Related
This question already has answers here:
JavaScript closure inside loops – simple practical example
(44 answers)
Closed 7 years ago.
I have this script:
for (var i = 1; i <= 2; i++) {
setTimeout(function() { alert(i) }, 100);
}
But 3 is alerted both times, instead of 1 then 2.
Is there a way to pass i, without writing the function as a string?
You have to arrange for a distinct copy of "i" to be present for each of the timeout functions.
function doSetTimeout(i) {
setTimeout(function() {
alert(i);
}, 100);
}
for (var i = 1; i <= 2; ++i)
doSetTimeout(i);
If you don't do something like this (and there are other variations on this same idea), then each of the timer handler functions will share the same variable "i". When the loop is finished, what's the value of "i"? It's 3! By using an intermediating function, a copy of the value of the variable is made. Since the timeout handler is created in the context of that copy, it has its own private "i" to use.
Edit:
There have been a couple of comments over time in which some confusion was evident over the fact that setting up a few timeouts causes the handlers to all fire at the same time. It's important to understand that the process of setting up the timer — the calls to setTimeout() — take almost no time at all. That is, telling the system, "Please call this function after 1000 milliseconds" will return almost immediately, as the process of installing the timeout request in the timer queue is very fast.
Thus, if a succession of timeout requests is made, as is the case in the code in the OP and in my answer, and the time delay value is the same for each one, then once that amount of time has elapsed all the timer handlers will be called one after another in rapid succession.
If what you need is for the handlers to be called at intervals, you can either use setInterval(), which is called exactly like setTimeout() but which will fire more than once after repeated delays of the requested amount, or instead you can establish the timeouts and multiply the time value by your iteration counter. That is, to modify my example code:
function doScaledTimeout(i) {
setTimeout(function() {
alert(I);
}, i * 5000);
}
(With a 100 millisecond timeout, the effect won't be very obvious, so I bumped the number up to 5000.) The value of i is multiplied by the base delay value, so calling that 5 times in a loop will result in delays of 5 seconds, 10 seconds, 15 seconds, 20 seconds, and 25 seconds.
Update
Here in 2018, there is a simpler alternative. With the new ability to declare variables in scopes more narrow than functions, the original code would work if so modified:
for (let i = 1; i <= 2; i++) {
setTimeout(function() {
alert(i)
}, 100);
}
The let declaration, unlike var, will itself cause there to be a distinct i for each iteration of the loop.
You can use an immediately-invoked function expression (IIFE) to create a closure around setTimeout:
for (var i = 1; i <= 3; i++) {
(function(index) {
setTimeout(function() { alert(index); }, i * 1000);
})(i);
}
This's Because!
The timeout function
callbacks are all running well after the completion of the loop. In fact,
as timers go, even if it was setTimeout(.., 0) on each iteration, all
those function callbacks would still run strictly after the completion
of the loop, that's why 3 was reflected!
all two of those functions, though they are defined
separately in each loop iteration, are closed over the same shared global
scope, which has, in fact, only one i in it.
the Solution's declaring a single scope for each iteration by using a self-function executed(anonymous one or better IIFE) and having a copy of i in it, like this:
for (var i = 1; i <= 2; i++) {
(function(){
var j = i;
setTimeout(function() { console.log(j) }, 100);
})();
}
the cleaner one would be
for (var i = 1; i <= 2; i++) {
(function(i){
setTimeout(function() { console.log(i) }, 100);
})(i);
}
The use of an IIFE(self-executed function) inside each iteration created a new scope for each
iteration, which gave our timeout function callbacks the opportunity
to close over a new scope for each iteration, one which had a variable
with the right per-iteration value in it for us to access.
The function argument to setTimeout is closing over the loop variable. The loop finishes before the first timeout and displays the current value of i, which is 3.
Because JavaScript variables only have function scope, the solution is to pass the loop variable to a function that sets the timeout. You can declare and call such a function like this:
for (var i = 1; i <= 2; i++) {
(function (x) {
setTimeout(function () { alert(x); }, 100);
})(i);
}
You can use the extra arguments to setTimeout to pass parameters to the callback function.
for (var i = 1; i <= 2; i++) {
setTimeout(function(j) { alert(j) }, 100, i);
}
Note: This doesn't work on IE9 and below browsers.
ANSWER?
I'm using it for an animation for adding items to a cart - a cart icon floats to the cart area from the product "add" button, when clicked:
function addCartItem(opts) {
for (var i=0; i<opts.qty; i++) {
setTimeout(function() {
console.log('ADDED ONE!');
}, 1000*i);
}
};
NOTE the duration is in unit times n epocs.
So starting at the the click moment, the animations start epoc (of EACH animation) is the product of each one-second-unit multiplied by the number of items.
epoc: https://en.wikipedia.org/wiki/Epoch_(reference_date)
Hope this helps!
You could use bind method
for (var i = 1, j = 1; i <= 3; i++, j++) {
setTimeout(function() {
alert(this);
}.bind(i), j * 100);
}
Well, another working solution based on Cody's answer but a little more general can be something like this:
function timedAlert(msg, timing){
setTimeout(function(){
alert(msg);
}, timing);
}
function yourFunction(time, counter){
for (var i = 1; i <= counter; i++) {
var msg = i, timing = i * time * 1000; //this is in seconds
timedAlert (msg, timing);
};
}
yourFunction(timeInSeconds, counter); // well here are the values of your choice.
I had the same problem once this is how I solved it.
Suppose I want 12 delays with an interval of 2 secs
function animate(i){
myVar=setTimeout(function(){
alert(i);
if(i==12){
clearTimeout(myVar);
return;
}
animate(i+1)
},2000)
}
var i=1; //i is the start point 1 to 12 that is
animate(i); //1,2,3,4..12 will be alerted with 2 sec delay
the real solution is here, but you need to be familiar with PHP programing language.
you must mix PHP and JAVASCRIPT orders in order to reach to your purpose.
pay attention to this :
<?php
for($i=1;$i<=3;$i++){
echo "<script language='javascript' >
setTimeout(function(){alert('".$i."');},3000);
</script>";
}
?>
It exactly does what you want, but be careful about how to make ralation between
PHP variables and JAVASCRIPT ones.
This question already has answers here:
JavaScript closure inside loops – simple practical example
(44 answers)
Closed 7 years ago.
I have this script:
for (var i = 1; i <= 2; i++) {
setTimeout(function() { alert(i) }, 100);
}
But 3 is alerted both times, instead of 1 then 2.
Is there a way to pass i, without writing the function as a string?
You have to arrange for a distinct copy of "i" to be present for each of the timeout functions.
function doSetTimeout(i) {
setTimeout(function() {
alert(i);
}, 100);
}
for (var i = 1; i <= 2; ++i)
doSetTimeout(i);
If you don't do something like this (and there are other variations on this same idea), then each of the timer handler functions will share the same variable "i". When the loop is finished, what's the value of "i"? It's 3! By using an intermediating function, a copy of the value of the variable is made. Since the timeout handler is created in the context of that copy, it has its own private "i" to use.
Edit:
There have been a couple of comments over time in which some confusion was evident over the fact that setting up a few timeouts causes the handlers to all fire at the same time. It's important to understand that the process of setting up the timer — the calls to setTimeout() — take almost no time at all. That is, telling the system, "Please call this function after 1000 milliseconds" will return almost immediately, as the process of installing the timeout request in the timer queue is very fast.
Thus, if a succession of timeout requests is made, as is the case in the code in the OP and in my answer, and the time delay value is the same for each one, then once that amount of time has elapsed all the timer handlers will be called one after another in rapid succession.
If what you need is for the handlers to be called at intervals, you can either use setInterval(), which is called exactly like setTimeout() but which will fire more than once after repeated delays of the requested amount, or instead you can establish the timeouts and multiply the time value by your iteration counter. That is, to modify my example code:
function doScaledTimeout(i) {
setTimeout(function() {
alert(I);
}, i * 5000);
}
(With a 100 millisecond timeout, the effect won't be very obvious, so I bumped the number up to 5000.) The value of i is multiplied by the base delay value, so calling that 5 times in a loop will result in delays of 5 seconds, 10 seconds, 15 seconds, 20 seconds, and 25 seconds.
Update
Here in 2018, there is a simpler alternative. With the new ability to declare variables in scopes more narrow than functions, the original code would work if so modified:
for (let i = 1; i <= 2; i++) {
setTimeout(function() {
alert(i)
}, 100);
}
The let declaration, unlike var, will itself cause there to be a distinct i for each iteration of the loop.
You can use an immediately-invoked function expression (IIFE) to create a closure around setTimeout:
for (var i = 1; i <= 3; i++) {
(function(index) {
setTimeout(function() { alert(index); }, i * 1000);
})(i);
}
This's Because!
The timeout function
callbacks are all running well after the completion of the loop. In fact,
as timers go, even if it was setTimeout(.., 0) on each iteration, all
those function callbacks would still run strictly after the completion
of the loop, that's why 3 was reflected!
all two of those functions, though they are defined
separately in each loop iteration, are closed over the same shared global
scope, which has, in fact, only one i in it.
the Solution's declaring a single scope for each iteration by using a self-function executed(anonymous one or better IIFE) and having a copy of i in it, like this:
for (var i = 1; i <= 2; i++) {
(function(){
var j = i;
setTimeout(function() { console.log(j) }, 100);
})();
}
the cleaner one would be
for (var i = 1; i <= 2; i++) {
(function(i){
setTimeout(function() { console.log(i) }, 100);
})(i);
}
The use of an IIFE(self-executed function) inside each iteration created a new scope for each
iteration, which gave our timeout function callbacks the opportunity
to close over a new scope for each iteration, one which had a variable
with the right per-iteration value in it for us to access.
The function argument to setTimeout is closing over the loop variable. The loop finishes before the first timeout and displays the current value of i, which is 3.
Because JavaScript variables only have function scope, the solution is to pass the loop variable to a function that sets the timeout. You can declare and call such a function like this:
for (var i = 1; i <= 2; i++) {
(function (x) {
setTimeout(function () { alert(x); }, 100);
})(i);
}
You can use the extra arguments to setTimeout to pass parameters to the callback function.
for (var i = 1; i <= 2; i++) {
setTimeout(function(j) { alert(j) }, 100, i);
}
Note: This doesn't work on IE9 and below browsers.
ANSWER?
I'm using it for an animation for adding items to a cart - a cart icon floats to the cart area from the product "add" button, when clicked:
function addCartItem(opts) {
for (var i=0; i<opts.qty; i++) {
setTimeout(function() {
console.log('ADDED ONE!');
}, 1000*i);
}
};
NOTE the duration is in unit times n epocs.
So starting at the the click moment, the animations start epoc (of EACH animation) is the product of each one-second-unit multiplied by the number of items.
epoc: https://en.wikipedia.org/wiki/Epoch_(reference_date)
Hope this helps!
You could use bind method
for (var i = 1, j = 1; i <= 3; i++, j++) {
setTimeout(function() {
alert(this);
}.bind(i), j * 100);
}
Well, another working solution based on Cody's answer but a little more general can be something like this:
function timedAlert(msg, timing){
setTimeout(function(){
alert(msg);
}, timing);
}
function yourFunction(time, counter){
for (var i = 1; i <= counter; i++) {
var msg = i, timing = i * time * 1000; //this is in seconds
timedAlert (msg, timing);
};
}
yourFunction(timeInSeconds, counter); // well here are the values of your choice.
I had the same problem once this is how I solved it.
Suppose I want 12 delays with an interval of 2 secs
function animate(i){
myVar=setTimeout(function(){
alert(i);
if(i==12){
clearTimeout(myVar);
return;
}
animate(i+1)
},2000)
}
var i=1; //i is the start point 1 to 12 that is
animate(i); //1,2,3,4..12 will be alerted with 2 sec delay
the real solution is here, but you need to be familiar with PHP programing language.
you must mix PHP and JAVASCRIPT orders in order to reach to your purpose.
pay attention to this :
<?php
for($i=1;$i<=3;$i++){
echo "<script language='javascript' >
setTimeout(function(){alert('".$i."');},3000);
</script>";
}
?>
It exactly does what you want, but be careful about how to make ralation between
PHP variables and JAVASCRIPT ones.
var funcs=[];
for(var i=0;i<3;i++){
funcs[i]=function(){
console.log(i);
};
}
for(var j=0;j<3;j++){
funcs[j]();
}
In this way ,I know it will alert 3 all.Because the functions were called after i was assigned to 3.
But in the below code, I can't understand why this happens.
for(var i = 0; i < 10; i++) {
setTimeout(function() {
console.log(i);
}, 2000);
}
In my oppinion, when i was assigned to 0, the first function setTimeout would be executed before i was assigned to 1.
Am I wrong in the order of this loop?
console.log(i) will be called after the for loop finished, the for loop which because of the way var is function scoped. Will be 10 by the time (2000ms) it reaches the console.log statement. One way to give the expected result is using let instead of var. However you are removing some browser support with using let.
Good reading on this topic would be getify's "You Don't Know Js" book.
for(let i = 0; i < 10; i++) {
setTimeout(function() {
console.log(i);
}, 2000);
}
You need to put the i variable into a closure, otherwise, it will be rewritten in each loop iteration. The same occurs for the setTimeout function, you should put the i variable into a closure as well.
var funcs=[];
for(var i=0;i<3;i++){
(function(i){
funcs[i]=function(){
console.log(i);
};
})(i);
}
for(var j=0;j<3;j++){
funcs[j]();
}
I've found some example in a tutorial (said it was the canonical example)
for (var i=1; i<=5 ; i++) {
setTimeout(function() {
console.log("i: " + i);
}, i*1000);
}
Now, I understand that, closure passes in the current scope in to the function, and I assume that it should output 1,2,3,4,5. But instead, it prints number 6 five times.
I ran it in the chrome debugger, and first it goes through the loop without going in to the function while doing the increment of the i value and only after that, it goes in to the inner function and execute it 5 times.
I'm not sure why its doing that, I know, the current scoped is passed in to the function because of closure, but why does it not execute each time the loop iterate?
By the time the timeout runs, the for loop has finished, and i is 6, that's why you're getting the output you see. You need to capture i during the loop:
for (var i=1; i<=5 ; i++) {
(function(innerI) {
setTimeout(function() {
console.log("i: " + innerI);
}, innerI*1000);
})(i);
}
This creates an inner function with it's own parameter (innerI), that gets invoked immediately and so captures the value of i for use within the timeout.
If you didn't want the complex-looking IIFE as explained in James' answer, you can also separate out the function using bind:
function count(i) {
console.log("i: " + i);
}
for (var i = 1; i <= 5; i++) {
setTimeout(count.bind(this, i), i * 1000);
}
Thank you for you help,
I found out another solution and it was a minor change.
On the top of the page I turned on the strict mode and also in the for loop, Instead of var, I used the "let" keyword.
I'm trying to set five staggered function calls (happening one second apart). That part works fine. What doesn't work is, I can't pass values 0 through 4 into the callback function. It just passes '5' each time. I can't seem to figure out why and how to fix it.
Code:
function callback(num)
{
console.log(num);
}
for (var i = 0, loadDelay = 1000; i < 5; ++ i, loadDelay += 1000)
setTimeout(function() { callback(i); }, loadDelay);
Result:
5
5
5
5
5
Desired result:
0
1
2
3
4
That's because you create a closure. So the function you pass to setTimeout share the same i instances. In the browser that supports the standards (not IE) you could have:
setTimeout(callback, loadDelay, i);
See:
http://www.whatwg.org/specs/web-apps/current-work/multipage/timers.html#timers
Otherwise you have to actually bind the argument to the function:
setTimeout(callback.bind(undefined, i), loadDelay);
See:
https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Function/bind
If the browser doesn't support ES5 bind method, you can either implement the shim present in the link above, or manually doing something like:
setTimeout(function(index){
return function() { callback(index) }
}(i), loadDelay);
But I would say it's more readable using bind and it's worthy to implement the shim. You can actually use this: https://github.com/kriskowal/es5-shim
To add es5 capabilities (where is possible) in the browser that don't support es5 natively.
Use a lambda / function expression to capture the current value. For example
for (var i = 0, loadDelay = 1000; i < 5; ++ i, loadDelay += 1000) {
var doCall = function (j) {
setTimeout(function() { callback(j); }, loadDelay);
}
doCall(i);
}
The problem here is that there is only 1 i value for all iterations of the loop. Variables in javascript have function scope even though you can declare them inside of a block. This means i is alive for the entire function.
To illustrate the problem consider the below code executes exactly the same as your sample
var i;
for (i = 0, loadDelay = 1000; i < 5; ++ i, loadDelay += 1000) {
...
}
My solution works because it introduces a new function and hence a new variable lifetime for j. This saves the current value of i in the function for use in the setTimeout callback
You needed a closure in order to pass i due to variable scoping. Check out this article, and this one as well for some good information on closures.
Live Demo
function callback(num)
{
console.log(num);
}
for (var i = 0, loadDelay = 1000; i < 5; ++ i, loadDelay += 1000)
setTimeout((function(num){return function(){
callback(num);
}
})(i), loadDelay);
setTimeout creates some odd scoping problems. Frame.js was designed to resolve some of this kind of confusion, this also works [updated]:
function callback(num) {
console.log(num);
}
for (var i = 0, loadDelay = 1000; i < 5; ++ i, loadDelay += 1000) {
Frame(function(next, i){
setTimeout(function() { callback(i); }, loadDelay);
next();
}, i);
}
Frame.init();