In the You Don't Know Javascript series, 1/3 of the way down IIFE's are described as not being closures themselves, but only if they're executed outside their lexical scope:
Chapter 3 introduced the IIFE pattern. While it is often said that
IIFE (alone) is an example of observed closure, I would somewhat
disagree, by our definition above.
This code "works", but it's not strictly an observation of closure.
Why? Because the function (which we named "IIFE" here) is not executed
outside its lexical scope. It's still invoked right there in the same
scope as it was declared (the enclosing/global scope that also holds
a). a is found via normal lexical scope look-up, not really via
closure.
var a = 2;
(function IIFE(){ // not actually a "closure"
console.log( a );
})();
In this SO post, the following snippet was given as an example of a closure:
for (var i = 0; i < someVar.length; i++)
(function (i) {
window.setTimeout(function () {
alert("Value of i was "+i+" when this timer was set" )
}, 10000);
})(i);
I am trying to understand this in terms of the definition of closure (as defined in this medium article):
To use a closure, simply define a function inside another function and
expose it. To expose a function, return it or pass it to another
function. ...
The inner function will have access to the variables in the outer
function scope, even after the outer function has returned.
I understand that a closure is a "stateful function", and that it is
a way to "remember" and continue to access a function's scope (its
variables) even once the function has finished running.
So in this example, I see that the loop's i is remembered when passed into the closing IIFE.
My question is:
Where is the "passing to another function or returning" portion happening? My guess is that the IIFE is able to remember the outer i for loop value at each iteration because the IIFE is passed to the window?
Basically, my understanding is that a closure is defined as remembering an outer scope's value after the Garbage collector cleans that outer scope up, and that the usage of it is to expose the closure by returning it and accessing it outside its lexical scope. Is this correct? So where is that "accessing it outside the lexical scope" happening?
in this example, I see that the loop's i is remembered when passed into the closing IIFE
No. The IIFE is only providing the scope for the i value to be remembered. As the quotes you cited state, the IIFE function is not a closure. The function expression that uses the i is the closure:
(function IIFE(i) {
// this is the scope of i
window.setTimeout(function closure() {
// this function closes over i
alert("Value of my local 'i' is still "+i+" even after 10 seconds");
}, 10000);
})(0);
// ^ this is some arbitrary value passed to the IIFE - it could have been a loop variable
Where is the "passing to another function or returning" portion happening?
It's the closure function being passed into setTimeout, which will call it back from a place where i would usually be no longer defined - if it wasn't a closure.
Related
Is the code below a closure? Why?
var getContact = (function(){
var person = {name: "John Doe"};
return {aFriend: person};
})();
console.log(getContact.aFriend.name);
//outputs: John Doe
No.
There is no function declared inside another function that is accessible after the outer function has finished executing.
In this example:
function createClosure() {
var foo = 0;
function bar() {
alert(foo);
}
return bar;
}
var myFunc = createClosure();
… the variable foo is closed over so there is a closure.
All JavaScript functions are closures; they keep a reference to the lexical environment object that's active when they're created. So technically there is briefly a closure created there, but in that code no closure endures for any length of time. The function is created, called, and then released; nothing maintains a reference to it, so it doesn't live on, and the lexical environment object it referenced can be reclaimed. Creating an object in a function does not give the object a reference to the function (or the environment object), so the function isn't retained, and so it doesn't retain its enclosing environment in memory.
Contrast with:
(function outer(x) {
setTimeout(function inner() {
alert(x);
}, 100);
})("foo");
There, we create two closures (outer and inner) but outer is released almost immediately (like your example); inner is released 100ms or so later after the timer fires and the timer subsystem releases its reference to it.
getContact is a IIFE ( immediately-invoked function expression) which returns an object. Here no inner function is created which refers to the outer function's environment variable. It should not be considered closure in my opinion, still waiting for explanation which can justify this as a closure.
function one() {
function two() {
alert(i);
}
return two;
}
(function() {
var i = 3;
var f = one();
f(); // Uncaught ReferenceError: i is not defined
}());
The articles I've read say that, when a function is called, a new execution context is added on to the stack and the scope chain is created by traversing up the stack. But surely that would result in 3 being alerted in the above code?
How does it work out the scope chain?
The scope chain is determined by the lexical nesting of scopes. The scope can be affected dynamically (via with), but that causes a lot of problems. It's definitely not the case that the current stack of function activations has anything to do with scope.
When you do var i = 3;, you are creating a local variable that exists only in the scope of the immediately-invoked function expression (IIFE).
Any functions that are defined in that scope can access the variable i.
one() (and therefore also two()) was declared outside the scope, so it has no idea what i is. It's looking up its scope chain to window.i, which doesn't exist.
The call stack (as mentioned keeps adding an execution context to the top of the stack when a function is invoked) and lexical scope (which allows access to variables from outer scope) are independent of each other.
Lexical scope depends only on where the function is created not where the function is invoked, so in your example function "two" is created inside function "one", so when variable "i" inside function "two" tries to access "i", it tries to find "i" in its own scope first, since there is no "i" defined it will go out to the scope where the function is created in your case its function "one" (not where the function is invoked in anonymous IIFE below) and it couldn't find variable "i" again inside function "one", now since function "one" is defined inside global scope (the default execution context) JS will try to find the value of "i" inside the global scope which results in "undefined".
In order to access variable "i" you need to move function "two" into anonymous IIFE as shown below, function two has outer lexical scope of function "one" which has an outer lexical scope of anonymous IIFE which has i assigned to 3 which is alerted.
(function () {
var i = 3;
function one() {
function two() {
alert(i); // will alert 3
}
return two;
}
var f = one();
f();
}());
Hope this is clear now :)
The call stack and the execution context stack are not the same. A function call adds a level to the call stack, but execution context stacks are not changed. However, the function that you call has a different execution context stack.
Each function scope has its own execution context stack which is determined by where the function is created, it doesn't inherit the execution context stack from the calling code.
If you nest functions in each other and only call the inner functions, like in the first example on the page, the call stack and the execution context stack will happen to correspond.
Execution stack is what happens on hardware (memory), regardless of whatever high level language is running. The scope chain model is managed by specific interpreter to implement programming language features such as functional programming, which itself is another program independent from the one you've written, the sequence of function call stacks via the interpreter is different from the actual order on physical hardware.
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Consider the following code:
function nepaliBuddha() {
var a = 20;
return function buddhaNepal() {
console.log(a);
}
}
var closure = nepaliBuddha();
closure(); // logs 20
Now when we invoke closure output is 20. This proves that the internal scope property ([[scope]]) was assigned to the inner function where it was defined or say when declared.If this wasn't assigned at declaration ,there was no way to log 20 as it gets invoked in different context
Invoking closure() the scope chain of a function context is created at function call and consists of the activation object or VO of the current context and the internal [[scope]] property of this function.
Invocation also creates [[scope]] property , this means that internal scope property is created at declaration as well as at execution isn't it?
Usually the definition says the [[scope]] property gets created at run time or at function call but this isn't true as [[scope]] property is already assigned at declaration as well.
What I think is the [[scope]] property might get updated after execution of function, is it? Please give clear definition of [[scope]] internal property. How and when it is created at declaration time or at execution time or at both time.
Wow, aren't you thoroughly confused. Alright, I'll try to explain closures as simply as possible.
First, we'll start with scopes. There are two types of scopes:
Block scopes
Function scopes
A block scope begins immediately when it occurs in the program. A function scope on the other hand does not begin until the function is called. Hence multiple calls to the same function result in multiple scopes being created.
JavaScript does not have block scopes. It only has function scopes. Hence to emulate a block scope we need to create a function expression and immediately execute it. This patttern is called an immediately invoked function expression (IIFE) and it looks like this:
(function () {
// this is the JS equivalent of a block scope
}());
Beside block scopes and function scopes there's another way to classify scopes. Hence we also have:
Lexical scopes
Dynamic scopes
This distinction only applies to function scopes because block scopes are always lexically scoped. JavaScript only has lexical scopes.
To understand the difference between lexical scopes and dynamic scopes we need to understand the difference between free and bound variables.
A free variable is a variable which is used within a function but which is not declared within that function.
A variable which is declared within a function is said to be bound to that function.
Consider the following program:
function add(x, y) {
return x + y; // x and y are bound to add
}
In the above program the variables x and y are bound to the function add because they are declared within add.
On the other hand the variables x and y in the following program are free within the function add because they are not declared within add but they are used within add:
function add() {
return x + y; // x and y are free within add
}
Now free variables are a problem. They need to be mapped to some value, but which value? This is where lexical and dynamic scopes come into picture. I won't go into the major details, but you can read about it on Wikipedia.
Scopes are a lot like prototypal inheritance. When a new scope begins it inherits from a parent scope forming a chain of scopes much like prototype chains in JavaScript.
Lexical scopes and dynamic scopes differ with respect to which parent scope a new scope inherits form.
In lexical scoping a new function scope inherits from the scope in which that function was defined (i.e. its lexical environment).
In dynamic scoping a new function scope inherits from the scope in which that function was called (i.e. the calling scope).
Since JavaScript only has lexical scoping we won't bother with dynamic scoping. Consider the following program:
var count = 0;
function incrementCount() {
return ++count;
}
(function () {
var count = 100;
alert(incrementCount()); // 1
}());
Here the function incrementCounter has one free variable - count. Since JavaScript has lexical scoping count will be mapped to the global variable count instead of the local count declared within the IIFE. Hence incrementCount returns 1 and not 101.
Now closures only work in languages which have lexical scoping. Consider the following program:
function getCounter() {
var count = 0;
return function () {
return ++count;
};
}
var counter = getCounter();
alert(counter()); // 1
alert(counter()); // 2
alert(counter()); // 3
In the above program the function returned by getCounter is a closure with respect to the variable count because:
The variable count is free within the returned function (i.e. counter).
The function is moved outside of the scope within which count is declared.
Both these conditions are necessary for a function to be called a closure. For more information read the following answer: https://stackoverflow.com/a/12931785/783743
Now the important thing to understand here is that the function counter will still be called a closure even though it may never be invoked. A closure is simply a function which closes over a variable (which is called the upvalue of the closure).
When we invoke getCounter we create a new scope (let's call this scope A), and each time we invoke the function returned by getCounter (i.e. counter) we create a new scope which inherits from scope A. That's all. No new closure is created.
A closure is a special kind of object that combines two things: a function, and the environment in which that function was created. The environment consists of any local variables that were in-scope at the time that the closure was created.
function makeFunc() {
var name = "Mozilla";
function displayName() {
alert(name);
}
return displayName;
}
Now call makeFunc()
var myFunc = makeFunc();
myFunc();
In this case, myFunc is a closure that incorporates both the displayName function and the "Mozilla" string that existed when the closure was created, MDN.
So, the scope created exactly when I called var myFunc = makeFunc(); and myFunc() (the result of the makeFunc() call) is now a closure. So, go to the first line where
A closure is a special kind of object that combines two things: a
function, and the environment in which that function was created.
Now consider these
function nepaliBuddha() {
var a = 1;
return function buddhaNepal() {
a = a+1;
console.log(a);
}
}
var closure1 = nepaliBuddha(); // An individual scope
var closure2 = nepaliBuddha(); // An individual scope
closure1(); // 1
closure1(); // 2
closure2(); // 1
closure2(); // 2
closure2(); // 3
Demo.
Which means, closure1() and closure2() are closures and both have their personal scope/environment and they have access to their own scope once they get it (in this case, every time you call nepaliBuddha you are creating a closure and giving/saving it to a variable).
Scope defines the area, where functions, variables and such are available. So, when you defined/declared the function buddhaNepal (inner function) inside the nepaliBuddha (outer function) the buddhaNepal (inner function) has been just separated from the global scope and nothing else. It can't access anything in the global scope but it has it's own scope, that's it. The nepaliBuddha (outer function) is the boundary of the buddhaNepal (inner function) and in this case the nepaliBuddha outer function's local scope/environment is the global scope for the buddhaNepal (inner function).
in JavaScript, this is known as Lexical Scopeing it defines how variable names are resolved in nested functions. Other names of Lexical Scope are Static Scoping or Closure. It means that the scope of an inner function contains the scope of a parent function.
When using Javascript Closures, is there some difference in using Object literal Vs Constructor based objects ?
Are there just syntax difference OR is there some other difference as well for Closures?
Any example explaining the 2 differences would be really helpful.
Closures are a feature of functional programming. They have nothing to do with objects or object literals.
Read the following answer - it explains closures really well: https://stackoverflow.com/a/12931785/783743
In general a closure is a function which closes over the variables in a nested function which moves out of the scope of the closure. For example:
function getCounter() {
var count = 0;
return function counter() {
return ++count;
};
}
var counter = getCounter();
counter(); // 1
counter(); // 2
counter(); // 3
Here the function getCounter becomes a closure because it closes over the variable count used in the nested function counter when the nested function is returned (moves out of the scope of getCounter).
The variable which is closed over (in this case count) is called an upvalue. Closures are important because they allow values which would otherwise go out of scope (be garbage collected) to remain alive. This is not possible in languages like C/C++ and Java.
Closure is more about the function scope of the variable. So the important thing to know is that the scope of a variable is the function it was defined in. Any function that run inside this scope will have access to its parent function per se. Parent function won't have access to a child function variable, because it's outside of that child's scope.
Therefore a variable in an object literal, would be scoped to the function that it is contained it. (If it's not in a function, then it's in the global scope). A constructor is a function so any variables that it defines, is scoped in itself and inaccessible outside. Any inner methods that are in the constructor has access to those defined variables.
Closures are created when a function has access to a variable that is outside its own scope and that variable may be changed or altered by something else...even well after the function has finished execution.
I hope that helped some what.
function f1(){
var n=999;
function f2(){
alert(n); // 999
}
}
is the function f2() a closure? if not? why?
but in this post. How do JavaScript closures work?
why it says:
function foo(x) {
var tmp = 3;
function bar(y) {
alert(x + y + (++tmp));
}
bar(10);
}
foo(2)
That is not a closure.A closure is when you return the inner function. The inner function will close-over the variables of foo before leaving. why?
but i don't know what's the difference between the example i made and another i cited, one is not a closure.but one is. i think the two example is the same.
Yes, f2 is a closure, since it accesses a variable (n) from an outer scope. (OK, it's not really a closure -- see update below).
n was declared inside f1, not f2; this makes it belong to f1's scope. So when you create the function f2 which references n, it is a closure by definition, since it uses someone else's variable.
UPDATE
Alright, if I understand the answer you've linked to correctly, it says that f2 is not a closure because it is merely accessing a variable within its scope (just like an if statement, which gets its own scope within the braces*, can use variables from the outer scope without needing a closure).
* Update: Turns out that only functions get their own scope in Javascript, not any old blocks. But my point still stands...
However, f2 would become a closure if it left f1's scope (e.g. by being returned); in that case, it would still have access to f1's variable n, even though f1's original scope would no longer exist (it's exited when control leaves the function). f2 would have "closed over" f1's variables, thus artificially extending the lifespan of f1's scope.
Personally, I would still call f2 a closure, even if it never leaves f1. If a function can become a closure simply by being used outside of its declaring scope, and its behaviour inside that scope is no different whether it's technically a closure or not, then I see no point in making a distinction. I would even go so far as to say that it's an implementation detail whether f2 is a closure or not, if it never leaves f1's scope.
On the other hand, ignoring that distinction would mean that any function that uses a global variable would be called a "closure", since it accesses a variable from an outer scope. So the fact that it becomes a closure only when it leaves the scope(s) that the variables it is using are defined in is a worthwhile (albeit subtle) distinction.
I guess the clearest answer I can give is that it's not a closure yet.
It is a closure, because it depends on the values of variables in an intermediate (i.e. not local or global) scope, and if the variable changes then so will the operation of the closure. That the value is always the same is incidental.
You may however want to return the closure to the caller of f1() so that it can be used elsewhere.