Develop Reference

Javascript Closures and self-executing anonymous functions

I was asked the below question during an interview, and I still couldn't get my head around it, so I'd like to seek your advice.
Here's the question:
var countFunctions = [];
for(var i = 0; i < 3; i++){
countFunctions[i] = function() {
document.getElementById('someId').innerHTML = 'count' + i;
};
}
//The below are executed in turns:
countFunctions[0]();
countFunctions[1]();
countFunctions[2]();
When asked what would be the output of the above, I said count0,count1 and count2 respectively. Apparently the answer was wrong, and that the output should all be count3, because of the concept of closures (which I wasn't aware of then). So I went through this article and realized that I should be using closure to make this work, like:
var countFunctions = [];
function setInner(i) {
return function(){
document.getElementById('someId').innerHTML = 'count' + i;
};
}
for(var i = 0; i < 3; i++){
countFunctions[i] = setInner(i);
}
//Now the output is what was intended:
countFunctions[0]();//count0
countFunctions[1]();//count1
countFunctions[2]();//count2
Now that's all well and good, but I remember the interviewer using something simpler, using a self-executing function like this:
var countFunctions = [];
for(var i = 0; i < 3; i++) {
countFunctions[i] = (function(){
document.getElementById('someId').innerHTML = 'count' + i;
})(i);
}
The way I understand the above code, we are skipping the declaration of a separate function and simply calling and executing the function within the for loop.
But when I ran the below:
countFunctions[0];
countFunctions[1];
countFunctions[2];
It didn't work, with all the output being stuck at count2.
So I tried to do the below instead:
for(var i = 0; i < 3; i++) {
countFunctions[i] = function(){
document.getElementById('someId').innerHTML = 'count' + i;
};
}
, and then running countFunctions[0](), countFunctions[1]() and countFunctions[2](), but it didn't work. The output is now being stuck at count3.
Now I really don't get it. I was simply using the same line of code as setInner(). So I don't see why this doesn't work. As a matter of fact, I could have just stick to the setInner kind of code structure, which does work, and is more comprehensive. But then I'd really like to know how the interviewer did it, so as to understand this topic a little better.

The relevant articles to read here are JavaScript closure inside loops – simple practical example and http://benalman.com/news/2010/11/immediately-invoked-function-expression/ (though you seem to have understood IEFEs quite well - as you say, they're "skipping the declaration of a separate function and simply calling and executing the function").
What you didn't notice is that setInner does, when called, return the closure function:
function setInner(i) {
return function() {
document.getElementById('someId').innerHTML = 'count' + i;
};
}
// then do
var countFunction = setInner("N"); // get the function
countFunction(); // call it to assign the innerHTML
So if you translate it into an IEFE, you still need to create (and return) the function that will actually get assigned to countFunctions[i]:
var countFunctions = [];
for(var i = 0; i < 3; i++) {
countFunctions[i] = (function(i){
return function() {
document.getElementById('someId').innerHTML = 'count' + i;
};
})(i);
}
Now, typeof countFunctions[0] will be "function", not "undefined" as in your code, and you can actually call them.

Take a look at these four functions:
var argument = 'G'; //global
function passArgument(argument){
alert(argument); //local
}
function noArguments(){
alert(argument); //global
}
function createClosure_1(argument){
return function (){
alert(argument); //local
};
}
function createClosure_2(argument){
var argument = argument; //local
return function (){
alert(argument); //local
};
}
passArgument('L'); //L
noArguments(); //G
createClosure_1('L') //L
createClosure_2('L') //L
alert(argument) //G
I think, first function is obvious.
In function noArguments you reference the global argument value;
The third and fourth functions do the same thing. They create a local argument variable that doesn't change inside them and return a function that references that local variable.
So, what was in the first and the last code snippet of your question is a creation of many functions like noArguments,
that reference global variable i.
In the second snippet your setInner works like createClosure_1. Within your loop you create three closures, three local variables inside them. And when you call functions inside countFunctions, they get the value of the local variable that was created inside the closure when they were created.
In the third one you assign the result of the execution of those functions to array elements, which is undefined because they don't return anything from that functions.

Javascript multiple dynamic addEventListener created in for loop - passing parameters not working

I want to use event listeners to prevent event bubbling on a div inside a div with onclick functions. This works, passing parameters how I intended:
<div onclick="doMouseClick(0, 'Dog', 'Cat');" id="button_id_0"></div>
<div onclick="doMouseClick(1, 'Dog', 'Cat');" id="button_id_1"></div>
<div onclick="doMouseClick(2, 'Dog', 'Cat');" id="button_id_2"></div>
<script>
function doMouseClick(peram1, peram2, peram3){
alert("doMouseClick() called AND peram1 = "+peram1+" AND peram2 = "+peram2+" AND peram3 = "+peram3);
}
</script>
However, I tried to create multiple event listeners in a loop with this:
<div id="button_id_0"></div>
<div id="button_id_1"></div>
<div id="button_id_2"></div>
<script>
function doMouseClick(peram1, peram2, peram3){
alert("doMouseClick() called AND peram1 = "+peram1+" AND peram2 = "+peram2+" AND peram3 = "+peram3);
}
var names = ['button_id_0', 'button_id_1', 'button_id_2'];
for (var i=0; i<names.length; i++){
document.getElementById(names[i]).addEventListener("click", function(){
doMouseClick(i, "Dog", "Cat");
},false);
}
</script>
It correctly assigns the click function to each div, but the first parameter for each, peram1, is 3. I was expecting 3 different event handlers all passing different values of i for peram1.
Why is this happening? Are the event handlers not all separate?

Problem is closures, since JS doesn't have block scope (only function scope) i is not what you think because the event function creates another scope so by the time you use i it's already the latest value from the for loop. You need to keep the value of i.
Using an IIFE:
for (var i=0; i<names.length; i++) {
(function(i) {
// use i here
}(i));
}
Using forEach:
names.forEach(function( v,i ) {
// i can be used anywhere in this scope
});

2022 edit
As someone is still reading and upvoting this answer 9 years later, here is the modern way of doing it:
for (const [i, name] of names.entries()) {
document.getElementById(name).addEventListener("click", () => doMouseClick(i, "Dog", "Cat"), false);
}
Using const or let to define the variables gives them block-level scope and the value of i passed to the handler function is different for each iteration of the loop, as intended.
The old ways will still work but are no longer needed.
2013 answer
As pointed out already the problem is to do with closures and variable scope. One way to make sure the right value gets passed is to write another function that returns the desired function, holding the variables within the right scope. jsfiddle
var names = ['button_id_0', 'button_id_1', 'button_id_2'];
function getClickFunction(a, b, c) {
return function () {
doMouseClick(a, b, c)
}
}
for (var i = 0; i < names.length; i++) {
document.getElementById(names[i]).addEventListener("click", getClickFunction(i, "Dog", "Cat"), false);
}
And to illustrate one way you could do this with an object instead:
var names = ['button_id_0', 'button_id_1', 'button_id_2'];
function Button(id, number) {
var self = this;
this.number = number;
this.element = document.getElementById(id);
this.click = function() {
alert('My number is ' + self.number);
}
this.element.addEventListener('click', this.click, false);
}
for (var i = 0; i < names.length; i++) {
new Button(names[i], i);
}
or slightly differently:
function Button(id, number) {
var element = document.getElementById(id);
function click() {
alert('My number is ' + number);
}
element.addEventListener('click', click, false);
}
for (var i = 0; i < names.length; i++) {
new Button(names[i], i);
}

It's because of closures.
Check this out: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures#Creating_closures_in_loops_A_common_mistake
The sample code and your code is essentially the same, it's a common mistake for those don't know "closure".
To put it simple, when your create a handler function, it does not just accesses the variable i from the outer environment, but it also "remembers" i.
So when the handler is called, it will use the i but the variable i is now, after the for-loop, 2.

I've been struggling with this problem myself for a few hours and now I've just now managed to solve it. Here's my solution, using the function constructor:
function doMouseClickConstructor(peram1, peram2, peram3){
return new Function('alert("doMouseClick() called AND peram1 = ' + peram1 + ' AND peram2 = ' + peram2 + ' AND peram3 = ' + peram3 + ');');
}
for (var i=0; i<names.length; i++){
document.getElementById(names[i]).addEventListener("click", doMouseClickConstructor(i,"dog","cat"));
};
Note: I havn't actually tested this code. I have however tested this codepen which does all the important stuff, so if the code above doesn't work I've probably just made some spelling error. The concept should still work.
Happy coding!

Everything is global in javascript. It is calling the variable i which is set to 3 after your loop...if you set i to 1000 after the loop, then you would see each method call produce 1000 for i.
If you want to maintain state, then you should use objects. Have the object have a callback method that you assign to the click method.
You mentioned doing this for event bubbling...for stopping event bublling, you really do not need that, as it is built into the language. If you do want to prevent event bubbling, then you should use the stopPropagation() method of the event object passed to the callback.
function doStuff(event) {
//Do things
//stop bubbling
event.stopPropagation();
}

Why couldn't I pass the value in javascript?

for (id = 50; id < 100; id++)
{
if($('#'+id).attr('class') == 'myField')
{
$('#'+id).bind('click', function() { install(id); } );
}
}
No idea why id can't reach 'install' in function(). I am trying to bind every button (id from 50 to 100) with a click event to trigger the install(id) function. But it seems the variable id cannot reach install function. While I hard code it:
for (id = 50; id < 100; id++)
{
if($('#'+id).attr('class') == 'myField')
{
$('#'+id).bind('click', function() { install( 56 ); });
}
}
it works! Please tell me why.

What you made is one of the most common mistakes when using Javascript closures.
By the way the very fact that this mistake is so common is IMO a proof that it's indeed a "bug" in the language itself.
Javascript supports read-write closures so when you capture a variable in a closure it's not the current value of the variable that is captured, but the variable itself.
This means that for example in
var arr = [];
for (var i=0; i<10; i++)
arr.push(function(){alert(i);});
each of the 10 functions in the array will contain a closure, but all of them will be referencing the same i variable used in the loop, not the value that this variable was having at the time the closure was created. So if you call any of them the output will be the same (for example 10 if you call them right after the loop).
Luckily enough the workaround is simple:
var arr = [];
for (var i=0; i<10; i++)
arr.push((function(i) {
return (function(){alert(i);});
})(i));
using this "wrapping" you are calling an anonymous function and inside that function the variable i is a different one from the loop and is actually a different variable for each invocation. Inside that function i is just a parameter and the closure returned is bound to that parameter.
In your case the solution is therefore:
for (id = 50; id < 100; id++)
{
if($('#'+id).attr('class') == 'myField')
{
$('#'+id).bind('click',
(function(id){
return (function() { install(id); });
})(id));
}
}

By not reaching the install(), I guess you mean you get all your install(id) behaves like install(100).
Reason why it doesn't work
This is caused by the javaSctipt closure. This line function() { install(id) } assign the id to the install() callback function. The id's value won't be resolved until install() is call when is far later after the loop is finished - the time when id has already reached 100.
The solution is create another closure the hold the current id value.
for (id = 50; id < 100; id++)
{
if($('#'+id).attr('class') == 'myField')
{
(function (id) {
$('#'+id).bind('click', function() { install(id); });
}) (id);
}
}
Here is a demonstration code:
var funcCollections = [];
for (id = 50; id < 100; id++)
{
if(true)
{
(function () {
var thatId = id;
funcCollections.push(function () {console.log(thatId,id)});
}) ();
}
}
// funcCollections[1]();
// 51 100
// undefined
// funcCollections[2]();
// 52 100

You can't pass a variable to the function you've bind. It loses the val. When you pass '56' it will be always 56, but when you pass a var, the JavaScript will not bind the value of the var in the loop.

When you loop over variables and you create anonymous functions(closure) that reference the loop variable they will reference the last value
also note that you don't limit scope the loop variable to the for loop(it's not declared with var) so that means that later modifications to that variable will be propagated to all closures.
take a look at this

It's down to variable scope.
The anonymous function you're binding to the click event of the $('#' + id) elements has no awareness of the id variable in the your sample code (assuming that your sample code is an excerpt from a function). Even if it did (e.g. you declared id outside of any function, giving it global scope), id would hold the value 100 when the click event was called, which isn't what you intend.
However, you could use $(this).attr('id') to get hold of the element's id value instead:
for (id = 50; id < 100; id++)
{
if($('#' + id).attr('class') == 'myField')
{
$('#' + id).bind('click', function()
{
install(parseInt($(this).attr('id')));
});
}
}
Check out the jQuery .bind() documentation, it shows how this can be used from within an event handler.

Javascript variable scope question

I'm having trouble resolving a scope issue with my javascript.
I have an array, dog[] that is defined from JSON, that I need access to from inside a nested function.
function blah(json) {
for (var u = 0; u < json[0][1][u].length; u ++ ) {
var dog = 'k' + json[0][1][u].doggies;
console.log(dog); // prints array of doggie strings
$('#puppy').click(function(dog) { // dog is passed in the function
console.log(dog); // Syntax error, unrecognized expression: #[object Object]
$('#' + dog).css('display, 'none');
});
}
}
when I dont pass dog into the click function: i get:
$('#puppy').click(function() {
console.log(dog) // (12) main.js:122k4c812e3a7275e10331000000 - this is the last value in the array - from safari console
$('#' dog).css('display', 'none);
}
Does anyone have any suggestions to get the array with every element passed into the click function?
Or am i calling the css method incorrectly to hide those divs?

Problem 1
Closures bind the entire function's scope, and not individual variables or values.
Take this code for example:
function foo() {
var i, func;
for (i = 0; i < 10; ++i) {
if (i == 0) {
func = function () {
alert(i);
}
}
}
func();
}
foo();
You may expect foo to cause 0 to be alerted. However, the value of i has changed since the function assigned to func was created; the call to func alerts "10".
Here is another example illustrating the concept:
function foo() {
var i = 42;
function func() {
alert(i);
}
for (i = 0; i < 10; ++i) {
// do nothing
}
func();
}
foo();
Try to figure out what will be alerted, and run the code as a test.
Problem 2
The second problem is that variables are bound at the function scope (and not the block scope as you expect).
Take this code:
function foo() {
var i;
for (i = 0; i < 10; ++i) {
var j = i;
}
alert(j);
}
foo();
You may expect this code to alert "undefined", throw a run-time error, or even throw a syntax error. However, "10" is alerted. Why? In JavaScript, the above code is translated into effectively:
function foo() {
var i;
var j;
for (i = 0; i < 10; ++i) {
j = i;
}
alert(j);
}
foo();
It should be more clear from this example that "10" is indeed alerted.
Solution
So how do you fix your problem? The simplest way is to change your logic: instead of attaching one event handler per dog, attack one event handler per collection of dogs. For example:
function blah(json) {
$('#puppy').click(function () {
var u, dog;
for (u = 0; u < json[0][1][u].length; u++) {
dog = 'k' + json[0][1][u].doggies;
console.log(dog);
$('#' + dog).css('display', 'none');
}
});
}
If you're interested in the "proper" transformation of your existing code (i.e. having the same behaviours, except with the bug fixed), I can give you an example of that as well. However, the solution I gave above is a much better solution and results in cleaner code.

Important Note:
You forgot to close your quote. This:
$('#' + dog).css('display, 'none');
Should be:
$('#' + dog).css('display', 'none');
An Improved Loop:
There are several problems with your script. I'll concentrate on the overall logical structure of the loop.
Instead of attaching many handlers to .click(), just attach one handler that iterates over you JSON using jQuery's .each(). The first argument of the callback of .each() is the index number and the second argument is the value. You can make use of those 2 by naming the arguments or by using arguments[0] and arguments[1]. I show the former method below:
I've added some more test output for demonstration purposes:
function blah(json) {
$('#puppy').click(function() {
// iterate over each json[0][1]
$.each(json[0][1], function(index, value) {
// Your original 2 lines
console.log(value);
$('#' + value).css('display', 'none');
// This is just test output, so you can see what is going
// on.
$("body").append("Number " + index + " is " + value ".<br/>");
});
});
}

Why not just give the doggies a class .dog and hide them when #puppy is clicked?
$("#puppy").click(function() {
$(".dog").hide();
});
Or since your dog's IDs seem to start with k, you might consider something like this:
$("#puppy").click(function() {
// hide everything with ID beginning with 'k'
$('[id^=k]').hide();
});

You can't pass the dog value into the jquery click event as you have done there. The click function signature is:
$(object).click(function(){
});
You can't pass dog in like this. Even if the function expected a parameter, naming it dog would cause issues. You may need to store the values of dog in a more global scope so that when the click event occurs, you still have access to it.

How to access this variable in an inline function?

Here is my dilemma.
I've got this section of code:
var list_of_numbers = new Array();
function AddToArray(func)
{
// Add to the *beginning* of the array
// essentially reversing the order
list_of_numbers.unshift(func);
}
function DisplayNumber(num)
{
document.write(num);
}
for(var i=0;i<5;++i)
{
AddToArray(function() { DisplayNumber(i); });
}
for(var i=0;i<5;++i)
{
list_of_numbers[i]();
}​
What is supposed to happen is that 5 inline functions will be added to the array - each taking a copy of i. However this does not happen.
Expected output:
43210
Actual output:
01234

You have two separate issues, both related to scope.
var list_of_numbers = new Array();
function AddToArray(func)
{
// Add to the *beginning* of the array
// essentially reversing the order
list_of_numbers.unshift(func);
}
function DisplayNumber(num)
{
document.write(num);
}
for(var i=0;i<5;++i)
{
(function(i)
{
AddToArray(function(){ DisplayNumber(i); });
})(i);
}
for(var j=0;j<5;++j)
{
list_of_numbers[j]();
}​
The anonymous function you're passing to AddToArray is bound to the variable i, not the current value. To address this, we create a new function, and pass in the current i.
JavaScript has function scope, so when you re-declare i in the second loop, you're still modifying the same variable. Thus, we rename it to j.
If only the first were an issue, you would get 55555, since all functions would use the same i, at that point 5. However, since you reuse i for the second index, i is set to the current loop index.