Output of one sample program of javascript is giving wrong answer - javascript

I was reading one book named 'Hands on node.js' by 'Pedro Teixiera'.
I was trying to execute one same program giving in that book that will call a function and that function is calling the same function recursively within some interval again and again.
But when I executed, it gives only one time '1' and stops
Please help me to figure it out why it is not able to call the same function again.
Sample program is as follows:
var schedule = function(timeout, callbackfunction) {
return {
start: function() {
setTimeout(callbackfunction, timeout)
}
};
};
(function()
{
var timeout = 10000; // 1 second
var count = 0;
schedule(timeout, function doStuff() {
console.log(++ count);
schedule(timeout, doStuff);
}).start(timeout);
})();

You aren't actually calling the function again. start() is the part that starts the timer.
schedule( timeout, function doStuff() {
console.log( ++count );
schedule( timeout, doStuff ).start(); // <--- added .start() here
}).start();
(Also note that the start() function doesn't take parameters.)

with some interval again and again
No, for that you would have used setInterval instead of setTimeout.
it gives only one time '1' and stops
Yes, your doStuff function doesn't put a new timeout. Your odd schedule function needs to be .start()ed!

Related

JavaScript: Set Interval not working

So basically I want to call a interval in a function while the function is being runned by another interval. This gives me the result I wasn't expecting. The console log should only be runned every third second but It's not.
My code:
// Define variable test
var test = 1;
// Setting function check on a 1000 ms interval
setInterval(function(){
check();
}, 1000)
// If our variable test is == 1, set another interval on 3000ms on next function
function check() {
if (test == 1) {
setInterval(function(){
execute();
}, 3000)
}
}
// Execute function
function execute() {
console.log("Called every 3rd second!");
}
How can I fix this, spent hours trying to find solution and I'm really exhausted and have no idea how to continue.
If you want to post a answer I really appreciate it! But try not to destroy the structure in the code.
Edit: If you have Chrome, you can run this script in the console. You have to wait a few seconds before it runs.
In case you need to run the function execute after every 3 seconds then you need to replace the first function setInterval with setTimeout. I am not sure why you need setInterval for the calling the check function.
Try out this code:
var test = 1;
// Setting function check on a 1000 ms interval
setTimeout(check, 1000)
// If our variable test is == 1, set another interval on 3000ms on next function
function check() {
if (test == 1) {
setInterval(execute, 3000)
}
}
// Execute function
function execute() {
console.log("Called every 3rd second!");
}

How to abort function by timeout at JavaScript

I need to execute some function but if timeout expired I need to abort this function. I tried to use setTimeout and setImmediate. I wrote this example and used setInterval in callback for checking of working but it did't help me:
function waiter(timeout, fun) {
var functionHandler = setImmediate(fun);
var timeoutHandler = setTimeout(() => {
console.log('stoped');
clearImmediate(functionHandler);
clearTimeout(timeoutHandler);
}, timeout);
fun();
}
waiter(5000, () => {
setInterval(() => {
console.log('work');
}, 500);
});
After clearImmediate and clearTimeout my interval still working. How I understood that method doesn't guarantee that my function will be aborted.
Does anybody have an idea how to abort function execution?
UPDATE:
Yes, I know that I should call clearInterval but it's just an example for work checking. For example I should parse some big data and if it doesn't can do it by timeout I need to cancel this function execution and call something else.
The clearXXX functions don't abort functions that are currently running, they just remove functions from the async queue, so that they never start to run. JavaScript always executes code to its completion. You cannot interrupt code that's running in the main thread.
You need to name your interval
and clear it by its name. So your interval is no longer anonymous, but you've got a reference for it.
let counter = 10;
let newYearCountdown = setInterval( () => {
console.log(counter);
counter--
if (counter === 0) {
console.log("HAPPY NEW YEAR!!");
// here i clear interval by its name.
clearInterval(newYearCountdown);
}
}, 1000);

JavaScript self invoking function

I'm trying to understand this example code, what is the function of line 15, why start(timeout)? (Sorry, I'm new to programming)
var schedule = function (timeout, callbackfunction) {
return {
start: function () {
setTimeout(callbackfunction, timeout)
}
};
};
(function () {
var timeout = 1000; // 1 second
var count = 0;
schedule(timeout, function doStuff() {
console.log(++count);
schedule(timeout, doStuff);
}).start(timeout);
})();
// "timeout" and "count" variables
// do not exist in this scope.
...why start(timeout)?
In that example, there's actually no reason for passing timeout into start, since start doesn't accept or use any arguments. The call may as well be .start().
What's happening is that schedule returns an object the schedule function creates, and one of the properties on that object is called start, which is a function. When start is called, it sets up a timed callback via setTimeout using the original timeout passed into schedule and the callback function passed into schedule.
The code calling schedule turns around and immediately calls the start function on the object it creates.
In the comments, Pointy points out (well, he would, wouldn't he?) that the callback function is calling schedule but not doing anything with the returned object, which is pointless — schedule doesn't do anything other than create and return the object, so not using the returned object makes the call pointless.
Here's the code with those two issues addressed:
var schedule = function (timeout, callbackfunction) {
return {
start: function () {
setTimeout(callbackfunction, timeout)
}
};
};
(function () {
var timeout = 1000; // 1 second
var count = 0;
schedule(timeout, function doStuff() {
console.log(++count);
schedule(timeout, doStuff).start(); // <== Change here
}).start(); // <== And here
})();
It's not very good code, though, frankly, even with the fixes. There's no particularly good reason for creating a new object every time, and frankly if the book is meant to be teaching, this example could be a lot clearer. Inline named function expressions and calls to methods on objects returned by a function...absolutely fine, but not for teaching. Still, I don't know the context, so those comments come with a grain of salt.
Here's a reworked version of using the schedule function by reusing the object it returns, and being clear about what bit is happening when:
(function () {
var timeout = 1000; // 1 second
var count = 0;
// Create the schedule object
var scheduleObject = schedule(timeout, doStuff);
// Set up the first timed callback
scheduleObject.start();
// This is called by each timed callback
function doStuff() {
// Show the count
console.log(++count);
// Set up the next timed callback
scheduleObject.start();
}
})();
The function schedule is executed as a function. That function returns an object. Like you can see with the { start... }. With the returned object it calls out the start function. This is called chaining. So the start function is executed after is set the function.
What is strange is that the timeout is passed to the start function which has no parameters.

how to do a jQuery timeout

using jQuery... how do I run a function and then run a second function every 2 minutes after.
eg:
function 1: runs once
function 2: runs every 2 minutes after function 1 has finished
Any help will be much appreciated.
C
function f2(){}
function f1(){
... some code ...
setInterval(f2, 2000*60);
}
//From somewhere in your code, call f1
f1();
setInterval also returns a handle which can be used to cancel further calling of that function.
jQuery's delay() function is not a replacement for javascript's setInterval or setTimeout. To run a function #1 once on page load and then function #2 every 2 minutes after:
function funcOne() {
// some javascript
setInterval('funcTwo()', 1000*60*2);
};
function funcTwo() {
// some other javascript
};
$(document).ready(function() {
funcOne();
});
Remember, you are using JQuery because it makes javascript more simple. JQuery is a javascript library, not a language, wich means you can perfectly use javascript functions on it.
For your problem, you only need to call, with the setTimeout, your second function, and put inside this function another setTimeout(ms);
Like this:
function f1(/*...*/){}
var t = setTimeout("f2()",2 * 60 * 1000);
And at the end of your f2() function you should include another setTimeout, in order to call that function every 2 minutes.
function f2(/*...*/){
//...
t = setTimeout("f2()",2 * 60 * 1000);
}
To cancel this callings to f2() is just as simple: clearTimeout(t);
I would rather use setTimeout(). It is supposed to be less demanding on the browser, more processor efficient.
Here's what your functions should look like:
function f2(){
t = setTimeout(f2, 2000 * 60);
// code for f2
}
function f1(){
// code for f1
setTimeout(f2, 2000 * 60);
}
Then wherever it is you want the whole thing to start, call the first function:
var t;
f1();
You can stop the loop anytime:
clearTimeout(t);
Be sure to trigger setTimeout at the beginning of f2, so that it fires exactly every 2 minutes. Any code before 'setTimeout', taking 'x' time to process would result in the next f2 call firing after 2min+x.
Heloo
if i get you right then this is a solution
function func1()
{}
function func2()
{}
window.onload = function()
{
func1();
var flag_first_call_is_after_2=0;
var interv = setInterval(
function()
{
if(flag_first_call_is_after_2==0)
{
flag_first_call_is_after_2=1;
}
else
{
func2();
}
}
,120000
);
}

Execute the setInterval function without delay the first time

It's there a way to configure the setInterval method of javascript to execute the method immediately and then executes with the timer
It's simplest to just call the function yourself directly the first time:
foo();
setInterval(foo, delay);
However there are good reasons to avoid setInterval - in particular in some circumstances a whole load of setInterval events can arrive immediately after each other without any delay. Another reason is that if you want to stop the loop you have to explicitly call clearInterval which means you have to remember the handle returned from the original setInterval call.
So an alternative method is to have foo trigger itself for subsequent calls using setTimeout instead:
function foo() {
// do stuff
// ...
// and schedule a repeat
setTimeout(foo, delay);
}
// start the cycle
foo();
This guarantees that there is at least an interval of delay between calls. It also makes it easier to cancel the loop if required - you just don't call setTimeout when your loop termination condition is reached.
Better yet, you can wrap that all up in an immediately invoked function expression which creates the function, which then calls itself again as above, and automatically starts the loop:
(function foo() {
...
setTimeout(foo, delay);
})();
which defines the function and starts the cycle all in one go.
I'm not sure if I'm understanding you correctly, but you could easily do something like this:
setInterval(function hello() {
console.log('world');
return hello;
}(), 5000);
There's obviously any number of ways of doing this, but that's the most concise way I can think of.
I stumbled upon this question due to the same problem but none of the answers helps if you need to behave exactly like setInterval() but with the only difference that the function is called immediately at the beginning.
Here is my solution to this problem:
function setIntervalImmediately(func, interval) {
func();
return setInterval(func, interval);
}
The advantage of this solution:
existing code using setInterval can easily be adapted by substitution
works in strict mode
it works with existing named functions and closures
you can still use the return value and pass it to clearInterval() later
Example:
// create 1 second interval with immediate execution
var myInterval = setIntervalImmediately( _ => {
console.log('hello');
}, 1000);
// clear interval after 4.5 seconds
setTimeout( _ => {
clearInterval(myInterval);
}, 4500);
To be cheeky, if you really need to use setInterval then you could also replace the original setInterval. Hence, no change of code required when adding this before your existing code:
var setIntervalOrig = setInterval;
setInterval = function(func, interval) {
func();
return setIntervalOrig(func, interval);
}
Still, all advantages as listed above apply here but no substitution is necessary.
You could wrap setInterval() in a function that provides that behavior:
function instantGratification( fn, delay ) {
fn();
setInterval( fn, delay );
}
...then use it like this:
instantGratification( function() {
console.log( 'invoked' );
}, 3000);
Here's a wrapper to pretty-fy it if you need it:
(function() {
var originalSetInterval = window.setInterval;
window.setInterval = function(fn, delay, runImmediately) {
if(runImmediately) fn();
return originalSetInterval(fn, delay);
};
})();
Set the third argument of setInterval to true and it'll run for the first time immediately after calling setInterval:
setInterval(function() { console.log("hello world"); }, 5000, true);
Or omit the third argument and it will retain its original behaviour:
setInterval(function() { console.log("hello world"); }, 5000);
Some browsers support additional arguments for setInterval which this wrapper doesn't take into account; I think these are rarely used, but keep that in mind if you do need them.
Here's a simple version for novices without all the messing around. It just declares the function, calls it, then starts the interval. That's it.
//Declare your function here
function My_Function(){
console.log("foo");
}
//Call the function first
My_Function();
//Set the interval
var interval = window.setInterval( My_Function, 500 );
There's a convenient npm package called firstInterval (full disclosure, it's mine).
Many of the examples here don't include parameter handling, and changing default behaviors of setInterval in any large project is evil. From the docs:
This pattern
setInterval(callback, 1000, p1, p2);
callback(p1, p2);
is identical to
firstInterval(callback, 1000, p1, p2);
If you're old school in the browser and don't want the dependency, it's an easy cut-and-paste from the code.
I will suggest calling the functions in the following sequence
var _timer = setInterval(foo, delay, params);
foo(params)
You can also pass the _timer to the foo, if you want to clearInterval(_timer) on a certain condition
var _timer = setInterval(function() { foo(_timer, params) }, delay);
foo(_timer, params);
For someone needs to bring the outer this inside as if it's an arrow function.
(function f() {
this.emit("...");
setTimeout(f.bind(this), 1000);
}).bind(this)();
If the above producing garbage bothers you, you can make a closure instead.
(that => {
(function f() {
that.emit("...");
setTimeout(f, 1000);
})();
})(this);
Or maybe consider using the #autobind decorator depending on your code.
You can set a very small initial delay-time (e.g. 100) and set it to your desired delay-time within the function:
var delay = 100;
function foo() {
console.log("Change initial delay-time to what you want.");
delay = 12000;
setTimeout(foo, delay);
}
To solve this problem , I run the function a first time after the page has loaded.
function foo(){ ... }
window.onload = function() {
foo();
};
window.setInterval(function()
{
foo();
}, 5000);
This example builds on #Alnitak's answer, but uses await Promise for finer granularity of control within the loop cycle.
Compare examples:
let stillGoing = true;
(function foo() {
console.log('The quick brown fox did its thing');
if (stillGoing) setTimeout(foo, 5000);
})();
foo();
In the above example we call foo() and then it calls itself every 5 seconds.
But if, at some point in the future, we set stillGoing to false in order to stop the loop, we'll still get an extra log line even after we've issued the stop order. This is because at any given time, before we set stillGoing to false the current iteration will have already created a timeout to call the next iteration.
If we instead use await Promise as the delay mechanism then we have an opportunity to stop the loop before calling the next iteration:
let stillGoing = true;
(async function foo() {
console.log('The quick brown fox did its thing');
await new Promise(resolve => setTimeout(resolve, 5000));
if (stillGoing) foo();
})();
foo();
In the second example we start by setting a 5000ms delay, after which we check the stillGoing value and decide whether calling another recursion is appropriate.
So if we set stillGoing to false at any point, there won't be that one extra log line printed after we set the value.
The caveat is this requires the function to be async, which may or may not be an option for a given use.
For Those using React, here is how I solve this problem:
const intervalRef = useRef(0);
useEffect(() => {
if (condition is true){
if (intervalRef.current === 0) {
callMyFunction();
}
const interval = setInterval(() => {
callMyFunction();
}, 5_000);
intervalRef.current = interval;
} else {
clearInterval(intervalRef.current);
}
}, [deps]);
// YCombinator
function anonymous(fnc) {
return function() {
fnc.apply(fnc, arguments);
return fnc;
}
}
// Invoking the first time:
setInterval(anonymous(function() {
console.log("bar");
})(), 4000);
// Not invoking the first time:
setInterval(anonymous(function() {
console.log("foo");
}), 4000);
// Or simple:
setInterval(function() {
console.log("baz");
}, 4000);
Ok this is so complex, so, let me put it more simple:
function hello(status ) {
console.log('world', ++status.count);
return status;
}
setInterval(hello, 5 * 1000, hello({ count: 0 }));
If you can use RxJS, there is something called timer():
import { Subscription, timer } from 'rxjs';
const INITIAL_DELAY = 1;
const INTERVAL_DELAY = 10000;
const timerSubscription = timer(INITIAL_DELAY, INTERVAL_DELAY)
.subscribe(() => {
this.updateSomething();
});
// when destroying
timerSubscription.unsubscribe();
With ES2017, it may be preferable to avoid setInterval altogether.
The following solution has a much cleaner execution flow, prevents issues if the function takes longer than the desired time to complete, and allows for asynchronous operations.
const timeout = (delayMs) => new Promise((res, _rej) => setTimeout(res, delayMs));
const DELAY = 1_000;
(async () => {
while (true) {
let start_time = Date.now();
// insert code here...
let end_time = Date.now();
await timeout(DELAY - (end_time - start_time));
}
})();
There's a problem with immediate asynchronous call of your function, because standard setTimeout/setInterval has a minimal timeout about several milliseconds even if you directly set it to 0. It caused by a browser specific work.
An example of code with a REAL zero delay wich works in Chrome, Safari, Opera
function setZeroTimeout(callback) {
var channel = new MessageChannel();
channel.port1.onmessage = callback;
channel.port2.postMessage('');
}
You can find more information here
And after the first manual call you can create an interval with your function.
actually the quickest is to do
interval = setInterval(myFunction(),45000)
this will call myfunction, and then will do it agaian every 45 seconds which is different than doing
interval = setInterval(myfunction, 45000)
which won't call it, but schedule it only

Categories