can someone try to fix my problem?
I am trying to get this output:
100x do C
done B
done A
But everytime i get this one:
10x do C
done B
done A
Is it possible to get it without using async/await feature?
function a() {
let myPromise = new Promise(function(myResolve, myReject) {
b();
myResolve()
});
myPromise.then(function() {
console.log('done A')
});
}
function b() {
let myPromise = new Promise(function(myResolve, myReject) {
for (i = 0; i < 10; i++) {
c();
}
myResolve()
});
myPromise.then(function() {
console.log('done B')
});
}
function c() {
for (i = 0; i < 10; i++) {
console.log('do C')
}
}
a()
The problem is that you're not declaring i, so you're creating an implicit global variable that both b and c write to. (I wrote an article about these years ago: The Horror of Implicit Globals.)
In the below, I'm going to make it three runs of three instead of ten runs of ten, just so we can see it more clearly (and so I don't have to increase the size of the in-snippet console's history — it has an option for that, but I don't recall the details).
I strongly recommend using strict mode so that this is the error it always should have been. Here's your code in strict mode and with rejection handlers added:
"use strict";
function a() {
let myPromise = new Promise(function(myResolve, myReject) {
b();
myResolve()
});
myPromise.then(function() {
console.log('done A')
}).catch(function(err) {
console.error(err);
});
}
function b() {
let myPromise = new Promise(function(myResolve, myReject) {
for (i = 0; i < 3; i++) {
c();
}
myResolve()
});
myPromise.then(function() {
console.log('done B')
}).catch(function(err) {
console.error(err);
});
}
function c() {
for (i = 0; i < 3; i++) {
console.log('do C')
}
}
a()
If you declare i (in both places), you'll get the result you expect.
function a() {
let myPromise = new Promise(function(myResolve, myReject) {
b();
myResolve()
});
myPromise.then(function() {
console.log('done A')
});
}
function b() {
let myPromise = new Promise(function(myResolve, myReject) {
for (let i = 0; i < 3; i++) { // ***
c();
}
myResolve()
});
myPromise.then(function() {
console.log('done B')
});
}
function c() {
for (let i = 0; i < 3; i++) { // ***
console.log('do C')
}
}
a()
but, beware that the reason you're getting the output you're getting is that everything except the calls to the fulfillment handlers is synchronous. If instead of a for loop you were doing some asynchronous process, there's nothing preventing you from getting done B or done A before all of the work done by c is done, because there's nothing connecting those promises together.
Let's see that. Suppose we have an asynchronous version of console.log that waits 10ms before writing the log entry (but we'll use console.log to write out done A and done B immediately, so you can see when they ran):
function asyncLog(...msgs) {
return new Promise(resolve => {
setTimeout(() => {
console.log(...msgs);
resolve();
}, 10);
});
}
Let's look at what happens:
function asyncLog(...msgs) {
return new Promise(resolve => {
setTimeout(() => {
console.log(...msgs);
resolve();
}, 10);
});
}
function a() {
let myPromise = new Promise(function(myResolve, myReject) {
b();
myResolve();
});
myPromise.then(function() {
console.log('done A');
});
}
function b() {
let myPromise = new Promise(function(myResolve, myReject) {
for (let i = 0; i < 3; i++) {
c();
}
myResolve()
});
myPromise.then(function() {
console.log('done B')
});
}
function c() {
for (let i = 0; i < 3; i++) {
asyncLog('do C');
}
}
a();
Notice how things aren't connected to each other.
To connect them, you'd have to make b wait for c's promise to be fulfilled, and make a wait for b's promise to be fulfilled. Something like this:
function asyncLog(...msgs) {
return new Promise(resolve => {
setTimeout(() => {
console.log(...msgs);
resolve();
}, 10);
});
}
function a() {
let myPromise = new Promise((resolve, reject) => {
resolve(b());
});
return myPromise
.then(() => {
console.log('done A');
});
}
function b() {
let myPromise = Promise.resolve();
for (let i = 0; i < 3; i++) {
myPromise = myPromise.then(c);
}
return myPromise
.then(() => {
console.log('done B')
});
}
function c() {
let p = Promise.resolve();
for (let i = 0; i < 3; i++) {
p = p.then(() => asyncLog('do C'));
}
return p;
}
a()
.then(() => {
console.log("all done");
})
.catch(error => {
console.error(`Error: ${error.message}`);
});
Related
I'm trying to show the process of how the backtracking algorithm solves a sudoku board, but I'm not sure how I can make sure that it only gets called every 500ms.
function solveBoard(board) {
// I tried doing setTimeout from here to the bottom, but it breaks the solver and just puts 9s everywhere.
let empty = findEmpty(board);
if (!empty) return true;
let row = empty[0];
let col = empty[1];
for (let i = 1; i < 10; i++) {
board[row][col] = i;
console.log(board[row][col]);
document.getElementById(`${row}-${col}`).value = i;
if (checkValid(board, row, col)) {
if (solveBoard(board)) {
return true;
}
}
board[row][col] = 0;
}
return false;
}
The first time I call solve board is just an event listener.
solveBtn.addEventListener("click", () => {
solveBoard(boardArray);
});
Call sleep in solveBoard
async function solveBoard(board) {
await sleep()
// …
}
function sleep(ms = 500) {
return new Promise(resolve => setTimeout(resolve, ms))
}
// Demo
(async () => {
console.log('a')
await sleep()
console.log('b')
await sleep()
console.log('c')
})();
New to js,I want to output 5 '!'s with a 1-second interval,and finally output "end".
I thought the problem was related to asynchronization.
I have tried a lot of times and a lot of methods such as "await,async" and "promise" ,but still failed.
class A {
constructor() {
this.cnt = 5;
}
real() {
this.cnt--;
console.log("!");
}
getCnt() {
return this.cnt;
}
}
class B {
constructor() {
this.a = new A();
}
fun() {
if (this.a.getCnt() > 0) {
this.a.real();
setTimeout(() => this.fun(), 1000);
}
}
}
class C {
constructor() {
this.b = new B();
}
f() {
this.b.fun();
console.log("end");
}
}
var c = new C();
c.f();
Skipping the complexity of the 3 classes involved, this is elegantly solved with async functions. (Without async functions, the cascade of setTimeouts in a loop becomes more difficult to manage.)
This can of course be wrapped into a trio of classes if required.
// Turns `setTimeout` into a promise you can `await`.
async function delay(ms) {
return new Promise((resolve) => setTimeout(resolve, ms));
}
// Calls a function `fun` `times` times, delaying for `delayTime` ms between each invocation.
// Since this is an async function, it returns a promise that will resolve as soon as it's done,
// which in turn can be awaited upon.
async function repeatWithDelay(fun, times, delayTime) {
for (let i = 0; i < times; i++) {
await fun(i);
await delay(delayTime);
}
}
// Prints the number passed in (could just print an exclamation mark).
function print(i) {
console.log(`${i}!`);
}
async function main() {
console.log("Go!");
await repeatWithDelay(print, 5, 500);
console.log("Done!");
}
// Assumes top-level `await` is not available in your environment.
// If it is, this too can be replaced with a simple `await`.
main().then(() => {
console.log("Main done.");
});
This prints out
Go!
0!
1!
2!
3!
4!
Done!
Main done.
Ultimately you need to make your method fun be able to report when it is finished as it is an asynchronous method (calls setTimeout). The best way to do this is return a Promise which allows calls to use await on it
fun() {
return new Promise(resolve => {
const exec = () => {
if (this.a.getCnt() > 0) {
this.a.real();
setTimeout(exec.bind(this), 1000);
}
else{
resolve();
}
}
exec.apply(this);
})
}
Once you've done that, also mark function f as async which allows you to call await inside it:
async f() {
await this.b.fun();
console.log("end");
}
And then all works as expected:
class A {
constructor() {
this.cnt = 5;
}
real() {
this.cnt--;
console.log("!");
}
getCnt() {
return this.cnt;
}
}
class B {
constructor() {
this.a = new A();
}
fun() {
return new Promise(resolve => {
const exec = () => {
if (this.a.getCnt() > 0) {
this.a.real();
setTimeout(exec.bind(this), 1000);
}
else{
resolve();
}
}
exec.apply(this);
})
}
}
class C {
constructor() {
this.b = new B();
}
async f() {
await this.b.fun();
console.log("end");
}
}
var c = new C();
c.f();
I tried to write a generic function that emulate typing, hope this help you understand async/await functions
let fakeConsole = document.getElementById('console');
async function sleep(time) {
await new Promise(r => setTimeout(r, time));
}
function type(text) {
fakeConsole.innerHTML += text;
}
async function emulateTyping(text, speed) {
// split the text into an array of characters;
const characters = text.split('').reverse();
if (characters.length > 0) {
// display first character right away
type(characters.pop());
while (characters.length > 0) {
// wait <speed> millisections
await sleep(speed);
// type one character
type(characters.pop());
}
}
}
async function main() {
// async function we wait for the end with "await"
await emulateTyping("!!!!!", 500);
// sync function
type(' END');
};
main();
<pre id="console"></pre>
If using promise, then you need to resolve the response in setTimeout too
class A {
constructor() {
this.cnt = 5;
}
real() {
this.cnt--;
console.log("!");
}
getCnt() {
return this.cnt;
}
}
class B {
constructor() {
this.a = new A();
}
fun() {
return new Promise((resolve) => {
if (this.a.getCnt() > 0) {
this.a.real();
setTimeout(() => resolve(this.fun()), 1000);
} else {
resolve()
}
})
}
}
class C {
constructor() {
this.b = new B();
}
f() {
this.b.fun().then(() => {
console.log("end");
})
}
}
var c = new C();
c.f();
For multiple loops:
var loop = [array of 100 items];
await Promise.all(
loop.map(async (item) => await this.b.fun();)
)
console.log("end");
I need a loop to play 10 sounds in sequence. My first attempt had the sounds overlapping, so I was told that I need to use Promise/await. The code below plays sound 0 then never continues the loop.
(The library I'm using (jscw) is for morse code. You pass it a string, it plays the morse equivalent. Its "onFinished" calls a user-defined function.)
async function playAll() {
for (let i = 0; i < 10; i++) {
playMorse(words[i]);
await playstate();
}
}
function playstate() {
playdone = true;
//console.log(playdone);
return new Promise((resolve) => {
window.addEventListener('playdone', resolve)
})
}
function playMorse(z) {
var m = new jscw();
playdone = false;
m.onFinished = function() {
playstate();
}
m.play(z);
}
It seems nothing is supposed to fire the playdone event you are listening for.
So a simple solution is to fire it in the onFinished callback.
const words = ["hello", "world"];
async function playAll() {
for (let i = 0; i < 2; i++) {
console.log("###READING", words[i]);
playMorse(words[i]);
await playstate();
}
}
function playstate() {
playdone = true;
//console.log(playdone);
return new Promise((resolve) => {
window.addEventListener('playdone', resolve, { once: true })
})
}
function playMorse(z) {
var m = new jscw();
playdone = false;
m.onFinished = function() {
dispatchEvent( new Event("playdone") );
}
m.play(z);
}
btn.onclick = playAll;
<script src="https://fkurz.net/ham/jscwlib/src/jscwlib.js"></script>
<button id="btn">play all</button>
But you don't need an event here, simply make playMorse return a Promise that will resolve in the onFinished callback:
const words = ["hello", "world"];
async function playAll() {
for (let i = 0; i < 2; i++) {
console.log("###READING", words[i]);
await playMorse(words[i]);
}
}
function playMorse(z) {
return new Promise( (resolve) => {
const m = new jscw();
m.onFinished = resolve;
m.play(z);
});
}
btn.onclick = playAll;
<script src="https://fkurz.net/ham/jscwlib/src/jscwlib.js"></script>
<button id="btn">play all</button>
I have an async function that has a loop that I need to be able to pause or unpause it. This is what I have so far.
I use a flag to pause the flow:
let flag = true;
function flag_func() {
flag = !flag;
}
$(document).ready(function () {
function sleep(ms) {
while (!flag) {
//...waiting.. but infinite loop
}
return new Promise(resolve => setTimeout(resolve, ms));
}
async function show_simulation(data) {
document.getElementById("solve-button").outerHTML = "<button type=\"button\" id='pause-button' onclick='flag_func()' class=\"btn btn-primary btn-lg\">Pause</button>";
//simulation
if (data.length === 0) {
console.log('stuff')
} else {
let i;
for (i = 0; i < data.length; i++) {
await sleep(40);
// do stuff
}
}
}
});
The problem is that is being paused, but due the while block the flow, I can't unpause the for loop.
Any idea about how I can solve this?
It might be a nice use case for async iterables. It involves a bit of boilerplate to create your async list, but then the code is much nicer. Basically you would have:
import AsyncList from './async-list.js'
const sleep = (ms) => new Promise(r => setTimeout(r, ms));
async function f(data) {
const list = new AsyncList(data);
document.getElementById("btn-toggle").addEventListener("click", function () {
if (list.paused) {
this.textContent = "Pause";
list.resume();
} else {
this.textContent = "Resume";
list.pause()
}
})
for await (let item of list) {
console.log(item)
await sleep(1000);
}
console.log("end of loop")
}
f([10, "hello", 1029, 90, 80, 209, 44])
A possible implementation of AsyncList could be:
export default class AsyncList {
constructor(array) {
// shallow copy
this._array = array.slice();
this._index = 0;
this._length = this._array.length;
this.paused = false;
this._resume = () => {}; // noop, in case `resume` is called before `pause`
}
[Symbol.asyncIterator]() {
return this;
}
pause() {
this.paused = true;
}
resume() {
this.paused = false;
this._resume();
}
next() {
if (this._index < this._length) {
const value = this._array[this._index++];
if (this.paused) {
return new Promise(r => this._resume = r.bind(null, { value }))
}
return Promise.resolve({ value })
} else {
return Promise.resolve({ done: true });
}
}
}
Just to give to you the idea, you could also encapsulate the private properties, and check more scenarios (here I assume data is an array, for example, not just an iterable).
I'd replace:
let i;
for (i = 0; i < data.length; i++) {
await sleep(40);
// do stuff
}
...with...
let i = 0;
const doStuff = () => {
// do stuff
if (++i < data.length) {
setTimeout(doStuff, 40);
}
};
setTimeout(doStuff, 40);
I am experimenting with async/await, I can't understand why this line :
resolvedValue = await this.tryToSolve()
gives me this error :
Unexpected token this
class Test {
constructor() {
this.method = 0
this.checkLink()
}
async checkLink() {
return new Promise((resolve, reject) => {
let resolvedValue
for (let i = 0; i < 3; i++) {
this.method = i
resolvedValue = await this.tryToSolve()
if (resolvedValue) break
}
console.log(`Method ${this.method} did the trick.`);
resolve(resolvedValue)
})
}
tryToSolve() {
return new Promise((resolve, reject) => { // Resolves if this.method==1
console.log(`Trying to solve with method ${this.method}...`);
setTimeout(() => {
resolve(!!this.method ? `http://www${this.method}.someurl.com` : false)
}, 1000)
})
}
}
const test = new Test()
Does anyone know the correct syntax to store the result of an async method in a variable?
Thanks in advance.
To keep things simple, it happens because when you create a Promise, in its' constructor you pass an arrow function, which contains await call. You must always put async keyword before the declaration of a function, that contains await.
So, instead of doing this
async checkLink() {
return new Promise((resolve, reject) => {
let resolvedValue
for (let i = 0; i < 3; i++) {
this.method = i
resolvedValue = await this.tryToSolve()
if (resolvedValue) break
}
console.log(`Method ${this.method} did the trick.`);
resolve(resolvedValue)
})
}
Do it like this
checkLink() {
return new Promise(async (resolve, reject) => {
let resolvedValue
for (let i = 0; i < 3; i++) {
this.method = i
resolvedValue = await this.tryToSolve()
if (resolvedValue) break
}
console.log(`Method ${this.method} did the trick.`);
resolve(resolvedValue)
})
}
More info: https://ponyfoo.com/articles/understanding-javascript-async-await#using-async-await
Drop the new Promise around the await! You want only
async checkLink() {
let resolvedValue
for (let i = 0; i < 3; i++) {
this.method = i
resolvedValue = await this.tryToSolve()
if (resolvedValue) break
}
console.log(`Method ${this.method} did the trick.`);
return resolvedValue;
}
or much simpler
async checkLink() {
for (let i = 0; i < 3; i++) {
const value = await this.tryToSolve()
if (value) {
console.log(`Method ${i} did the trick.`);
return value;
}
}
}