I have a very simple test page that tests jquery (1.4.2) queue and delay.
for (var i = 1; i <= 5; i++) {
$('#test')
//.delay(50)
.queue(function(next) {
console.log(i);
next();
});
}
Now when I run this code in FF with firebug, I get what I expected, 1 ~ 5.
However, if I un-comment delay, I got 6 five times instead?
Can someone please help me clarify this?
The i is a single variable stored one time and shared by all iterations of the loop. Without the .delay() you're using the value of i right then, so it's what you expect. With the .delay() however, you're using what the value is later...and later it's what it ended up as at the end of the loop, 6.
#Nick provides an excellent explanation for why it behaves like this.
For completeness, you can "fix" this by capturing the current value of i in a new scope. JavaScript has only function scope, so you have to use a function to capture the value. E.g. you can use an immediate function:
for (var i = 1; i <= 5; i++) {
$('#test')
.delay(50)
.queue((function(index) {
return function(next) {
console.log(index);
next();
}
}(i))); // <- function gets called directly with `i` and the returned
// function is passed to queue.
}
DEMO
Well for the following code in firebug console on stackoverflow
for (var i = 1; i <= 5; i++) {
$('#custom-header')
//.delay(50)
.queue(function(next) {
console.log(i);
next();
});
}
On console, you get result
1
2
3
4
5
[div#custom-header]
And for code
for (var i = 1; i <= 5; i++) {
$('#custom-header')
.delay(50)
.queue(function(next) {
console.log(i);
next();
});
}
You get result
[div#custom-header]
6
6
6
6
6
[Explanation]:
From this we can conclude that dealay(50) delays the the evaluation of the function inside queue so [div#custom-header] is printed first and i is printed which is all 6 (by the moment) because the loop (which is not delayed) is evaluated first (only the printing function inside queue is delayed).
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.
I wanna use Chrome or Firefox Developer Tools to execute code on a website.
When I execute a "window.RR" variable lonely, it shows me the right value (it is a measure of the site's server time (in milliseconds) and every time I execute it gives me a different value). When I use it in a loop (variable c), then the variable is constant in all loops and it equals to the value of the first loop.
My code is:
var i;
b=window.RR;
for (i=0 ; i<400000 ; i++) {
c=window.RR;
if (c!==b) {
alert(c)
}
}
I expect to receive the correct value when I use it in the loop, how can I achieve this?
I assume the the window.RR variable is updated by an interval. This means that the variable is updated asynchronous.
For example, lets say that the window.RR holds the timestamp (in milliseconds) of the server, and the code below updates the window.RR every millisecond.
setInterval(function () {
window.RR += 1;
}, 1);
If you then run a loop, that may take more than 1ms to be executed
var c = window.RR;
for (var i = 0; i < 999999999999999; i++){
if (c != window.RR) { // This is always false
console.log('This will never be printed');
}
}
the window.RR will not change during the execution of the loop, because the javascript asynchronous codes does not run in parallel.
Thus, if the loop takes more than 1 ms to be executed window.RR update code will happen after the loop finish (it will have to wait any other code fired to finish before that code can be executed).
More info:
This code does not let any other code to run while the for loop is running.
var c = window.RR;
for (var i = 0; i < 999999999999999; i++){
if (c != window.RR) { // This is always false
console.log('This will never be printed');
}
}
In order to let other code to run, you will have to make each loop async.
var c = window.RR;
var loop = function(i) {
if (i < 999999999999999) {
i++;
if (c != window.RR) {
console.log('This will be printed!');
}
else {
// Allow other javascript codes to run
// So that the window.RR can be updated
setTimeout(function () {loop(i);}, 0);
}
}
};
loop(0);
Background (You might want to skip this)
I'm working on a web app that animates the articulation of English phonemes, while playing the sound. It's based on the Interactive Sagittal Section by Daniel Currie Hall, and a first attempt can be found here.
For the next version, I want each phoneme to have it's own animation timings, which are defined in an array, which in turn, is included in an object variable.
For the sake of simplicity for this post, I have moved the timing array variable from the object into the function.
Problem
I set up a for loop that I thought would reference the index i and array t to set the milliseconds for each setTimeout.
function animateSam() {
var t = [0, 1000, 2000, 3000, 4000];
var key = "key_0";
for (var i = 0; i < t.length; i++) {
setTimeout(function() {
console.log(i);
key = "key_" + i.toString();
console.log(key);
//do stuff here
}, t[i]);
}
}
animateSam()
However, it seems the milliseconds are set by whatever i happens to be when the function gets to the top of the stack.
Question: Is there a reliable way to set the milliseconds from the array?
The for ends before the setTimeout function has finished, so you have to set the timeout inside a closure:
function animateSam(phoneme) {
var t = [0,1000,2000,3000,4000];
for (var i = 0; i < t.length; i++) {
(function(index) {
setTimeout(function() {
alert (index);
key = "key_" + index.toString();
alert (key);
//do stuff here
}, t[index]);
})(i);
}
}
Here you have the explanation of why is this happening:
https://hackernoon.com/how-to-use-javascript-closures-with-confidence-85cd1f841a6b
The for loop will loop all elements before the first setTimeout is triggered because of its asynchronous nature. By the time your loop runs, i will be equal to 5. Therefore, you get the same output five times.
You could use a method from the Array class, for example .forEach:
This ensures that the function is enclosed.
[0, 1000, 2000, 3000, 4000].forEach((t, i) => {
setTimeout(function() {
console.log(i);
console.log(`key_${i}`);
//do stuff here
}, t)
});
Side note: I would advise you not to use alert while working/debugging as it is honestly quite confusing and annoying to work with. Best is to use a simple console.log.
Some more clarifications on the code:
.forEach takes in as primary argument the callback function to run on each of element. This callback can itself take two arguments (in our previous code t was the current element's value and i the current element's index in the array):
Array.forEach(function(value, index) {
});
But you can use the arrow function syntax, instead of defining the callback with function(e,i) { ... } you define it with: (e,i) => { ... }. That's all! Then the code will look like:
Array.forEach((value,index) => {
});
This syntax is a shorter way of defining your callback. There are some differences though.
I would suggest using a function closure as follows:
function animateSam(phoneme) {
var t = [0,1000,2000,3000,4000];
var handleAnimation = function (idx) {
return function() {
alert(idx);
key = "key_" + idx.toString();
alert(key);
//do stuff here
};
}
for (var i = 0; i < t.length; i++) {
setTimeout(handleAnimation(i), t[i]);
}
}
I this example you wrap the actual function in a wrapper function which captures the variable and passes on the value.
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.