This question may be answered elsewhere but I wasn't even sure how to begin searching for the answer. I'm new to JavaScript so this one is a struggle for me to understand.
Given the following code:
function multiple(n) {
function f(x) {
return x * n;
}
return f;
}
var triple = multiple(3);
var quadruple = multiple(4);
When I pass the following into the console:
console.log(triple(5));
I get what I expect, that is, 15. Likewise with any number, it will be tripled (or quadrupled if I used the second function).
But when I type triple into the console I get the following code:
f(x) {
return x * n;
}
Shouldn't the console return...
f(x) {
return x * 3;
}
...since 3 is coded into the function by virtue of the following code:
var triple = multiple(3);
3 is not hard-coded into the function. The code of f refers to the variable n, not the number 3. In your code, it so happens that there is no way to modify n, but imagine some code where there is a way to modify n:
function multiple(n) {
function f(x) {
return x * n;
}
function modifyN(newN) {
n = newN;
}
return { f: f, modifyN: modifyN };
}
var resultFor3 = multiple(3);
var triple = resultFor3.f;
var modifyTripleN = resultFor3.modifyN;
As you can see n is not hard-coded; it's no different from any other variable. In your specific example there is no way to modify n after the termination of multiple, but that does not make the values inside of the closure created by the invocation of multiple in any way "hard-coded".
Typing triple simply shows the source code of your function.
since 3 is coded into the function by virtue of the following code
That is an incorrect statement. You're not changing the source code of the function. To change the source code, you would have to redefine the entire function. All you're doing is passing in a parameter. The console is giving you the correct output.
When you're passing in a parameter to a function, on a high-level, at runtime it's just looking for whatever value is stored at the memory address for that variable (or something like that). But it is not rewriting the source code of the function.
Well, it is the same as it would if you had:
var n = 3;
function f(x) {
return n * x;
}
If you log f you will see the function above, but when you call it n will get its value from the closest variable called n in the scope chain, in our case n = 3, the value for the n declared in the global scope.
I don't know the exact way in which the JS engine stores the n variable inside that closure (in your case the closure created by the multiple function), but what is the important thing is that variables inside a closure are saved by reference not by value.
Related
I was asked what happens when we assign a value to a variable?
Meaning, let's say we have a variable data and we assign some value to it,
var data = 5
What actually happens in the background? What is the initial value of data and what happens, how value is assigned?
I tried to google articles for it, but I couldn't find anything related to working, may be I was not using correct keywords. If anyone have any article that I could follow or provide me some explanation that would be really great.
Because you are using var, the variable named data is first created by the interpreter at the start of the containing function block and its initial value at that point is undefined. This concept of moving the declaration to the start of the function block is called variable hoisting. This data variable will be a piece of memory internal to the interpreter. It will probably be allocated within a stack frame object.
A reference on hoisting:
Hoisting on MDN
Basics of hoisting in Javascript
Note also that functions declared like this:
function x() {
// code here
}
Are also hoisted so two functions in the same scope can call each other without worrying about declaration order.
When the interpreter gets to the point of executing the code where the declaration/assignment line is located, it will assign the value of 4 to the variable.
So, if your code was like this:
function myFunction() {
var sum = 3 * 4;
console.log(sum * 3);
var data = 5;
console.log(sum * data);
}
myFunction();
Then, what the interpreter actually does internally is this:
function myFunction() {
var sum = undefined, data = undefined;
sum = 3 * 4;
console.log(sum * 3);
data = 5;
console.log(sum * data);
}
myFunction();
In fact, you can even write this (somewhat ugly) code:
function myFunction() {
var sum = 3 * 4;
data = 6; // this will not be an error
console.log(data);
console.log(sum * 3);
var data = 5; // the declaration of data is hoisted
console.log(sum * data);
}
myFunction();
because, this is how the interpreter handles that:
function myFunction() {
var sum = undefined, data = undefined;
sum = 3 * 4;
data = 6; // this will not be an error
console.log(data);
console.log(sum * 3);
data = 5;
console.log(sum * data);
}
myFunction();
Note that let and const are block-scoped, not function-scoped and, if you try to reference them before their declaration, it is an error whereas var lets you do that. var is somewhat legacy and there is rarely (if ever) a reason to use var any more. It is safer to code with let and const instead. If you want function scope, you can still put your let or const at the top level of your function. But, you should generally declare your variables within the block that they are used/needed (restrict the scope to only what is needed).
The cpu will try to find a location in memory to hold the value of 5 and store the value in that address and store the address in the variable declared by var.
When looking at the code for the addSuffix function
const addSuffix = (x) => {
const concat = (y) => {
return y + x
}
return concat;
}
let add_ful = addSuffix("ful");
If I were to console.log(add_ful); why do I get
(y) => {
return y + x
}
instead of
(y) => {
return y + "ful"
}
?
This is the most confusing point about closures for me, I understand that I can return a function, but I'm not sure where the reference to "ful" is being stored in the function.
Where is the value of x being stored in the add_ful function?
The value is not stored in the code. It does not get inlined at every place where x is used in the closure - the code still contains a variable x that is dynamically resolved when encountered (it's also still writable).
The value is stored in the scope object containing the variable, and this is referenced by the function object (that's what we call a closure). You can also inspect this hidden [[Scope]] slot in the devtools when you are logging the function object (and not just the function code produced by .toString()):
console.log logs a string representation of the function itself. It doesn't enquire into the state of the function and display the current values that are being referenced. It's just showing the syntax you used to create the function.
However, if you set a breakpoint and walk through the execution, you can see the state of the variables in scope using your debugging tools.
This question already has answers here:
JavaScript closure inside loops – simple practical example
(44 answers)
Closed 8 years ago.
While reading a different topic I came across this piece of code:
function loadme() {
var arr = ["a", "b", "c"];
var fs = [];
for (var i in arr) {
var x = arr[i];
var f = function() { console.log(x) };
f();
fs.push(f);
}
for (var j in fs) {
fs[j]();
}
}
This will output: a,b,c,c,c,c. The problem here as explained by the author is that the x var is hoisted at the start of the function and therefore does not keeps it's value when it is used in the loop. What I do not understand is why is c assigned to x on the second console.log for 3 times? Can someone explain?
This situation is, in fact, quite easy to explain and it occurs in many programming languages, not just JavaScript.
Let's first take the output sequense a,b,c,c,c,c and discard the first three elements - a,b,c because this is what gets printed when you execute your function f() inside the first for loop.
So, we're left with the sequense c,c,c which gets printed when we iterate over the fs array. The reason why you don't get a,b,c printed is because the x parameter that you have inside your first for loop is captured by reference (not sure if the definition is valid, though) by your f() closure (remember, every function in JavaScript is a closure of some sort).
Now, since the console.log(x) expression is evaluated when you call the f(), the current value of the captured x parameter will be passed as an argument. While the code works, as seems to be expected, when you invoke f() inside the first for loop, the x is what has just been assigned from arr[i] so you get a,b,c. But when you exit the loop, the x (even though it's unavailable to the outer scope) is still captured by f() but after the first loop it has a value c (the one that it got on the last iteration) and that is what gets evaluated when you iterated over your functions the second time.
This is, in fact, can be a source of many confusing bugs and so some languages can detect this and notify the developer (for example, C# compiler, when it sees constructs like this, produces the following warning: Access to modified closure).
To fix the problem, you just need to capture the current value of x inside your first for loop:
for (var i in arr) {
var x = arr[i];
var f = (function(x) {
return function() { console.log(x) };
})(x);
f();
fs.push(f);
}
Here we use IIFE (Immediately-Invoked Function Expression) to capture the current value of x.
Hope this helps.
var x = arr[i];
var f = function() { console.log(x) };
In these two lines, console.log(x) thinks that it just has to print x. So, all the three functions are created that way only. So, when you immediately execute the functions, the value of x is different in each of the iterations. But when the looping is over, the variable x retains the last value it held and when the dynamically created functions are executed, they simply print the value of x which is c.
So, in order to fix this, you have to have your own copy of x, for each dynamically created function. Normally, we retain the current state of the variable which changes in the loop, with a function parameter, like this
var f = function(x) { return function() { console.log(x) } }(x);
Now, we are creating a function and executing it immediately, which returns another function and that returned function actually prints the value of x.
function(x) {
return function() {
console.log(x)
}
}(x);
We are passing the value of x as a parameter to the wrapper function and now the current value of x is retained.
Hope somebody finds the time to explain little about functions in functions and scoping.
I am trying to understand little more on functions and scope of variables and found a quite good tutorial, but this part I just do not get.
The task:
Create a function sum that will work like that: sum(a)(b) = a+b and accepts any number of brackets. Examples:
sum(1)(2) == 3
sum(5)(-1)(2) == 6
The solution:
function sum(a) {
var sum = a;
function f(b){
sum += b;
return f;
}
f.toString = function() { return sum };
return f; //line 12
}
alert( sum(1)(2) ); // 3e
The Explanation:
To make sum(1) callable as sum(1)(2), it must return a function.
The function can be either called or converted to a number with valueOf.
The solution is really self-explanatory:
My interpretation:
This f in function f(b) returned to the scope, which is from line 02 - 12.
The f in f.toString, is the currently returned f from function(b)
The next return f returns to the scope which is outside the function sum(a).
Problem:
I cannot figure out, where I need to think differently, because like I described above, the function would not be called again, so where is the part of the code, that make the 'several parentheses' possible?
Moreover, did I correctly assume where the fs are returned? Would be great if somebody would give some explanations.
The function sum returns a function, which we refer to as f.
The function f also returns a function: in fact, the function f returns itself.
When the function f is defined inside of sum, it gets permanent access to all variables currently visible in the scope chain. Here, it includes the locally-defined variable sum (the local running sum tally) and f (the function itself). (A "closure" is what we call the functional code of f along with all its in-scope variables.)
Because f returns itself, you can chain f with repeated calls:
var this_is_f = sum(1);
var same_f_again = this_is_f(2);
var f_a_third_time = same_f_again(3);
Or simply:
sum(1)(2)(3);
It is important to note that in my first example, I don't create new functions; instead, I just refer to the exact same function object with three different identifiers.
Each call to sum creates a brand-new f with a new local sum in its scope (here, I mean the local sum defined on the first line of the function named sum). However, calling the function sum does not clobber any old f, because each call to sum instantiates a new f (and knows nothing about any other fs that have been created on prior calls to sum). That way, you can have multiple tallies running:
var first_tally = sum(1)(2); // first: 3
var second tally = sum(4)(5); // second: 9
first_tally(3); // first: 6
second_tally(6); // second: 15
The reason you're able to see a meaningful result at any time is that f stingifies to the value of sum, instead of showing you its source code.
If you take this code and simplify it to the bare minimum I would be easier to understand. Take a function add that sums only 2 numbers:
function add(x,y) {
return x + y;
}
The above is a "normal" function. If you don't pass all the arguments you'll get unexpected results.
Now, if you want a function that adds 2 to any number, you could partially apply an argument to add, for example:
function add2(x) {
return add(2, x);
}
But in JavaScript we have first class functions (objects that can be passed around), so a function can take functions as inputs and return other functions. This is where "currying" comes in handy. While "partial application" lets you fix function arguments, "currying" takes a function of many arguments and breaks it down into a function of a single argument that returns another function of one single argument until all the arguments have been evaluated, in order, and then returns the result. For example:
function add(x) {
return function(y) {
return x + y;
}
}
Now you can create a function add2 by currying the function add:
var add2 = add(2);
add2(1); //=> 3
The normal function and the curried one have equivalent computations where:
add(1, 2) === add(1)(2)
This is what makes "several parentheses" possible.
In JavaScript "scope" and "closure" refer to functions but they're different concepts. A "scope" determines the reach/privacy of your variables while a "closure" lets you encapsulate code and carry it around. In the curried function above the variable x is kept in memory through closure because it's referenced inside the returned function object.
A particular limitation of "currying" is that you can't have functions of dynamic arity; the number of arguments must be fixed. This is not the case in the code you post where your sum function must be able to add a new number to the previous sum indefinitely.
Similarly to "currying" there's the idea of "generators"; a pattern to model sequences of lazy computation, in this case just adding a number to the previous sum on demand.
function sum(a) { // begin closure
var sum = a; // kept in memory...
function f(b) {
sum += b; //...because you use it here
return f;
}
f.toString = function() { return sum };
return f;
} // end closure
A different (maybe more clear) way to express your code would be to return an object to chain the computations, where you can either add a new number to the previous sum, or get the total sum so far:
function add(a) {
var sum = a;
return {
plus: function(b) {
sum += b;
return this;
},
sum: function() {
return sum;
}
}
}
add(1).plus(2).plus(3).sum(); //=> 6
In your code the returned function f acts as plus and toString as sum which retrieves the value.
Line 01-13 defines the function sum within the global scope.
Line 15 calls sum and changes scope to inside the function.
The variables a and sum and the closure function f are all defined in the function's scope - with the sum variable hiding the function definition from the global scope.
I'll skip over the toString bit here as its not important till later (when it gets very important).
The function sum finally returns a reference to the closure function f to the global scope as an anonymous function (since f can only be referenced by name from within its containing function's scope).
Back to line 15 - sum(1)(2) is the same as (sum(1))(2) and since sum(1) returns f then this can be reduced to (f)(2) or, more simply, f(2).
Calling f(2) adds 2 to sum - sum is defined in two scopes: as a function in the global scope; and as a variable (currently assigned the value of 1) within that function which hides the definition from the global scope - so, in f, the sum variable is set to 1+2=3.
Finally, in f, the function returns itself.
Back, again, to line 15 - f(2) returns f (although the named function f cannot be referenced in the global scope and, again , is treated as an anonymous function)
alert() is processed and the anonymous function (referred to as f) is converted to a string to be alerted; normally when alert() is called on a function it will display the source for the function (try commenting out Line 10 to see this). However, since f has a toString method (Line 10) then this is invoked and the value of sum (as defined in the function containing f) is returned and 3 is alerted.
Let's step through the function line by line:
function sum(a) {
This part is pretty self-explanatory; we have a function named sum which accepts an argument a.
var sum = a
Here we have a local variable called sum that is set to the value of the argument that is passed in.
function f(b) {
sum += b
return f
}
This is an inner function called f that accepts an argument called b. This b is then added to the value of sum from the outer scope (i.e., inside the scope of the function that is also called sum). After that, the function returns itself.
f.toString = function() { return sum }
This is the fun part! When you normally console.log a function, it will just spit out the function's source. Here we are redefining the toString method to instead be a function that spits out the value of sum. This is why you end up seeing the running total that is displayed instead of the function's source even though what you are returning is still a function.
Finally we have:
return f
So you basically have a function sum, returning a function that returns itself. The function sum basically sets everything up, but after calling sum(3), you then work with f, which you repeatedly call.
So the first time you call sum, you essentially get back this function:
function f(b) {
sum += b; //value of sum is 3
return f;
}
In this context, the value of sum will be the value of a that you passed into the initial call of the function sum. However, since the toString of f has been redefined, you only see the value 3. Then let's say you do sum(3)(4).
You get back, as before:
function f(b) {
sum += b; //value of sum is 3
return f;
}
But then you are actually calling f with an argument of 4 (basically f(4)). Since f is an inner function, it has full access to the scope of its parent function. This parent function (sum) is maintaining the the running total in a variable called sum, which is accessible to f. So when you now call f(4), you have b set to 4 and sum having a value of 3:
function f(b) { //b is 4
sum += b; //value of sum is 3 + 4, which is 7
return f;
}
So with each subsequent pair parentheses you are making repeated calls to the same f which is maintaining a running tally.
Another way is to think of sum as a kind of factory that can give you different f's, all of which keep their own running tallies (basically behaving like accumulators):
var firstSum = sum(4);
var secondSum = sum(2);
firstSum(5); //equivalent to sum(4)(5) returns 9
secondSum(2); //equivalent to sum(2)(2) returns 4
There are 3 concepts being played here
closures (no scope).
functions are first class OBJECTS in javascript (allows for chaining f(a)(b)(c)). There's no need to save a handle to the function to call it later.
I'll give you a run-down and then add some extra explanations afterwards
function sum(a) {
// when sum is called, sumHolder is created
var sumHolder = a;
// the function f is created and holds sumHolder (a closure on the parent environment)
function f(b) {
// do the addition
sumHolder += b;
// return a FUNCTION
return f;
}
// change the functions default toString method (another closure)
f.toString = function() {return sumHolder;}
// return a FUNCTION
return f
}
/*
* ok let's explain this piece by piece
*
* you call sum with a parameter
* - parameter is saved into sumHolder
* - a function is returned
*
* you call the returned function f with another parameter
* EXPLANATION: { sum(a) } returns f, so let's call f as this => {...}()
* - this private (priviledged) function adds whatever it's been passed
* - returns itself for re-execution, like a chain
*
* when it all ends {{{{}(a)}(b)}(c)} the remainder is a FUNCTION OBJECT
* this remainder is special in that it's toString() method has been changed
* so we can attempt to cast (juggle) it to string for (loose) comparison
*
*/
The concept behind closures is quite easy to understand but it's application make's your head spin until you get used to the idea that there is no function scope in javascript, there is closures and these are powerfull critters indeed
// this anonymous function has access to the global environment and window object
(function()
{// start this context
var manyVars, anotherFunc;
function someFunc() {
// has access to manyVars and anotherFunc
// creates its own context
};
anotherFunc = function () {
// has access to the same ENVIRONMENT
// creates its own context
};
}// whatever is in these keys is context that is not destroyed and
// will exist within other functions declared inside
// those functions have closure on their parent's environment
// and each one generates a new context
)();
Functions are first class objects. What does this mean? I'm not sure myself but let me explain with some further examples:
// how about calling an anonymous function just as its created
// *cant do the next line due to a language constraint
// function(){}()
// how about a set of parens, this way the word "function" is not the first expression
(function(){}());
// the function was created, called and forgotten
// but the closure inside MAY STILL EXIST
function whatDoIReturn() {
return function (){alert('this is legal');return 'somevalue';}();// and executed
}// returns 'somevalue'
Don't take this word for word. Go look for other people's code, check the Crockford and ask any and all questions that come up
I'm trying to wrap my head around closures in Javascript.
Here is an example from a tutorial:
function greeter(name, age) {
var message = name + ", who is " + age + " years old, says hi!";
return function greet() {
console.log(message);
};
}
// Generate the closure
var bobGreeter = greeter("Bob", 47);
// Use the closure
bobGreeter();
The author said that this is an effective way of using closure to make private variables, but I don't get the point.
Could someone enlighten the benefits of coding like this?
A closure is a pair of a function and the environment in which it was defined (assuming lexical scoping, which JavaScript uses). Thus, a closure's function can access variables in its environment; if no other function has access to that environment, then all of the variables in it are effectively private and only accessible through the closure's function.
The example you provided demonstrates this reasonably well. I've added inline comments to explain the environments.
// Outside, we begin in the global environment.
function greeter(name, age) {
// When greeter is *invoked* and we're running the code here, a new
// environment is created. Within this environment, the function's arguments
// are bound to the variables `name' and `age'.
// Within this environment, another new variable called `message' is created.
var message = name + ", who is " + age + " years old, says hi!";
// Within the same environment (the one we're currently executing in), a
// function is defined, which creates a new closure that references this
// environment. Thus, this function can access the variables `message', `name',
// and `age' within this environment, as well as all variables within any
// parent environments (which is just the global environment in this example).
return function greet() { console.log(message); };
}
When var bobGreeter = greeter("Bob", 47); is run, a new closure is created; that is, you've now got a new function instance along with the environment in which it was created. Therefore, your new function has a reference to the `message' variable within said environment, although no one else does.
Extra reading: SICP Ch 3.2. Although it focuses on Scheme, the ideas are the same. If you understand this chapter well, you'll have a good foundation of how environments and lexical scoping work.
Mozilla also has a page dedicated to explaining closures.
The purpose of a closure is so that the variables you use inside a given function are guaranteed to be "closed" which means they do not depend on external variables - they only depend on and use their arguments. This makes your Javascript methods closer to a pure function, that is, one that returns the same value for the same given arguments.
Without using closures, your functions will be like Swiss cheese, they will have holes in them. A closure plugs up those holes so the method doesn't depend on variables higher in the scope chain.
Now, up until this point, my answer has been simply about organizing your code and style. So take this simple example. At the line with the comment, I invoke a function and the value of the variable a is captured for future use.
var a = "before";
var f = function(value) {
return function()
{
alert(value);
}
} (a); //here I am creating a closure, which makes my inner function no longer depend on this global variable
a = "after";
f(); //prints "before"
Now, why would you need to do this? Well, here's a practical example. Consider the following code that uses jQuery to add 5 links to the document. When you click a link, you would expect it to alert the number associated with the link, so clicking the first you would think would alert 0, and so on. But, this is not the case, each link will alert the value of 5. This is because the function I define depends on the variable i which is being modified outside the context of the function. The function I pass into bind is a Swiss cheese function.
for (var i = 0; i < 5; i++)
{
var a = $('<a>test link</a>').bind('click', function(){
alert(i);
});
$(a).appendTo('body');
}
Now, let's fix this by creating a closure so each link will alert its correct number.
for (var i = 0; i < 5; i++)
{
var fn = function (value) {
return function() {
alert(value);
};
} (i); //boom, closure
var a = $('<a>test link</a>').bind('click', fn);
$(a).appendTo('body');
}
I don't think this is a good example for private variables, because there are no real variables. The closure part is that the function greet can see message (which is not visible to the outside, hence private), but it (or anyone else) is not changing it, so it is more of a constant.
How about the following example instead?
function make_counter(){
var i =0;
return function(){
return ++i;
}
}
var a = make_counter();
console.log(a()); // 1
console.log(a()); // 2
var b = make_counter();
console.log(b()); // 1
console.log(a()); // 3
A better example may be
function add(start, increment) {
return function() {
return start += increment;
}
}
var add1 = add(10, 1);
alert(add1()); // 11
alert(add1()); // 12
Here, every time you call the returned function, you add 1. The internals are encapsulated.
The returned function still has access to its parents variables (in this case, start and increment).
On a lower level of thinking, I think it means that the function's stack is not destroyed when it returns.
Once you "get it" you will wonder why it took you so long to understand it. That's the way way I felt anyways.
I think function scope in Javascript can be expressed fairly concisely.
The function body will have access to any variables that were visible in the lexical environment of the function declaration, and also any variables created via the function's invocation -- that is, any variables declared locally, passed through as arguments or otherwise provided by the language (such as this or arguments).
It's called "closures" because they are "closed" around free variables, and there are much more ways to use it then only hiding state. For example, in functional programming, where closures came from, they are often used to reduce parameters number or set some constant for a function. Let's say you need function goodEnough() that will test if some result is better then some threshold. You can use function of 2 variables - result and threshold. But you can also "enclose" your constant inside function:
function makeThresholdFunction(threshold) {
return function(param) {
return (param > threshold);
}
}
var goodEnough = makeThresholdFunction(0.5);
...
if (goodEnough(calculatedPrecision)) {
...
}
With closures you can also use all the tricks with functions such as their composition:
function compose(f1, f2) {
return function(arg) {
return f1(f2(arg));
}
}
var squareIncremented = compose(square, inc);
squareIncremented(5); // 36
More on closure design and usage can be found at SICP.
I found this a pretty helpful article.
When is a function not a function?
//Lets start with a basic Javascript snippet
function generateCash() {
var denomination = [];
for (var i = 10; i < 40; i += 10) {
denomination.push(i);
}
return denomination;
}
This a basic function statement in Javascript that returns an array of [10,20,30]
//--Lets go a step further
function generateCash() {
var denomination = [];
for (var i = 10; i < 40; i += 10) {
denomination.push(console.log(i));
}
return denomination;
}
This will print 10, 20 ,30 sequentialy as the loop iterates, but will return an array of [undefined, undefined, undefined], the major reason being we are not pushing the actual value of i, we are just printing it out, hence on every iteration the javascript engine will set it to undefined.
//--Lets dive into closures
function generateCash() {
var denomination = [];
for (var i = 10; i < 40; i += 10) {
denomination.push(function() {
console.log(i)
});
}
return denomination;
}
var dn = generateCash();
console.log(dn[0]());
console.log(dn[1]());
console.log(dn[2]());
This is a little tricky, what do you expect the output will be, will it be [10,20,30]? The answers is no, Lets see how this happens. First a Global execution context is created when we create dn, also we have the generatecash() function. Now we see that as the for loop iterates, it creates three anonymous function objects, it might be tempting to think that the console.log within the push function is getting fired too, but in reality it is not. We haved invoked generateCash(), so the push function is just creating three anonymous function objects, it does not trigger the function. At the end of the iteration, the current local context is popped from the execution stack and it leaves the state of i : 40 and arr:[functionobj0(), functionob1(), functionobj2()].
So when we start executing the last three statements, all of them output 40, since it is not able to get the value of i from the current scope, it goes up the scope chain and finds that the value of i has been set to 40. The reason all of them will fire 40 is beacause every single component of dn lies in the same execution context and all of them on being not able to find the value of i in their current scope will go up the scope chain and find i set as 40 and output it respectively