I understand that every function in JavaScript is a first-class object and it has an internal property [[scope]] which hosts the binding records of the function's free variables. However, there are two special cases.
Is the function created by Function constructor also a closure? The function object created by Function constructor is special, because its [[scope]] may not refer to the lexical environments of its outer functions, but only the global context. For example,
var a = 1;
var fn = (function outer() {
var a = 2;
var inner = new Function('alert(a); ');
return inner;
})();
fn(); // will alert 1, not 2.
This is unintuitive. Is this also called closure?
If an inner function doesn't have any free variables, can we say a closure is formed when the inner function is created? For example,
// This is a useless case only for academic study
var fn = (function outer() {
var localVar1 = 1,
localVar2 = 2;
return function() {};
})();
In this case, fn refers to an empty function object which was created as an inner function. It has no free variables. In this case can we say a closure is formed?
Is the function created by Function constructor also a closure?
Yes, it closes over the global scope. That might be unintuitive because all other JavaScript closures close over their lexical scope, but it still matches our definition of a closure. In your example, a is a free variable, and resolves to the a in an other scope when the inner/fn function is called somewhere.
If an inner function doesn't have any free variables, can we still call it a closure?
Depends on whom you ask. Some say Yes, others call them "uninteresting closures", personally I say No because they don't reference an outer scope.
Note: Functions created with the Function constructor do not create
closures to their creation contexts; they always are created in the
global scope. When running them, they will only be able to access
their own local variables and global ones, not the ones from the scope
in which the Function constructor was called. This is different from
using eval with code for a function expression.
from https://developer.mozilla.org
Related
I understand that every function in JavaScript is a first-class object and it has an internal property [[scope]] which hosts the binding records of the function's free variables. However, there are two special cases.
Is the function created by Function constructor also a closure? The function object created by Function constructor is special, because its [[scope]] may not refer to the lexical environments of its outer functions, but only the global context. For example,
var a = 1;
var fn = (function outer() {
var a = 2;
var inner = new Function('alert(a); ');
return inner;
})();
fn(); // will alert 1, not 2.
This is unintuitive. Is this also called closure?
If an inner function doesn't have any free variables, can we say a closure is formed when the inner function is created? For example,
// This is a useless case only for academic study
var fn = (function outer() {
var localVar1 = 1,
localVar2 = 2;
return function() {};
})();
In this case, fn refers to an empty function object which was created as an inner function. It has no free variables. In this case can we say a closure is formed?
Is the function created by Function constructor also a closure?
Yes, it closes over the global scope. That might be unintuitive because all other JavaScript closures close over their lexical scope, but it still matches our definition of a closure. In your example, a is a free variable, and resolves to the a in an other scope when the inner/fn function is called somewhere.
If an inner function doesn't have any free variables, can we still call it a closure?
Depends on whom you ask. Some say Yes, others call them "uninteresting closures", personally I say No because they don't reference an outer scope.
Note: Functions created with the Function constructor do not create
closures to their creation contexts; they always are created in the
global scope. When running them, they will only be able to access
their own local variables and global ones, not the ones from the scope
in which the Function constructor was called. This is different from
using eval with code for a function expression.
from https://developer.mozilla.org
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.
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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.
Javascript closure definition says :
A "closure" is an expression (typically a function) that can have free
variables together with an environment that binds those variables
(that "closes" the expression).
Can some one explain to me the concept of free variables ? Is this concept Javascript specific or applies to other languages also ?
Free variables are simply the variables that are neither locally declared nor passed as parameter.
Source :
In computer programming, the term free variable refers to variables
used in a function that are not local variables nor parameters of that
function.1 The term non-local variable is often a synonym in this
context.
In javascript closures, those are simply the variables that the function takes (read and write) in the enclosing scope where is declared the closure or in a parent scope.
Look at this real world example :
Gol.prototype._ensureInit = function() {
...
var _this = this;
var setDim = function() {
_this.w = _this.canvas.clientWidth;
_this.h = _this.canvas.clientHeight;
_this.canvas.width = _this.w;
_this.canvas.height = _this.h;
_this.dimChanged = true;
_this.draw();
};
setDim();
window.addEventListener('resize', setDim);
...
};
In this example a closure points from the setDim function towards the variable _this declared in the enclosing scope (the _ensureInit function). This variable isn't declared in setDim nor passed. It's a "free variable".
Note that _this doesn't become a variable of the function setDim : another function declared in the same scope would share the same variable.
A "free-translation" could be: "out of scope" - variables.
Since ECMAscript uses lexical scoping, a free variable is a variable which was defined in a parent-scope and gets looked-up by a scope-chain search.
(function _outerScope() {
var foo = 42;
(function _innerScope() {
var bar = 100;
console.log( foo + bar ); // 142
}());
}());
In the above example, foo is a free variable within the context of _innerScope. it becomes very obvious if we have a quick glance into the underlying concepts of ECMAscript.
A Context is linked to an Activation Object (in ES3), respectively a Lexical Enviroment Record (in ES5), which contains things like: function declarations, variables declared with var and formal paramters, as well as a reference to all parent Activation Objects / Lexical Environments. If a variable needs to be accessed, the ECMAscript engine will first look into the AOs / LEs from the current Context itself; if it can't be found there, it looks into the parent AO's / LE's.
Since any Context stores this data in an array-like structure (don't forget we're talking about implementation level here, not Javascript itself), we are talking about Lexical Scope, because we search through all parent Contexts in order.
As an example:
var myModule = (function (){
var moduleVar; // closure variable
return function(){
// actual function
}
})();
the variable defined there is a closure variable. it can be used all over the closure itself but is not part of a global namespace.
I don't remember how many times I've read JS books and revise some topic in which the closure topic is there.
The answer is just an addition to the accepted answer.
The following example is excerpted from Modern JavaScript for the Impatient,
Cay Horstmann
// text is a free variable of the arrow function.
const sayLater = (text, when) => {
// vv
let task = () => console.log(text) // (heed no 'text' variable here) => console.log(text)
setTimeout(task, when)
}
Now you can grasp the meaning of
The term free variable refers to variables used in a function that are
not local variables nor parameters of that function.