How to execute asynchronous js code (written in node.js) every X seconds and ensure that the new execute/call won't be started before the callback (success, error) from asynchronous function is executed. Code example:
asyncFunc(function(successData){
// do something with successData
}, function(errorData){
// do something with errorData
});
setInterval comes to my mind firstly, but I think it won't ensure what I want, if the callback doesn't execute inside interval, it will go to the queue.
EDIT: To simplify case, repeat async function call 1 second after its callback finishes.
You can use setTimeout like this:
(function loop() {
asyncFunc(function(successData){
setTimeout(loop, X);
}, function(errorData){
// If you still want to continue:
setTimeout(loop, X);
});
})(); // execute immediately
This will start the delay when the async call triggers the callback.
Consider abstracting the problem with a re-usable method that'll allow you to specify the function to call, the interval at which it's called, and the callbacks to invoke:
function repeatAsync(func, interval, success, error) {
(function loop() {
func(
() => {
success();
setTimeout(loop, interval);
},
() => {
error();
setTimeout(loop, interval);
}
);
})(); // invoke immediately
}
If you want the chain of calls to be broken on error, replace the second argument to func with just error, i.e. func( () => { ... }, error)
Related
I am trying to execute an API call 3 seconds after the last modification of a variable in Vue.js.
I implemented this using setTimeout and clearTimeout with a watcher on my variable :
data () return {
timeoutDuration: null,
}
watch: {
queryGeocodage: function (value) {
if (this.timeoutDuration) clearTimeout(this.timeoutDuration)
await // async stuff
this.timeoutDuration = setTimeout(() => { console.log('timeout') }, 2000)
}
}
The console.log still execute as many time as queryGeocodage change like if the timeout wasn't clear.
Why the timeout isn't cleared?
Guess:
Because queryGeocodage() is async function, when queryGeocodage change before await // async stuff completed, this.timeoutDuration is not assgined. So clearTimeout() is not executed.
You can try to log the value of this.timeoutDuration to explore it.
Recently, I've tried to understand callback functions in JavaScript, however, this concept is still far away from my understanding. I have code like this:
function exampleFunc(callback) {
console.log("Starting...");
setTimeout(() => {
console.log("Logged after 3 secs.");
}, 3000);
callback();
}
function displayMessage() {
console.log("Log it!");
}
exampleFunc(() =>
{
console.log("Further code.");
});
displayMessage();
I've expected that after calling exampleFunc(), program will wait 3 seconds, and then call the callback and rest of code. But instead, the sequence of code is:
Starting...
Further code.
Log it!
Logged after 3 secs.
Why does it happen? I've expected that program will output "Starting", then wait and log "Logged after 3 secs.", then go to callback and output "Further code", and in the end, "Log it!".
Calling a setTimeout does not cause further execution of the code in that function to delay. setTimeout schedules a timeout, then continues executing the rest of the code in the function immediately.
You need to call callback inside the setTimeout callback, so that it runs only after the 3 seconds are up.
You also need to put the call of displayMessage inside the callback passed to exampleFunc.
function exampleFunc(callback) {
console.log("Starting...");
setTimeout(() => {
console.log("Logged after 3 secs.");
callback();
}, 3000);
}
function displayMessage() {
console.log("Log it!");
}
exampleFunc(() => {
console.log("Further code.");
displayMessage();
});
I'm having a synchronization and loading issues with some JS modules when the program starts. This error only shows up once at the beginning and then everything works, so it is an obvious sync problem.
The code:
//pyramid of doom
function initGame(){
initWorld(function(){
initPlayer(function(){
initBots(function(){
console.log("Game Loaded!");
update();
})
})
});
}
function initWorld(callback){
world.init(worldParams);
callback&&callback();
}
function initPlayer(callback){
player.init(scene,playerParams,world.getPhysicModel());
callback&&callback();
}
function initBots(callback){
bots.init(scene,botsParams,world.getPhysicModel());
callback&&callback();
}
function update() {
world.update(1/60);
player.update();
bots.update();
}
initGame();
The following is the error I'm getting.
Bots.js:112 Uncaught TypeError: Cannot read property 'mixer' of undefined
at Bots.update (Bots.js:112)
at update (Final.html:160)
What am I doing wrong? How can I synchronize the execution of the init functions?
(What I think that is going on is that the execution of initBots doesn't reach it end before the udpdate function starts to run.)
You can find the Bots.js module in my repository at ( 1 )
In bots.init you execute new THREE.ColladaLoader().load which looks to be asynchronous.
In its callback, you fill your _bots array (self._bots[modelLoaded] = bot;).
However, you execute bots.init() and do not wait for these asynchronous calls to complete before executing the initBots function callback. In the case of initGame execution, this callback executes update(), which in turn executes bots.update(), which tries to access this._bots[i].mixer with i index up to this._botsParams.length, i.e. a pre-defined value that does not account for how many items have been actually filled in _bots array.
Hence your error message: the array has no items yet at some indices, and trying to read a property on undefined throws an error.
Conclusion: common asynchronous issue.
You need to be passing the callbacks into the init functions. Guessing from a brief look at your bots code, they are not expecting to receive callbacks so you might be in for a rebuild.
You can't tell from the outside of an async function if it is done yet!
Equivalent to what you are doing:
let result = undefined
function takeTime () {
setTimeout(function() {
result = 'hello!'
}, 100)
}
function callback () {
console.log(result)
}
function run (callback) {
takeTime()
callback && callback()
}
run(callback) // undefined! takeTime has not finished yet
What you need to be doing:
let result = undefined
function takeTime (callback) {
setTimeout(function() {
result = 'hello!'
callback()
}, 100)
}
function callback () {
console.log(result)
}
function run (callback) {
callback && takeTime(callback)
}
run(callback) // 'hello!', the callback was only called once the takeTime function completed
function firstFunction(_callback){
// do some asynchronous work
// and when the asynchronous stuff is complete
_callback();
}
function secondFunction(){
// call first function and pass in a callback function which
// first function runs when it has completed
firstFunction(function() {
console.log('huzzah, I\'m done!');
});
}
This is an example from this site, I would like help understanding it.
If I have a function that sums 2 number and the other returns it. So:
var z = 0;
function firstf (x, y, callback){
z = x + y;
callback();
}
function secondf () {
console.log(z);
}
I dont get how this works? How do I make it so that secondf waits until firstf is done using callbacks?
After define 2 function, you call:
firstf(2,3,secondf);
Follow : z=2+3 then call function callback. And now, function callback ~ secondf :
z=2+3 ;
secondf();
If you want to the second block to wait until the first block is done. Then using callback makes no sense.
Because the main concept of callback is to provide an asynchronous platform.
A callback is a function call at the completion of a given task, hence prevents any blocking which may might occur if the first block takes long time to load data.
So, if you want both the blocks to work asynchronously the use callback, and to achieve what you are asking simply call the second function after the task of block one is done.
For better understanding go through this link,
https://docs.nodejitsu.com/articles/getting-started/control-flow/what-are-callbacks/
Best of luck!
You can use "promise" concept to ensure that the secondf waits until firstf is done:
function firstf(x,y){
return new Promise(
function (resolve, reject) {
resolve(x+y);
});
}
function secondf(){
firstf(x,y).then ( function (result){
console.log(result);
});
}
By re-ordering your code:
edit Made code async for demo purposes
var z = 0;
function firstf(x, y, callback) {
console.log("Inside firstf");
z = x + y;
console.log("Will call callback in 1 second");
// Lets make this function async.
setTimeout(function() {
console.log("Calling callback");
callback();
}, 1000);
}
function secondf() {
console.log("Inside secondf");
console.log(z);
console.log("Leaving secondf");
}
firstf(1, 2, secondf);
Why doesn't this function return true?
function test(str) {
window.setTimeout(function() {
if(str == 'ok') {
return true;
}
}, 1000);
}
console.log(test('ok'));
That's not exactly what i want to do.
I have a function nammed test() who does some actions after 1 second.
I want to execute next function, when test() is finished (so after the timeout).
How i can know when test i finished ?
Tracing your code, here's what happens.
test() is invoked.
setTimeout schedules a function to be called 1000 ms later.
test() concludes execution, no return statement was executed, so undefined is returned instead.
about 1000 ms later the scheduled function fires.
The scheduled function returns true to nothing.
In other words, it just doesn't work that way. The JS interpreter does not pause, it continues over the timeout. You cannot pause execution in JS.
Instead you typically use callbacks:
function test(str, callback) {
window.setTimeout(function() {
if (str === 'ok') {
callback(true);
}
}, 1000);
}
// logs 'true' 1000 ms later
test('ok', function(result) {
console.log(result);
});
// logs nothing, callback never fires
test('NOTOK!', function(result) {
console.log(result);
});
This code will do more what you seem to have expected.
It does not return true because the setTimeout call is asynchronous. Also, the return value true in your code comes from an inner function.
The normal way of handling such program flow is to pass a callback to the asynchronous function.
function test(str, callback) {
window.setTimeout(function() {
callback(str == 'ok');
}, 1000);
}
test('ok', function (result) {
console.log(result);
});
The function passed as the second argument to test() will be called when setTimeout executes the code. The argument to the callback function will tell if str is was ok or not.
For starters, settimeout is an asynchronous method, so the actual function test() will have finished and returned before the settimout code runs.
Secondly however, you are only returning true from the settimeout function not the test function, so you will never get anything other than false.
It doesn't return true because asynchronous function setTimeout() will execute after 1000 ms and console.log will execute in normal fashion without waiting of your 'test' function.