I'm facing a question here that I need to run, for example, 2 function per time, wait just 2 minutes and start run these two function again, just like a loop with a pause between the batches.
I've tried something here, and I know that my issue is, every time I start the myFunction with start function as callback I get a duplicated call. But right now, I really don't know what is the best approach to follow.
var count = 0;
function myFunction() {
console.log('function called', count);
start();
}
function batch () {
for (i = 0; i < 2; i++) {
myFunction();
console.log('batch', i);
}
console.log('--------------');
}
function start() {
console.log('started')
setTimeout(function() {
batch();
}, 2000);
count++;
}
start();
Thank you for suggestions ;)
From what I understand, you want to execute two functions every two minutes.
Have you tried using window.setInterval?
const func1 = () => console.log('func1 called');
const func2 = () => console.log('func2 called');
window.setInterval(() => {
func1();
func2();
}, 1000);
Now, func1 and func2 will be called every second.
window.onbeforeunload = function() {
if (document.getElementById("parentpan").style.display == "block") {
return "You are logged out.";
Logout();
}
};
I want the logout() function to be called after the return statement, is it possible?
You can't execute anything after a return statement.
edit: the finally statement allows code execution after a return for cleanup purposes.
(This is a good example for an XY-Question: You are asking about Y while never telling us for what X you actually need it).
The best possible way and most efficient way is try, catch and finally
catch is optional in this
`try{
// do something
return;
} finally {
// call function after return
}`
https://youtu.be/Is_o_L-ZIS8 this is helpful for you
The return statement ends a function, you cannot execute code after it. You could do this:
ret = "You are logged out.";
Logout();
return ret;
What you need is to execute Logout asynchronously. This can be easily achieve in JavaScript by using the setTimeout function as others have said. Here's a method I commonly use to call functions asynchronously:
Function.prototype.async = function () {
setTimeout.bind(null, this, 0).apply(null, arguments);
};
This method pushes a function call onto the event loop immediately (after 0 ms). Hence the function is executed after the current code completes (which for you is after you return). Here's a simple example of how to use it:
alert.async("This will be displayed later.");
alert("This will be displayed first.");
Since the first alert is called asynchronously it will execute after the second call to alert. As simple as preceding your function call with async. This is what you would do in your case:
window.onbeforeunload = function () {
if (document.getElementById("parentpan").style.display === "block") {
Logout.async();
return "You are logged out.";
}
};
What's the disadvantage? Since the function is blocked on the event loop it may never get the chance to execute (hence the user will never logout). Such a situation may arise. It usually occurs when the control goes into an infinite loop, or hangs because of a blocking AJAX request.
Good news for you however, this happens on a very rare occasion. So don't worry about it. Just use setTimeout like everyone else is bantering you to and you'll do just fine. Personally I think you should log out before returning a message that "You are logged out.", but it's your application.
Happy New Year. Cheers!
In general if you want something to be executed after the function has returned, you can set a timer:
function myFunction() {
if (document.getElementById("parentpan").style.display == "block") {
setTimeout(Logout, 50); // Logout will be called 50ms later
return "You are logged out.";
}
};
However, as noted in comments, this is not a good idea for onbeforeunload, as the timer event will not be fired if the page finished unloading first.
Most of the other answerers are missing what you are trying to do here. You want window.onbeforeunload to act like window.confirm(). There is no way to act on the ok action in the onbeforeunload event.
What you would have to do is hook it up on onunload to do the action.
window.onbeforeunload = function () {
return "Your session will be logged out"
};
window.onunload = function () {
logout();
}
Problem with this is modern day browsers will kill a lot of processes that run in unload/beforeunload to "speed up" the browser so it is faster. So if it is asynchronous, you will have a race condition.
return means you are returning from the execution of the called function.When return statement is executed, system understands that the function execution is over and it will switch to the main program from which the function is called.
In the program, you can see a statement after return.But the system wont check that even.
If you have jquery in your project you can use defered mechanism. You can return promise object for ongoing tasks like this :
function task() {
var defered = $.Deferred();
setTimeout(defered.resolve , 5000);
return defered.promise();
}
function task2() {
var defered = $.Deferred();
setTimeout(defered.resolve , 10000);
return defered.promise();
}
function run() {
return $.when(task(),task2());
}
var promise = run();
promise.done(function(){
alert("All tasks has been completed");
});
Demo
You can use setTimeout to achieve this. Your code should be as below
window.onbeforeunload = function() {
if (document.getElementById("parentpan").style.display == "block") {
setTimeout(function(){
Logout();
}, 0);
return "You are logged out.";
}
};
This will make sure that Logout is executed after return statement.
var a = 10;
function b(){
a = 25;
return;
function a(){}
}
b();
document.write(a);
try it
I found two ways to approach this.
The first one is as stated above by Bhavsar Japan
1. Example with try, catch and finally
const example1 = () => {
try {
return console.log('will execute first')
} finally{
console.log('will execute second')
}
return 'will never execute'
}
const main = () => {
const message = example1();
console.log(message)
}
main()
2. Example with Promise.resolve
const example2 = () => {
Promise.resolve()
.then(() => console.log('will execute after return'));
return 'will execute first'
}
const main = () => {
const message = example2();
console.log(message);
}
main();
I just written a way to return a result and then call a callback, like this:
function after_return(result, callback) {
function returner(resolve) {
if (!resolve) {
new Promise((res) => returner(res)).then(callback);
return result;
} else {
resolve();
}
}
return returner(undefined);
}
function main(a, b) {
return after_return(a + b, (_) => {
console.log("DONE");
});
}
console.log(main(5, 4));
I'm guessing that Logout is a time-intensive process and you want to provide feedback to the user before executing it:
setTimeout(Logout,1);
return "You are logged out.";
I need to create a function which can be executed only once, in each time after the first it won't be executed. I know from C++ and Java about static variables that can do the work but I would like to know if there is a more elegant way to do this?
If by "won't be executed" you mean "will do nothing when called more than once", you can create a closure:
var something = (function() {
var executed = false;
return function() {
if (!executed) {
executed = true;
// do something
}
};
})();
something(); // "do something" happens
something(); // nothing happens
In answer to a comment by #Vladloffe (now deleted): With a global variable, other code could reset the value of the "executed" flag (whatever name you pick for it). With a closure, other code has no way to do that, either accidentally or deliberately.
As other answers here point out, several libraries (such as Underscore and Ramda) have a little utility function (typically named once()[*]) that accepts a function as an argument and returns another function that calls the supplied function exactly once, regardless of how many times the returned function is called. The returned function also caches the value first returned by the supplied function and returns that on subsequent calls.
However, if you aren't using such a third-party library, but still want a utility function (rather than the nonce solution I offered above), it's easy enough to implement. The nicest version I've seen is this one posted by David Walsh:
function once(fn, context) {
var result;
return function() {
if (fn) {
result = fn.apply(context || this, arguments);
fn = null;
}
return result;
};
}
I would be inclined to change fn = null; to fn = context = null;. There's no reason for the closure to maintain a reference to context once fn has been called.
Usage:
function something() { /* do something */ }
var one_something = once(something);
one_something(); // "do something" happens
one_something(); // nothing happens
[*] Be aware, though, that other libraries, such as this Drupal extension to jQuery, may have a function named once() that does something quite different.
Replace it with a reusable NOOP (no operation) function.
// this function does nothing
function noop() {};
function foo() {
foo = noop; // swap the functions
// do your thing
}
function bar() {
bar = noop; // swap the functions
// do your thing
}
Point to an empty function once it has been called:
function myFunc(){
myFunc = function(){}; // kill it as soon as it was called
console.log('call once and never again!'); // your stuff here
};
<button onClick=myFunc()>Call myFunc()</button>
Or, like so:
var myFunc = function func(){
if( myFunc.fired ) return;
myFunc.fired = true;
console.log('called once and never again!'); // your stuff here
};
// even if referenced & "renamed"
((refToMyfunc)=>{
setInterval(refToMyfunc, 1000);
})(myFunc)
UnderscoreJs has a function that does that, underscorejs.org/#once
// Returns a function that will be executed at most one time, no matter how
// often you call it. Useful for lazy initialization.
_.once = function(func) {
var ran = false, memo;
return function() {
if (ran) return memo;
ran = true;
memo = func.apply(this, arguments);
func = null;
return memo;
};
};
Talking about static variables, this is a little bit like closure variant:
var once = function() {
if(once.done) return;
console.log('Doing this once!');
once.done = true;
};
once(); // Logs "Doing this once!"
once(); // Logs nothing
You could then reset a function if you wish:
once.done = false;
once(); // Logs "Doing this once!" again
You could simply have the function "remove itself"
function Once(){
console.log("run");
Once = undefined;
}
Once(); // run
Once(); // Uncaught TypeError: undefined is not a function
But this may not be the best answer if you don't want to be swallowing errors.
You could also do this:
function Once(){
console.log("run");
Once = function(){};
}
Once(); // run
Once(); // nothing happens
I need it to work like smart pointer, if there no elements from type A it can be executed, if there is one or more A elements the function can't be executed.
function Conditional(){
if (!<no elements from type A>) return;
// do stuff
}
var quit = false;
function something() {
if(quit) {
return;
}
quit = true;
... other code....
}
simple decorator that easy to write when you need
function one(func) {
return function () {
func && func.apply(this, arguments);
func = null;
}
}
using:
var initializer= one( _ =>{
console.log('initializing')
})
initializer() // 'initializing'
initializer() // nop
initializer() // nop
try this
var fun = (function() {
var called = false;
return function() {
if (!called) {
console.log("I called");
called = true;
}
}
})()
From some dude named Crockford... :)
function once(func) {
return function () {
var f = func;
func = null;
return f.apply(
this,
arguments
);
};
}
Reusable invalidate function which works with setInterval:
var myFunc = function (){
if (invalidate(arguments)) return;
console.log('called once and never again!'); // your stuff here
};
const invalidate = function(a) {
var fired = a.callee.fired;
a.callee.fired = true;
return fired;
}
setInterval(myFunc, 1000);
Try it on JSBin: https://jsbin.com/vicipar/edit?js,console
Variation of answer from Bunyk
Here is an example JSFiddle - http://jsfiddle.net/6yL6t/
And the code:
function hashCode(str) {
var hash = 0, i, chr, len;
if (str.length == 0) return hash;
for (i = 0, len = str.length; i < len; i++) {
chr = str.charCodeAt(i);
hash = ((hash << 5) - hash) + chr;
hash |= 0; // Convert to 32bit integer
}
return hash;
}
var onceHashes = {};
function once(func) {
var unique = hashCode(func.toString().match(/function[^{]+\{([\s\S]*)\}$/)[1]);
if (!onceHashes[unique]) {
onceHashes[unique] = true;
func();
}
}
You could do:
for (var i=0; i<10; i++) {
once(function() {
alert(i);
});
}
And it will run only once :)
Initial setup:
var once = function( once_fn ) {
var ret, is_called;
// return new function which is our control function
// to make sure once_fn is only called once:
return function(arg1, arg2, arg3) {
if ( is_called ) return ret;
is_called = true;
// return the result from once_fn and store to so we can return it multiply times:
// you might wanna look at Function.prototype.apply:
ret = once_fn(arg1, arg2, arg3);
return ret;
};
}
If your using Node.js or writing JavaScript with browserify, consider the "once" npm module:
var once = require('once')
function load (file, cb) {
cb = once(cb)
loader.load('file')
loader.once('load', cb)
loader.once('error', cb)
}
If you want to be able to reuse the function in the future then this works well based on ed Hopp's code above (I realize that the original question didn't call for this extra feature!):
var something = (function() {
var executed = false;
return function(value) {
// if an argument is not present then
if(arguments.length == 0) {
if (!executed) {
executed = true;
//Do stuff here only once unless reset
console.log("Hello World!");
}
else return;
} else {
// otherwise allow the function to fire again
executed = value;
return;
}
}
})();
something();//Hello World!
something();
something();
console.log("Reset"); //Reset
something(false);
something();//Hello World!
something();
something();
The output look like:
Hello World!
Reset
Hello World!
A simple example for turning on light only once.
function turnOnLightOnce() {
let lightOn = false;
return function () {
if (!lightOn) {
console.log("Light is not on...Turning it on for first and last time");
lightOn = true;
}
};
}
const lightOn = turnOnLightOnce();
lightOn() // Light is not on...Turning it on for first and last time
lightOn()
lightOn()
lightOn()
lightOn()
https://codesandbox.io/s/javascript-forked-ojo0i?file=/index.js
This happens due to closure in JavaScript.
function once (fn1) {
var ran = false
var memo = null
var fn = function(...args) {
if(ran) {return memo}
ran = true
memo = fn1.apply(null, args)
return memo
}
return fn
}
I'm using typescript with node and it was #I Hate Lazy's answer that inspired me. I just assigned my function to a noop function.
let printName = (name: string) => {
console.log(name)
printName = () => {}
}
printName('Sophia') // Sophia
printName('Nico') // Nothing Happens
https://jsbin.com/yuzicek/edit?js,console
FOR EVENT HANDLER
If the function is a callback for an event listener, there is already a built-in option in the addEventListner method for just executing the callback once.
It can accept 3 parameters
Type
callback
options
options is an object that has a property called once
ex:
const button = document.getElementById('button');
const callbackFunc = () => {
alert('run')
}
button.addEventListener('click', callbackFunc, { once: true })
<button id="button">Click Once</button>
Trying to use underscore "once" function:
var initialize = _.once(createApplication);
initialize();
initialize();
// Application is only created once.
http://underscorejs.org/#once
var init = function() {
console.log("logges only once");
init = false;
};
if(init) { init(); }
/* next time executing init() will cause error because now init is
-equal to false, thus typing init will return false; */
if (!window.doesThisOnce){
function myFunction() {
// do something
window.doesThisOnce = true;
};
};
If you're using Ramda, you can use the function "once".
A quote from the documentation:
once Function
(a… → b) → (a… → b)
PARAMETERS
Added in v0.1.0
Accepts a function fn and returns a function that guards invocation of fn such that fn can only ever be called once, no matter how many times the returned function is invoked. The first value calculated is returned in subsequent invocations.
var addOneOnce = R.once(x => x + 1);
addOneOnce(10); //=> 11
addOneOnce(addOneOnce(50)); //=> 11
keep it as simple as possible
function sree(){
console.log('hey');
window.sree = _=>{};
}
You can see the result
JQuery allows to call the function only once using the method one():
let func = function() {
console.log('Calling just once!');
}
let elem = $('#example');
elem.one('click', func);
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div>
<p>Function that can be called only once</p>
<button id="example" >JQuery one()</button>
</div>
Implementation using JQuery method on():
let func = function(e) {
console.log('Calling just once!');
$(e.target).off(e.type, func)
}
let elem = $('#example');
elem.on('click', func);
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div>
<p>Function that can be called only once</p>
<button id="example" >JQuery on()</button>
</div>
Implementation using native JS:
let func = function(e) {
console.log('Calling just once!');
e.target.removeEventListener(e.type, func);
}
let elem = document.getElementById('example');
elem.addEventListener('click', func);
<div>
<p>Functions that can be called only once</p>
<button id="example" >ECMAScript addEventListener</button>
</div>
Tossing my hat in the ring for fun, added advantage of memoizing
const callOnce = (fn, i=0, memo) => () => i++ ? memo : (memo = fn());
// usage
const myExpensiveFunction = () => { return console.log('joe'),5; }
const memoed = callOnce(myExpensiveFunction);
memoed(); //logs "joe", returns 5
memoed(); // returns 5
memoed(); // returns 5
...
You can use IIFE. IIFE means Immediately Invoked Function Expression and the result is to call a function only once by the time is created.
Your code will be like this:
(function () {
//The code you want to execute only one time etc...
console.log("Hello world");
})()
Additionally, this way the data in the function remains encapsulated.
Of course and you can return values from the function and stored them into a new variable, by doing:
const/let value = (function () {
//The code you want to execute only one time etc...
const x = 10;
return x;
})()
function x()
{
let a=0;
return function check()
{
if(!a++)
{
console.log("This Function will execute Once.")
return;
}
console.log("You Can't Execute it For the Second Time.")
return;
}
}
z=x()
z() //Op - This Function will execute once
z() //OP - You can't Execute it for the second time.
I find it useful to just have a simple function that just returns true once, so you can keep the side effects higher up.
let once = () => !! (once = () => false);
once() // true
once() // false
Use like this:
if (once()) {
sideEffect()
}
This exploits the fact that you can coerce an assignment expression to return true while changing the same function into a function that returns false.
If you must have it execute a function, it can be adapted using a ternary:
let once = (x) => !! (once = () => false) ? x() : false;
Now it accepts a single function as an argument. Fun fact, the second false is never reached.
// This is how function in JavaScript can be called only once
let started = false;
if (!started) {
start() { // "do something" }
}
started = true;
}