
resolve(request);
};
chrome.runtime.onMessage.addListener(listener);
});
const result = await message;
console.log(result); // Logs true
});
I've extended this into a function chrome.tabs.executeAsyncFunction (as part of chrome-extension-async, which 'promisifies' the whole API):
function setupDetails(action, id) {
// Wrap the async function in an await and a runtime.sendMessage with the result
// This should always call runtime.sendMessage, even if an error is thrown
const wrapAsyncSendMessage = action =>
`(async function () {
const result = { asyncFuncID: '${id}' };
try {
result.content = await (${action})();
}
catch(x) {
// Make an explicit copy of the Error properties
result.error = {
message: x.message,
arguments: x.arguments,
type: x.type,
name: x.name,
stack: x.stack
};
}
finally {
// Always call sendMessage, as without it this might loop forever
chrome.runtime.sendMessage(result);
}
})()`;
// Apply this wrapper to the code passed
let execArgs = {};
if (typeof action === 'function' || typeof action === 'string')
// Passed a function or string, wrap it directly
execArgs.code = wrapAsyncSendMessage(action);
else if (action.code) {
// Passed details object https://developer.chrome.com/extensions/tabs#method-executeScript
execArgs = action;
execArgs.code = wrapAsyncSendMessage(action.code);
}
else if (action.file)
throw new Error(`Cannot execute ${action.file}. File based execute scripts are not supported.`);
else
throw new Error(`Cannot execute ${JSON.stringify(action)}, it must be a function, string, or have a code property.`);
return execArgs;
}
function promisifyRuntimeMessage(id) {
// We don't have a reject because the finally in the script wrapper should ensure this always gets called.
return new Promise(resolve => {
const listener = request => {
// Check that the message sent is intended for this listener
if (request && request.asyncFuncID === id) {
// Remove this listener
chrome.runtime.onMessage.removeListener(listener);
resolve(request);
}
// Return false as we don't want to keep this channel open https://developer.chrome.com/extensions/runtime#event-onMessage
return false;
};
chrome.runtime.onMessage.addListener(listener);
});
}
chrome.tabs.executeAsyncFunction = async function (tab, action) {
// Generate a random 4-char key to avoid clashes if called multiple times
const id = Math.floor((1 + Math.random()) * 0x10000).toString(16).substring(1);
const details = setupDetails(action, id);
const message = promisifyRuntimeMessage(id);
// This will return a serialised promise, which will be broken
await chrome.tabs.executeScript(tab, details);
// Wait until we have the result message
const { content, error } = await message;
if (error)
throw new Error(`Error thrown in execution script: ${error.message}.
Stack: ${error.stack}`)
return content;
}
This executeAsyncFunction can then be called like this:
const result = await chrome.tabs.executeAsyncFunction(
tab.id,
// Async function to execute in the page
async function() {
// Do lots of things with await
return true;
});
This wraps the chrome.tabs.executeScript and chrome.runtime.onMessage.addListener, and wraps the script in a try-finally before calling chrome.runtime.sendMessage to resolve the promise.

Passing promises from page to content script doesn't work, the solution is to use chrome.runtime.sendMessage and to send only simple data between two worlds eg.:
function doSomethingOnPage(data) {
fetch(data.url).then(...).then(result => chrome.runtime.sendMessage(result));
}
let data = JSON.stringify(someHash);
chrome.tabs.executeScript(tab.id, { code: `(${doSomethingOnPage})(${data})` }, () => {
new Promise(resolve => {
chrome.runtime.onMessage.addListener(function listener(result) {
chrome.runtime.onMessage.removeListener(listener);
resolve(result);
});
}).then(result => {
// we have received result here.
// note: async/await are possible but not mandatory for this to work
logger.error(result);
}
});

For anyone who is reading this but using the new manifest version 3 (MV3), note that this should now be supported.
chrome.tabs.executeScript has been replaced by chrome.scripting.executeScript, and the docs explicitly state that "If the [injected] script evaluates to a promise, the browser will wait for the promise to settle and return the resulting value."

How to wait until a predicate condition becomes true in JavaScript?

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;
}