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>
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')
})();
I'm trying to understand JavaScript Throttling. I implemented a very basic throttle for the knowledge that I have so far.
const display = (msg) => {
return msg;
}
const throttleDisplay = (func, limit) => {
let flag = true;
return function() {
if(flag) {
func.apply(this, arguments);
flag = false;
setTimeout(() => flag = true, limit);
}
}
}
for(let i=1; i<=5; i++) {
setTimeout(() => {
const result = throttleDisplay(display("Hi"), 6000);
console.log(result)
}, i*1000);
}
My console.log is returning [Function] instead of the message Hi. Also, it is returning [Function] 5 times. Shouldn't it ignore the next call until the limit, 6000ms, has passed?
Thanks a lot!
Throttling works differently.
First, you should name your throttle function just throttle, as it is not specifically linked with display. Then, you should call this throttle to get a function that is specific to display. And only then you would use that secondary function in the actual use case you have for it, i.e. with the timer.
Here is your code corrected:
const throttle = (func, limit) => {
let flag = true;
return function() {
if(flag) {
func.apply(this, arguments);
flag = false;
setTimeout(() => flag = true, limit);
}
}
};
const throttleDisplay = throttle(() => console.log("Hi"), 6000);
for(let i=1; i<=10; i++) {
setTimeout(throttleDisplay, i*1000);
}
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 was practicing Express 4.x and noticed the following:
app.get('/fake', function(req, res) {
var obj = [];
for (let i = 0; i < 3; i++) {
jsf.resolve(fakeSchema).then(function(iter) {
obj.push(iter);
});
}
res.send(obj);
});
So, going to that route, I get "[ ]", while I was expecting to receive an array of 3 (fake) documents.
FYI, when logging each loop, I can clearly see the documents generated, even inside the array.
Any explanation?
Your jsf.resolve functiion is async so you can use async/await for this to perform task in sync manner.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function
app.get('/fake', async function(req, res) {
var obj = [];
for (let i = 0; i < 3; i++) {
try {
var iter = await jsf.resolve(fakeSchema);
obj.push(iter);
} catch (e) {}
}
res.send(obj);
});
Although #Nishant's provided answer works, I suggest using this approach.
let jsf = {};
// faking your jsf.resolve method
jsf.resolve = (param) => {
return new Promise((resolve) => {
setTimeout(() => {
resolve(Math.random());
}, 1000);
})
};
let fakeSchema = {};
let obj = [];
let promises = [];
for (let i = 0; i !== 3; i++) {
promises.push(jsf.resolve(fakeSchema).then(function (iter) {
obj.push(iter);
}));
}
Promise.all(promises).then(() => {
console.log(obj);
});
This allows all the promises to run concurrently, imagine your jsx.resolve takes a long time to complete, using await would freeze your entire appp.
As opposed to this. Note the runtime.
(async () => {
let jsf = {};
// faking your jsf.resolve method
jsf.resolve = (param) => {
return new Promise((resolve) => {
setTimeout(() => {
resolve(Math.random());
}, 1000);
})
};
let fakeSchema = {};
let obj = [];
for (let i = 0; i !== 3; i++) {
obj.push(await jsf.resolve(fakeSchema));
}
console.log(obj);
})();
#Nishant Dixit's answer also correct!
You can try this simple solution also, if you like :
app.get('/fake', function(req, res) {
var obj = [];
for (let i = 0; i < 3; i++) {
try {
jsf.resolve(fakeSchema).then(function(iter) {
obj.push(iter);
res.send(obj);
} catch (e) {
res.send(e);
}
});
};
});
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;
}
}
}