Can you clarify the concept of closures and how they bind the loop counter to the function scope?

Question

Can you clarify the concept of closures and how they bind the loop counter to the function scope?

I have observed programmers setting up event listeners inside loops, utilizing the counter. The syntax that I have come across is as follows:

for(var i=0; i < someArray.length; i++){
   someArray[i].onclick = (function(i){/* Some code using i */})(i);
}

I am curious about the reasoning behind this approach and the peculiar syntax used here. This is new to me:

(function(i))(i);

Thank you in advance for taking the time to explain this.

javascript function loops closures

Answer 1

Answer №1

The technique of using (function(i))(i) creates an anonymous function and then immediately runs it.

This is commonly done to generate a unique function each time the loop iterates, ensuring that each event handler has its own version of the variable instead of all event handlers sharing the same one.

For instance:

for(int i = 0; i < 10; i++)
    buttons[i].click = function() { doFoo(i); };

This scenario can be confusing for many people, as clicking any button will trigger doFoo(10).

On the contrary:

for(int i = 0; i < 10; i++)
    buttons[i].click = (function(i){ return function() { doFoo(i); };)(i);

Each iteration generates a new variant of the inner function (with its specific value of i), ensuring proper functionality.

Answer 2

The technique of using (function(i))(i) creates an anonymous function and then immediately runs it.

This is commonly done to generate a unique function each time the loop iterates, ensuring that each event handler has its own version of the variable instead of all event handlers sharing the same one.

For instance:

for(int i = 0; i < 10; i++)
    buttons[i].click = function() { doFoo(i); };

This scenario can be confusing for many people, as clicking any button will trigger doFoo(10).

On the contrary:

for(int i = 0; i < 10; i++)
    buttons[i].click = (function(i){ return function() { doFoo(i); };)(i);

Each iteration generates a new variant of the inner function (with its specific value of i), ensuring proper functionality.

Answer 3

Answer №2

One reason for this is due to the fact that JavaScript operates under function scope, not block scope. This means that any variable you declare within a loop will be within the function's scope, allowing every closure to access the same variable.

In order to create a new scope, it is necessary to invoke a function, which is exactly what

(function(i){/* Some code using i */}(i))

accomplishes.

It is important to note that in your example, a crucial component is missing: The immediate function must return another function that serves as the click handler:

someArray[i].onclick = (function(i){
    return function() {
       /* Some code using i */
    }
}(i));

The immediate function is essentially just a way to combine function definition and function call. It can also be replaced by a regular function call:

function getClickHandler(i) {
    return function() {
         /* Some code using i */
    }
}

for(var i=0; i < someArray.length; i++){
   someArray[i].onclick = getClickHandler(i);
}

Answer 4

One reason for this is due to the fact that JavaScript operates under function scope, not block scope. This means that any variable you declare within a loop will be within the function's scope, allowing every closure to access the same variable.

In order to create a new scope, it is necessary to invoke a function, which is exactly what

(function(i){/* Some code using i */}(i))

accomplishes.

It is important to note that in your example, a crucial component is missing: The immediate function must return another function that serves as the click handler:

someArray[i].onclick = (function(i){
    return function() {
       /* Some code using i */
    }
}(i));

The immediate function is essentially just a way to combine function definition and function call. It can also be replaced by a regular function call:

function getClickHandler(i) {
    return function() {
         /* Some code using i */
    }
}

for(var i=0; i < someArray.length; i++){
   someArray[i].onclick = getClickHandler(i);
}

Can you clarify the concept of closures and how they bind the loop counter to the function scope?

Answer №1

Answer №2

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