function buildList( list ) {
var i = 0;
var first = function () {
console.log( "in" )
console.log( i );
}
var Second = function () {
console.log( "out" )
first();
}
return Second;
}
var a = buildList( [1, 2, 3] )
console.dir( a );
a(); // Here closure is created which has function first ,Here first also has one closure of itself that means recursive closure
When i see my console in Chrome it has a closure which has function first which also has a closure of itself ie it has repetitive loop of its own function in closure, Does anyone knows whats happening here, I am very much confused, Why there is infinte closure loop
A closureis a special kind of object that combines two things: a function, and the environment in which that function was created.
No need to be confused, the behavior is same as expected to this code. Here what happening is that when you do console.dir( a ); in your code it returns the Second function, i think it is clear for you.
Now when you will expand this function it will show you in Closure the parent function (environment function) of Second, which is buildList. In you code it is doing the same thing.
Now next thing is to expand this function buildList, what it will show you is the child objects of it, Which are var i = 0; and the function first. Your console is showing as expected.
Now again when you open first() it will show you in Closure the parent function(environment function) of first, which is buildList. (same it did in step 2).
Now it repeats the step 3 again and then step 4. and so onnn...
May be you understand what is happening here.
The developer tools displays the variable a which is a variable that points to a anonymous function/closure.
In javascript a function is defined in a scope and also it can define a scope by its body block. A scope "knows" all variables inside the defining block and all variables that are defined outside of the function but in the hierachy of scopes the scope is defined in.
The tools show you the scope of the returned function (a in this case). The function first is defined in the scope of the function a.
The variable first is also known in the scope of the anonymous function you assigned to the variable first.
And that what what you get on your screen: first is a variable containing a function. In the scope of this function a variable first is known that points to a function. In the scope of this function ...
You see?
Related
I run into a bit of confusion regarding scoping with respect to where a callback is defined.
function test(){
var b = 3
var fun = function(){
var printingFunction = function(){
console.log(b)
}
printingFunction()
}
return fun
}
test()() //prints 3, as expected because of closures
However, the following doesnt work
function test(){
var b = 3
var fun = function(cb){
cb()
}
return fun
}
test()(function(){console.log(b)}) //b is not defined
I would expect that since the function is passed as an argument and has not been defined before, its definition takes place inside 'fun' and therefore it would have access to b. Instead, it looks a lot like the function is first defined in the scope where its passed and THEN passed as an argument. Any ideas/pointers?
EDIT: Some extra pointers.
someFunction("a")
We couldn't possibly claim that "a" is a definition. What happens here implicitly is that "a" is assigned to a variable named by the argument name so var argumentNameInDefintion = "a". This happens in the body of someFunction.
Similarly we cant claim {} is a definition in : someFunction({}). So why would:
someFunction(function(){})
decide that function(){} is a definition is beyond me. Had it been
var a = function(){}
someFunction(a)
everything would make perfect sense. Maybe its just how the language works.
Scoping in JavaScript is lexical. If you look at your examples, you can see that where printingFunction is defined, lexically (e.g., in the source text) b is declared in a containing scope. But in your second example, it isn't. That's why b can't be resolved in your second example but can in your first.
The way it works is that when a function is created, it has a reference to a conceptual object containing the variables and such in the scope in which it's created (which has a fancy name: "Lexical Environment object"); and that object has a reference to the one that contains it. When looking up a variable reference, the JavaScript engine looks at the current lexical environment object and, if it finds the variable, uses it; otherwise, it looks to the previous one in the chain, and so on up to the global one.
More details can be found:
In this SO question's answers
In this post on my anemic little blog
The issue stems from me not understanding that function(){} on its own is a definition and so is "a" and {}. Since these are definitions then the definition scope of the function passed is appropriately placed where it is and the world makes sense again.
In first case, it is forming a closure and has access to the variable "b" but in second case it does not form a closure at all. If you put a debugger just before the cb() inside the function, you will notice that there is no closure formed and the reason of that being the callback function is suplied to the function as an argument and it becomes local to that function which does not have any knowledge of the variable b.
Think of it as two different functions, one in which it has a local variable "b" and in other no local variable but we are trying to access it which throws the reference error.
Disclaimer: This question is purely curiosity driven and has to do a lot with how the javascript works.
I understand why the following code works. Due to closures, foo has access to the scope where a resides. This makes sense.
var a = 10
var foo = function(){
console.log(a);
}
setTimeout(foo,1000)
However, i wonder why the following also works (explained right after).
var a = 10
setTimeout(function(){
console.log(a);
},1000)
The function is defined in the argument of the function receiving it and essentially was never a closure to the scope that contains a. We know that when a function receives an argument, it creates a local variable for that argument so for example
var outerVar="5"
var bar = function(a){
//implicitly, var a = outerVar happens here
console.log(a)
}
bar(something);
So following that logic, the function passed to setTimeout couldnt have access to a and yet it does.
Im suspecting that when a function is defined in the argument space what happens is, it realy is defined before being assigned as an argument but have no proof of that. Any pointers highly appreciated.
Thanks a bunch.
It's not exactly closure, but it's close.
Strictly speaking, closure is when a variable's scope ends, but is still enclosed in an inner function that still lives on:
function createTimer() {
let counter = 0;
return function() {
return counter++;
}
}
const timer = createTimer(); // function() { ... }
console.log(timer(), timer(), timer()); // 0, 1, 2
The function in which counter is defined has returned, the scope ended, and under normal circumstances, counter should have died and garbage collected. But the inner function returned from createTimer() still has a reference to it, from the enclosed scope, that is a closure.
In JavaScript, every function has access to all of the scopes of all of its ancestors, this is what you're seeing here.
The function passed to setTimeout() has access to a because a is defined in the scope around it.
When you look at a javascript code and see how it works, the best way according to me is first understand the javascript engine how it works.
First it traverses the code and assigns all the variables to the scope and further more in the second trace it assigns the values based on the scopes.
So in your first code,
The engine first traverses and assigns
var a to global scope,
var foo as global scope,
Then when setTimeout runs it calls foo function and logs the value of a as that of the global a as it doesnt have any local “a” so checks the lexical scoping.
In your second code,
Var a is again global scoped
and no other declaration this time.
In second traverse it assigns the value 10 to a and interprets the settimeout and prints the value
In your third code,
Same as the second one except the fact that instead what “foo” was giving to the settimeout function, you wrote your callback function then n there itself.
By the time l it executed the setTimeout,
Each of your codes have the value for “a” in the global scope that they are accessing.
First of all - yes, I know there are plenty of posts regarding this, or at least very similar ones. Having looked through those I still haven't found the answer to what I'm looking for:
I've learned that there are two main ways to create functions in javascript:
var test = function(a){
console.log(a);
}
Which is created at runtime, and:
function test(a){
console.log(a);
}
Which is created before runtime.
Today I saw this one:
(function test(a){
console.log(a);
})();
I have never seen that before. What separates this one from the two above?
This is immediately-invoke function, it will call itself immediately after declaration.
You can read more about Immediately-invoked function expression at wikipedia.
What separates them is that the first 2 just define the function, whilst the last one defines and executes the function. Typical use for the last one is to reduce global namespace pollution as variables will be scoped to the function not the window.
Your function examples would be considered in Javascript as follows:
Function expression
Function declaration
Immediately invoked function also called a Immediately invoked function expression "IIFE" and self-executing anonymous function.
What happens in what you call 'runtime' is two things. The Javascript engine makes two passes over your code. First pass could be called the declaration pass where variables and function declaration are identified. However, a funny thing happens with variables in this first pass they are said to be 'hoisted' but not initialized. The second pass is the execution pass. Those are the two things that happen in what I think you are referring to as 'runtime'.
The first function what will happen is the same behavior that happens to variables since is assigned to a variable it will be "hoisted" to the top of it functional context and declared but not initialized. So JS will know its a variable but its recognized as undefined until it reaches the section of code that says 'oh' initialize this variable to a function. So you have to wait until after the initialization step to use.
/* This is how function expression looks after
first pass identified but not assigned(initialized) to function
its hoisted and undefined just like a reg variable. */
var test;
//test is undefined here
console.log(test);
//test is initialize here
var test = function (a) {
console.log(a);
}
//test is recognized as a function here
console.log(test);
The second function will be declared(identified) as function in the first pass and you can use it since JS will know right that this function is a function.
//this is recognized as function
console.log(test);
function test (a) {
console.log(a);
}
//test is recognized as function here as well
console.log(test);
The third function will execute itself right away after being declared in the first pass and create it own execution context, self containing.
//test is not recognized here
console.log(test);
(function test(a) {
console.log(a);
//surprise test is recognized here
console.log(test);
})();
//test is not recognized here
console.log(test);
You also have anonymous functions, and named function expressions. Functions are very powerful in JS. Happy Coding!!
This question already has answers here:
What is the (function() { } )() construct in JavaScript?
(28 answers)
Closed 9 years ago.
I see this all the time in javascript sources but i've never really found out the real reason this construct is used. Why is this needed?
(function() {
//stuff
})();
Why is this written like this? Why not just use stuff by itself and not in a function?
EDIT: i know this is defining an anonymous function and then calling it, but why?
This defines a function closure
This is used to create a function closure with private functionality and variables that aren't globally visible.
Consider the following code:
(function(){
var test = true;
})();
variable test is not visible anywhere else but within the function closure where it's defined.
What is a closure anyway?
Function closures make it possible for various scripts not to interfere with each other even though they define similarly named variables or private functions. Those privates are visible and accessible only within closure itself and not outside of it.
Check this code and read comments along with it:
// public part
var publicVar = 111;
var publicFunc = function(value) { alert(value); };
var publicObject = {
// no functions whatsoever
};
// closure part
(function(pubObj){
// private variables and functions
var closureVar = 222;
var closureFunc = function(value){
// call public func
publicFunc(value);
// alert private variable
alert(closureVar);
};
// add function to public object that accesses private functionality
pubObj.alertValues = closureFunc;
// mind the missing "var" which makes it a public variable
anotherPublic = 333;
})(publicObject);
// alert 111 & alert 222
publicObject.alertValues(publicVar);
// try to access varaibles
alert(publicVar); // alert 111
alert(anotherPublic); // alert 333
alert(typeof(closureVar)); // alert "undefined"
Here's a JSFiddle running code that displays data as indicated by comments in the upper code.
What it actually does?
As you already know this
creates a function:
function() { ... }
and immediately executes it:
(func)();
this function may or may not accept additional parameters.
jQuery plugins are usually defined this way, by defining a function with one parameter that plugin manipulates within:
(function(paramName){ ... })(jQuery);
But the main idea is still the same: define a function closure with private definitions that can't directly be used outside of it.
That construct is known as a self-executing anonymous function, which is actually not a very good name for it, here is what happens (and why the name is not a good one). This:
function abc() {
//stuff
}
Defines a function called abc, if we wanted an anonymous function (which is a very common pattern in javascript), it would be something along the lines of:
function() {
//stuff
}
But, if you have this you either need to associate it with a variable so you can call it (which would make it not-so-anonymous) or you need to execute it straight away. We can try to execute it straight away by doing this:
function() {
//stuff
}();
But this won't work as it will give you a syntax error. The reason you get a syntax error is as follows. When you create a function with a name (such as abc above), that name becomes a reference to a function expression, you can then execute the expression by putting () after the name e.g.: abc(). The act of declaring a function does not create an expression, the function declaration is infact a statement rather than an expression. Essentially, expression are executable and statements are not (as you may have guessed). So in order to execute an anonymous function you need to tell the parser that it is an expression rather than a statement. One way of doing this (not the only way, but it has become convention), is to wrap your anonymous function in a set of () and so you get your construct:
(function() {
//stuff
})();
An anonymous function which is immediately executed (you can see how the name of the construct is a little off since it's not really an anonymous function that executes itself but is rather an anonymous function that is executed straight away).
Ok, so why is all this useful, one reason is the fact that it lets you stop your code from polluting the global namespace. Because functions in javascript have their own scope any variable inside a function is not visible globally, so if we could somehow write all our code inside a function the global scope would be safe, well our self-executing anonymous function allows us to do just that. Let me borrow an example from John Resig's old book:
// Create a new anonymous function, to use as a wrapper
(function(){
// The variable that would, normally, be global
var msg = "Thanks for visiting!";
// Binding a new function to a global object
window.onunload = function(){
// Which uses the 'hidden' variable
alert( msg );
};
// Close off the anonymous function and execute it
})();
All our variables and functions are written within our self-executing anonymous function, our code is executed in the first place because it is inside a self-executing anonymous function. And due to the fact that javascript allows closures, i.e. essentially allows functions to access variables that are defined in an outer function, we can pretty much write whatever code we like inside the self-executing anonymous function and everything will still work as expected.
But wait there is still more :). This construct allows us to solve a problem that sometimes occurs when using closures in javascript. I will once again let John Resig explain, I quote:
Remember that closures allow you to reference variables that exist
within the parent function. However, it does not provide the value of
the variable at the time it is created; it provides the last value of
the variable within the parent function. The most common issue under
which you’ll see this occur is during a for loop. There is one
variable being used as the iterator (e.g., i). Inside of the for loop,
new functions are being created that utilize the closure to reference
the iterator again. The problem is that by the time the new closured
functions are called, they will reference the last value of the
iterator (i.e., the last position in an array), not the value that you
would expect. Listing 2-16 shows an example of using anonymous
functions to induce scope, to create an instance where expected
closure is possible.
// An element with an ID of main
var obj = document.getElementById("main");
// An array of items to bind to
var items = [ "click", "keypress" ];
// Iterate through each of the items
for ( var i = 0; i < items.length; i++ ) {
// Use a self-executed anonymous function to induce scope
(function(){
// Remember the value within this scope
var item = items[i];
// Bind a function to the element
obj[ "on" + item ] = function() {
// item refers to a parent variable that has been successfully
// scoped within the context of this for loop
alert( "Thanks for your " + item );
};
})();
}
Essentially what all of that means is this, people often write naive javascript code like this (this is the naive version of the loop from above):
for ( var i = 0; i < items.length; i++ ) {
var item = items[i];
// Bind a function to the elment
obj[ "on" + item ] = function() {
alert( "Thanks for your " + items[i] );
};
}
The functions we create within the loop are closures, but unfortunately they will lock in the last value of i from the enclosing scope (in this case it will probably be 2 which is gonna cause trouble). What we likely want is for each function we create within the loop to lock in the value of i at the time we create it. This is where our self-executing anonymous function comes in, here is a similar but perhaps easier to understand way of rewriting that loop:
for ( var i = 0; i < items.length; i++ ) {
(function(index){
obj[ "on" + item ] = function() {
alert( "Thanks for your " + items[index] );
};
})(i);
}
Because we invoke our anonymous function on every iteration, the parameter we pass in is locked in to the value it was at the time it was passed in, so all the functions we create within the loop will work as expected.
There you go, two good reasons to use the self-executing anonymous function construct and why it actually works in the first place.
It's used to define an anonymous function and then call it. I haven't tried but my best guess for why there are parens around the block is because JavaScript needs them to understand the function call.
It's useful if you want to define a one-off function in place and then immediately call it. The difference between using the anonymous function and just writing the code out is scope. All the variables in the anonymous function will go out of scope when the function's over with (unless the vars are told otherwise, of course). This can be used to keep the global or enclosing namespace clean, to use less memory long-term, or to get some "privacy".
It is an "anonymous self executing function" or "immediately-invoked-function-expression". Nice explanation from Ben Alman here.
I use the pattern when creating namespaces
var APP = {};
(function(context){
})(APP);
Such a construct is useful when you want to make a closure - a construct helps create a private "room" for variables inaccessible from outside. See more in this chapter of "JavaScript: the good parts" book:
http://books.google.com/books?id=PXa2bby0oQ0C&pg=PA37&lpg=PA37&dq=crockford+closure+called+immediately&source=bl&ots=HIlku8x4jL&sig=-T-T0jTmf7_p_6twzaCq5_5aj3A&hl=lv&ei=lSa5TaXeDMyRswa874nrAw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBUQ6AEwAA#v=onepage&q&f=false
In the example shown on top of page 38, you see that the variable "status" is hidden within a closure and cannot be accessed anyway else than calling the get_status() method.
I'm not sure if this question is answered already, so apologies if I'm just repeating stuff.
In JavaScript, only functions introduce new scope. By wrapping your code in an immediate function, all variables you define exist only in this or lower scope, but not in global scope.
So this is a good way to not pollute the global scope.
There should be only a few global variables. Remember that every global is a property of the window object, which already has a lot of properties by default. Introducing a new scope also avoids collisions with default properties of the window object.
Why does calling my JavaScript function throw an error when I call it like this
wysiwyg2();
var wysiwyg2 = function()
{
alert(1);
}
but work when I do this?
wysiwyg2();
function wysiwyg2 ()
{
alert(1);
}
You need to define your function variable first, i.e.
var wysiwyg2 = function()
{
alert(1);
}
wysiwyg2();
For a good explanation of the difference, see Why can I use a function before it’s defined in Javascript?
In the first snippet, you're trying to invoke a variable before the variable is defined.
You'd get the same problem from the following code:
test.toString();
var test = new Date;
In the second snippet, you're declaring the function without assigning it to a variable, and this results in a global declaration that is usable in earlier code.
You can think of your javascript as though it's evaluated in two passes. The first pass builds all the objects and names (and remember: functions are objects), and places them "in scope", so to speak. It's kind of like a compilation step. Then the second pass executes the code.
So your second sample works because the first pass built and "scoped" the function before execution. The first sample does not work because the function object is created as part of a variable assignment, and so it's not in scope yet when you try to call it.
You mention another situation in the comments, where the function call and definition are separated into two script blocks. That doesn't work because the engine completes both steps of one block before moving on to the next, and you tried to call the function in a block that is executed before the block where it's defined. You can call function across script blocks, but not until they are defined.
In the first one, you're declaring a function and assigning it to a variable. Thus you won't be able to call it through that variable until it's actually assigned to it.
In the second, you're declaring a named function. And can call that function from wherever (as long as it's in scope).
When entering a new execution context (which is either a function call or global code), JavaScript first goes through a variable instantiation phase during which all variable declarations and function declarations within the global code or function body are examined and create as properties of the current variable object, which is effectively a collection of all the objects that are in the current scope. In particular, any function declaration such as
function wysiwyg2 ()
{
alert(1);
}
... is fully created during this phase, while any variable declaration such as
var a = 2;
... only leads to the creation of a variable called a with a value of undefined during this phase. This is also true of a variable declaration with an assignment to a function expression, such as
var wysiwyg2 = function()
{
alert(1);
}
Only the variable instantiation takes place at this point. Once this phase is complete the rest of the code (including variable assignment) is executed sequentially.