Related
In javascript, when would you want to use this:
(function(){
//Bunch of code...
})();
over this:
//Bunch of code...
It's all about variable scoping. Variables declared in the self executing function are, by default, only available to code within the self executing function. This allows code to be written without concern of how variables are named in other blocks of JavaScript code.
For example, as mentioned in a comment by Alexander:
(function() {
var foo = 3;
console.log(foo);
})();
console.log(foo);
This will first log 3 and then throw an error on the next console.log because foo is not defined.
Simplistic. So very normal looking, its almost comforting:
var userName = "Sean";
console.log(name());
function name() {
return userName;
}
However, what if I include a really handy javascript library to my page that translates advanced characters into their base level representations?
Wait... what?
I mean, if someone types in a character with some kind of accent on it, but I only want 'English' characters A-Z in my program? Well... the Spanish 'ñ' and French 'é' characters can be translated into base characters of 'n' and 'e'.
So someone nice person has written a comprehensive character converter out there that I can include in my site... I include it.
One problem: it has a function in it called 'name' same as my function.
This is what's called a collision. We've got two functions declared in the same scope with the same name. We want to avoid this.
So we need to scope our code somehow.
The only way to scope code in javascript is to wrap it in a function:
function main() {
// We are now in our own sound-proofed room and the
// character-converter library's name() function can exist at the
// same time as ours.
var userName = "Sean";
console.log(name());
function name() {
return userName;
}
}
That might solve our problem. Everything is now enclosed and can only be accessed from within our opening and closing braces.
We have a function in a function... which is weird to look at, but totally legal.
Only one problem. Our code doesn't work.
Our userName variable is never echoed into the console!
We can solve this issue by adding a call to our function after our existing code block...
function main() {
// We are now in our own sound-proofed room and the
// character-converter libarary's name() function can exist at the
// same time as ours.
var userName = "Sean";
console.log(name());
function name() {
return userName;
}
}
main();
Or before!
main();
function main() {
// We are now in our own sound-proofed room and the
// character-converter libarary's name() function can exist at the
// same time as ours.
var userName = "Sean";
console.log(name());
function name() {
return userName;
}
}
A secondary concern: What are the chances that the name 'main' hasn't been used yet? ...so very, very slim.
We need MORE scoping. And some way to automatically execute our main() function.
Now we come to auto-execution functions (or self-executing, self-running, whatever).
((){})();
The syntax is awkward as sin. However, it works.
When you wrap a function definition in parentheses, and include a parameter list (another set or parentheses!) it acts as a function call.
So lets look at our code again, with some self-executing syntax:
(function main() {
var userName = "Sean";
console.log(name());
function name() {
return userName;
}
}
)();
So, in most tutorials you read, you will now be bombarded with the term 'anonymous self-executing' or something similar.
After many years of professional development, I strongly urge you to name every function you write for debugging purposes.
When something goes wrong (and it will), you will be checking the backtrace in your browser. It is always easier to narrow your code issues when the entries in the stack trace have names!
Self-invocation (also known as
auto-invocation) is when a function
executes immediately upon its
definition. This is a core pattern and
serves as the foundation for many
other patterns of JavaScript
development.
I am a great fan :) of it because:
It keeps code to a minimum
It enforces separation of behavior from presentation
It provides a closure which prevents naming conflicts
Enormously – (Why you should say its good?)
It’s about defining and executing a function all at once.
You could have that self-executing function return a value and pass the function as a param to another function.
It’s good for encapsulation.
It’s also good for block scoping.
Yeah, you can enclose all your .js files in a self-executing function and can prevent global namespace pollution. ;)
More here.
Namespacing. JavaScript's scopes are function-level.
I can't believe none of the answers mention implied globals.
The (function(){})() construct does not protect against implied globals, which to me is the bigger concern, see http://yuiblog.com/blog/2006/06/01/global-domination/
Basically the function block makes sure all the dependent "global vars" you defined are confined to your program, it does not protect you against defining implicit globals. JSHint or the like can provide recommendations on how to defend against this behavior.
The more concise var App = {} syntax provides a similar level of protection, and may be wrapped in the function block when on 'public' pages. (see Ember.js or SproutCore for real world examples of libraries that use this construct)
As far as private properties go, they are kind of overrated unless you are creating a public framework or library, but if you need to implement them, Douglas Crockford has some good ideas.
I've read all answers, something very important is missing here, I'll KISS. There are 2 main reasons, why I need Self-Executing Anonymous Functions, or better said "Immediately-Invoked Function Expression (IIFE)":
Better namespace management (Avoiding Namespace Pollution -> JS Module)
Closures (Simulating Private Class Members, as known from OOP)
The first one has been explained very well. For the second one, please study following example:
var MyClosureObject = (function (){
var MyName = 'Michael Jackson RIP';
return {
getMyName: function () { return MyName;},
setMyName: function (name) { MyName = name}
}
}());
Attention 1: We are not assigning a function to MyClosureObject, further more the result of invoking that function. Be aware of () in the last line.
Attention 2: What do you additionally have to know about functions in Javascript is that the inner functions get access to the parameters and variables of the functions, they are defined within.
Let us try some experiments:
I can get MyName using getMyName and it works:
console.log(MyClosureObject.getMyName());
// Michael Jackson RIP
The following ingenuous approach would not work:
console.log(MyClosureObject.MyName);
// undefined
But I can set an another name and get the expected result:
MyClosureObject.setMyName('George Michael RIP');
console.log(MyClosureObject.getMyName());
// George Michael RIP
Edit: In the example above MyClosureObject is designed to be used without the newprefix, therefore by convention it should not be capitalized.
Scope isolation, maybe. So that the variables inside the function declaration don't pollute the outer namespace.
Of course, on half the JS implementations out there, they will anyway.
Is there a parameter and the "Bunch of code" returns a function?
var a = function(x) { return function() { document.write(x); } }(something);
Closure. The value of something gets used by the function assigned to a. something could have some varying value (for loop) and every time a has a new function.
Here's a solid example of how a self invoking anonymous function could be useful.
for( var i = 0; i < 10; i++ ) {
setTimeout(function(){
console.log(i)
})
}
Output: 10, 10, 10, 10, 10...
for( var i = 0; i < 10; i++ ) {
(function(num){
setTimeout(function(){
console.log(num)
})
})(i)
}
Output: 0, 1, 2, 3, 4...
Short answer is : to prevent pollution of the Global (or higher) scope.
IIFE (Immediately Invoked Function Expressions) is the best practice for writing scripts as plug-ins, add-ons, user scripts or whatever scripts are expected to work with other people's scripts. This ensures that any variable you define does not give undesired effects on other scripts.
This is the other way to write IIFE expression. I personally prefer this following method:
void function() {
console.log('boo!');
// expected output: "boo!"
}();
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/void
From the example above it is very clear that IIFE can also affect efficiency and performance, because the function that is expected to be run only once will be executed once and then dumped into the void for good. This means that function or method declaration does not remain in memory.
One difference is that the variables that you declare in the function are local, so they go away when you exit the function and they don't conflict with other variables in other or same code.
First you must visit MDN IIFE , Now some points about this
this is Immediately Invoked Function Expression. So when your javascript file invoked from HTML this function called immediately.
This prevents accessing variables within the IIFE idiom as well as polluting the global scope.
Self executing function are used to manage the scope of a Variable.
The scope of a variable is the region of your program in which it is defined.
A global variable has global scope; it is defined everywhere in your JavaScript code and can be accessed from anywhere within the script, even in your functions. On the other hand, variables declared within a function are defined only within the body of the function.
They are local variables, have local scope and can only be accessed within that function. Function parameters also count as local variables and are defined only within the body of the function.
As shown below, you can access the global variables variable inside your function and also note that within the body of a function, a local variable takes precedence over a global variable with the same name.
var globalvar = "globalvar"; // this var can be accessed anywhere within the script
function scope() {
alert(globalvar);
var localvar = "localvar"; //can only be accessed within the function scope
}
scope();
So basically a self executing function allows code to be written without concern of how variables are named in other blocks of javascript code.
Since functions in Javascript are first-class object, by defining it that way, it effectively defines a "class" much like C++ or C#.
That function can define local variables, and have functions within it. The internal functions (effectively instance methods) will have access to the local variables (effectively instance variables), but they will be isolated from the rest of the script.
Self invoked function in javascript:
A self-invoking expression is invoked (started) automatically, without being called. A self-invoking expression is invoked right after its created. This is basically used for avoiding naming conflict as well as for achieving encapsulation. The variables or declared objects are not accessible outside this function. For avoiding the problems of minimization(filename.min) always use self executed function.
(function(){
var foo = {
name: 'bob'
};
console.log(foo.name); // bob
})();
console.log(foo.name); // Reference error
Actually, the above function will be treated as function expression without a name.
The main purpose of wrapping a function with close and open parenthesis is to avoid polluting the global space.
The variables and functions inside the function expression became private (i.e) they will not be available outside of the function.
Given your simple question: "In javascript, when would you want to use this:..."
I like #ken_browning and #sean_holding's answers, but here's another use-case that I don't see mentioned:
let red_tree = new Node(10);
(async function () {
for (let i = 0; i < 1000; i++) {
await red_tree.insert(i);
}
})();
console.log('----->red_tree.printInOrder():', red_tree.printInOrder());
where Node.insert is some asynchronous action.
I can't just call await without the async keyword at the declaration of my function, and i don't need a named function for later use, but need to await that insert call or i need some other richer features (who knows?).
It looks like this question has been answered all ready, but I'll post my input anyway.
I know when I like to use self-executing functions.
var myObject = {
childObject: new function(){
// bunch of code
},
objVar1: <value>,
objVar2: <value>
}
The function allows me to use some extra code to define the childObjects attributes and properties for cleaner code, such as setting commonly used variables or executing mathematic equations; Oh! or error checking. as opposed to being limited to nested object instantiation syntax of...
object: {
childObject: {
childObject: {<value>, <value>, <value>}
},
objVar1: <value>,
objVar2: <value>
}
Coding in general has a lot of obscure ways of doing a lot of the same things, making you wonder, "Why bother?" But new situations keep popping up where you can no longer rely on basic/core principals alone.
You can use this function to return values :
var Test = (function (){
const alternative = function(){ return 'Error Get Function '},
methods = {
GetName: alternative,
GetAge:alternative
}
// If the condition is not met, the default text will be returned
// replace to 55 < 44
if( 55 > 44){
// Function one
methods.GetName = function (name) {
return name;
};
// Function Two
methods.GetAge = function (age) {
return age;
};
}
return methods;
}());
// Call
console.log( Test.GetName("Yehia") );
console.log( Test.GetAge(66) );
Use of this methodology is for closures. Read this link for more about closures.
IIRC it allows you to create private properties and methods.
Learning about IIFEs i understand that using them does:
not pollute the global scope
shields your code from others.
Could someone please help me understand it better and give a real example of when i could get in trouble when using regular function statement (instead od IIFE?)
Let's say you have a decent sized codebase (a few thousand lines or more). Putting every function on the top level won't be a good idea, because then it'll be pretty easy to accidentally write a function name twice, which will cause bugs due to the name collision. For example:
// code around validating user registration
function validate(username) {
return /someValidationRegex/.test(username);
}
// do stuff with validate
// And then, far elsewhere in the code,
// you need to validate an input for sending to the server:
function validate(input) {
return /someOtherValidationRegex/.test(input);
}
// do stuff with validate
This will not work, because the last validate function will overwrite the first validate function, and the code won't work as expected.
Put each segment of code into an IIFE instead, to avoid the possibility of name collisions:
(() => {
// code around validating user registration
function validate(username) {
return /someValidationRegex/.test(username);
}
// do stuff with validate
})();
(() => {
// code around validating an input for sending to the server:
function validate(input) {
return /someOtherValidationRegex/.test(input);
}
// do stuff with validate
})();
This is the technique that Stack Overflow's Javascript uses (at least in some parts).
Another reason to use IIFEs even if you're careful not to duplicate function names is that you may accidentally duplicate a window property. For example, the following is a somewhat common bug people run into:
// name is defined to be a number...
var name = 5;
// but it's actually a string???
console.log(typeof name);
The problem here is that you're accidentally referring to / overwriting the window.name property on the top level, and window.name may only be a string. There are a whole bunch of functions and variables defined on the top level. To avoid name collisions, put everything into an IIFE instead:
(() => {
// name is defined to be a number...
var name = 5;
// And it's still a number, good
console.log(typeof name);
})();
Still, if your codebase is large enough that name collisions are a decent possibility, rather than manually writing IIFEs, I'd highly recommend using a module system instead, like with Webpack. This will allow you to write modules inside their own encapsulated scope, without leakage or name collisions, and will be more maintainable, when each module only contains exactly the code it needs to run, and nothing more. This makes identifying bugs and extending features much easier than one huge long <script> that you have to navigate through manually.
The common advantage of IIFE is that any "Function or Variable" defined inside IIFE, cannot be accessed outside the IIFE block, thus preventing global scope from getting polluted.
IIFE's are used to define and execute functions on fly and use them on the spot without extra line of Code.
(function(){
var firstName = 'Jhon';
console.log(firstName);
})(); // will be executed on the fly
function test(){
var firstName = 'Jhon';
console.log(firstName);
}
test(); // has to call manually
The IIFE will actually run (immediately-invoked function expression), they don't need a trigger or function call to initiate them, so the variable will be set to its response.
Here's a Fiddle to demonstrate this:
var iffe = (function() {
return 'foo';
})();
var func = function() {
return 'bar';
};
console.log('iife = ' + iffe);
console.log('non-iife = ' + func);
In your JS console you'll see something similar to:
iife = foo and
non-iife = function () {
return 'bar'; }
Also in case we need to pass something in the IFFE scope from the outer scope we need to pass them as a parameter and receive this inside the function defined in the IFFE wrap, something like this -:
var iffe = (function(doc) {
return 'foo';
})(document);
This is a link for some more reference - http://benalman.com/news/2010/11/immediately-invoked-function-expression/
In javascript, when would you want to use this:
(function(){
//Bunch of code...
})();
over this:
//Bunch of code...
It's all about variable scoping. Variables declared in the self executing function are, by default, only available to code within the self executing function. This allows code to be written without concern of how variables are named in other blocks of JavaScript code.
For example, as mentioned in a comment by Alexander:
(function() {
var foo = 3;
console.log(foo);
})();
console.log(foo);
This will first log 3 and then throw an error on the next console.log because foo is not defined.
Simplistic. So very normal looking, its almost comforting:
var userName = "Sean";
console.log(name());
function name() {
return userName;
}
However, what if I include a really handy javascript library to my page that translates advanced characters into their base level representations?
Wait... what?
I mean, if someone types in a character with some kind of accent on it, but I only want 'English' characters A-Z in my program? Well... the Spanish 'ñ' and French 'é' characters can be translated into base characters of 'n' and 'e'.
So someone nice person has written a comprehensive character converter out there that I can include in my site... I include it.
One problem: it has a function in it called 'name' same as my function.
This is what's called a collision. We've got two functions declared in the same scope with the same name. We want to avoid this.
So we need to scope our code somehow.
The only way to scope code in javascript is to wrap it in a function:
function main() {
// We are now in our own sound-proofed room and the
// character-converter library's name() function can exist at the
// same time as ours.
var userName = "Sean";
console.log(name());
function name() {
return userName;
}
}
That might solve our problem. Everything is now enclosed and can only be accessed from within our opening and closing braces.
We have a function in a function... which is weird to look at, but totally legal.
Only one problem. Our code doesn't work.
Our userName variable is never echoed into the console!
We can solve this issue by adding a call to our function after our existing code block...
function main() {
// We are now in our own sound-proofed room and the
// character-converter libarary's name() function can exist at the
// same time as ours.
var userName = "Sean";
console.log(name());
function name() {
return userName;
}
}
main();
Or before!
main();
function main() {
// We are now in our own sound-proofed room and the
// character-converter libarary's name() function can exist at the
// same time as ours.
var userName = "Sean";
console.log(name());
function name() {
return userName;
}
}
A secondary concern: What are the chances that the name 'main' hasn't been used yet? ...so very, very slim.
We need MORE scoping. And some way to automatically execute our main() function.
Now we come to auto-execution functions (or self-executing, self-running, whatever).
((){})();
The syntax is awkward as sin. However, it works.
When you wrap a function definition in parentheses, and include a parameter list (another set or parentheses!) it acts as a function call.
So lets look at our code again, with some self-executing syntax:
(function main() {
var userName = "Sean";
console.log(name());
function name() {
return userName;
}
}
)();
So, in most tutorials you read, you will now be bombarded with the term 'anonymous self-executing' or something similar.
After many years of professional development, I strongly urge you to name every function you write for debugging purposes.
When something goes wrong (and it will), you will be checking the backtrace in your browser. It is always easier to narrow your code issues when the entries in the stack trace have names!
Self-invocation (also known as
auto-invocation) is when a function
executes immediately upon its
definition. This is a core pattern and
serves as the foundation for many
other patterns of JavaScript
development.
I am a great fan :) of it because:
It keeps code to a minimum
It enforces separation of behavior from presentation
It provides a closure which prevents naming conflicts
Enormously – (Why you should say its good?)
It’s about defining and executing a function all at once.
You could have that self-executing function return a value and pass the function as a param to another function.
It’s good for encapsulation.
It’s also good for block scoping.
Yeah, you can enclose all your .js files in a self-executing function and can prevent global namespace pollution. ;)
More here.
Namespacing. JavaScript's scopes are function-level.
I can't believe none of the answers mention implied globals.
The (function(){})() construct does not protect against implied globals, which to me is the bigger concern, see http://yuiblog.com/blog/2006/06/01/global-domination/
Basically the function block makes sure all the dependent "global vars" you defined are confined to your program, it does not protect you against defining implicit globals. JSHint or the like can provide recommendations on how to defend against this behavior.
The more concise var App = {} syntax provides a similar level of protection, and may be wrapped in the function block when on 'public' pages. (see Ember.js or SproutCore for real world examples of libraries that use this construct)
As far as private properties go, they are kind of overrated unless you are creating a public framework or library, but if you need to implement them, Douglas Crockford has some good ideas.
I've read all answers, something very important is missing here, I'll KISS. There are 2 main reasons, why I need Self-Executing Anonymous Functions, or better said "Immediately-Invoked Function Expression (IIFE)":
Better namespace management (Avoiding Namespace Pollution -> JS Module)
Closures (Simulating Private Class Members, as known from OOP)
The first one has been explained very well. For the second one, please study following example:
var MyClosureObject = (function (){
var MyName = 'Michael Jackson RIP';
return {
getMyName: function () { return MyName;},
setMyName: function (name) { MyName = name}
}
}());
Attention 1: We are not assigning a function to MyClosureObject, further more the result of invoking that function. Be aware of () in the last line.
Attention 2: What do you additionally have to know about functions in Javascript is that the inner functions get access to the parameters and variables of the functions, they are defined within.
Let us try some experiments:
I can get MyName using getMyName and it works:
console.log(MyClosureObject.getMyName());
// Michael Jackson RIP
The following ingenuous approach would not work:
console.log(MyClosureObject.MyName);
// undefined
But I can set an another name and get the expected result:
MyClosureObject.setMyName('George Michael RIP');
console.log(MyClosureObject.getMyName());
// George Michael RIP
Edit: In the example above MyClosureObject is designed to be used without the newprefix, therefore by convention it should not be capitalized.
Scope isolation, maybe. So that the variables inside the function declaration don't pollute the outer namespace.
Of course, on half the JS implementations out there, they will anyway.
Is there a parameter and the "Bunch of code" returns a function?
var a = function(x) { return function() { document.write(x); } }(something);
Closure. The value of something gets used by the function assigned to a. something could have some varying value (for loop) and every time a has a new function.
Here's a solid example of how a self invoking anonymous function could be useful.
for( var i = 0; i < 10; i++ ) {
setTimeout(function(){
console.log(i)
})
}
Output: 10, 10, 10, 10, 10...
for( var i = 0; i < 10; i++ ) {
(function(num){
setTimeout(function(){
console.log(num)
})
})(i)
}
Output: 0, 1, 2, 3, 4...
Short answer is : to prevent pollution of the Global (or higher) scope.
IIFE (Immediately Invoked Function Expressions) is the best practice for writing scripts as plug-ins, add-ons, user scripts or whatever scripts are expected to work with other people's scripts. This ensures that any variable you define does not give undesired effects on other scripts.
This is the other way to write IIFE expression. I personally prefer this following method:
void function() {
console.log('boo!');
// expected output: "boo!"
}();
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/void
From the example above it is very clear that IIFE can also affect efficiency and performance, because the function that is expected to be run only once will be executed once and then dumped into the void for good. This means that function or method declaration does not remain in memory.
One difference is that the variables that you declare in the function are local, so they go away when you exit the function and they don't conflict with other variables in other or same code.
First you must visit MDN IIFE , Now some points about this
this is Immediately Invoked Function Expression. So when your javascript file invoked from HTML this function called immediately.
This prevents accessing variables within the IIFE idiom as well as polluting the global scope.
Self executing function are used to manage the scope of a Variable.
The scope of a variable is the region of your program in which it is defined.
A global variable has global scope; it is defined everywhere in your JavaScript code and can be accessed from anywhere within the script, even in your functions. On the other hand, variables declared within a function are defined only within the body of the function.
They are local variables, have local scope and can only be accessed within that function. Function parameters also count as local variables and are defined only within the body of the function.
As shown below, you can access the global variables variable inside your function and also note that within the body of a function, a local variable takes precedence over a global variable with the same name.
var globalvar = "globalvar"; // this var can be accessed anywhere within the script
function scope() {
alert(globalvar);
var localvar = "localvar"; //can only be accessed within the function scope
}
scope();
So basically a self executing function allows code to be written without concern of how variables are named in other blocks of javascript code.
Since functions in Javascript are first-class object, by defining it that way, it effectively defines a "class" much like C++ or C#.
That function can define local variables, and have functions within it. The internal functions (effectively instance methods) will have access to the local variables (effectively instance variables), but they will be isolated from the rest of the script.
Self invoked function in javascript:
A self-invoking expression is invoked (started) automatically, without being called. A self-invoking expression is invoked right after its created. This is basically used for avoiding naming conflict as well as for achieving encapsulation. The variables or declared objects are not accessible outside this function. For avoiding the problems of minimization(filename.min) always use self executed function.
(function(){
var foo = {
name: 'bob'
};
console.log(foo.name); // bob
})();
console.log(foo.name); // Reference error
Actually, the above function will be treated as function expression without a name.
The main purpose of wrapping a function with close and open parenthesis is to avoid polluting the global space.
The variables and functions inside the function expression became private (i.e) they will not be available outside of the function.
Given your simple question: "In javascript, when would you want to use this:..."
I like #ken_browning and #sean_holding's answers, but here's another use-case that I don't see mentioned:
let red_tree = new Node(10);
(async function () {
for (let i = 0; i < 1000; i++) {
await red_tree.insert(i);
}
})();
console.log('----->red_tree.printInOrder():', red_tree.printInOrder());
where Node.insert is some asynchronous action.
I can't just call await without the async keyword at the declaration of my function, and i don't need a named function for later use, but need to await that insert call or i need some other richer features (who knows?).
It looks like this question has been answered all ready, but I'll post my input anyway.
I know when I like to use self-executing functions.
var myObject = {
childObject: new function(){
// bunch of code
},
objVar1: <value>,
objVar2: <value>
}
The function allows me to use some extra code to define the childObjects attributes and properties for cleaner code, such as setting commonly used variables or executing mathematic equations; Oh! or error checking. as opposed to being limited to nested object instantiation syntax of...
object: {
childObject: {
childObject: {<value>, <value>, <value>}
},
objVar1: <value>,
objVar2: <value>
}
Coding in general has a lot of obscure ways of doing a lot of the same things, making you wonder, "Why bother?" But new situations keep popping up where you can no longer rely on basic/core principals alone.
You can use this function to return values :
var Test = (function (){
const alternative = function(){ return 'Error Get Function '},
methods = {
GetName: alternative,
GetAge:alternative
}
// If the condition is not met, the default text will be returned
// replace to 55 < 44
if( 55 > 44){
// Function one
methods.GetName = function (name) {
return name;
};
// Function Two
methods.GetAge = function (age) {
return age;
};
}
return methods;
}());
// Call
console.log( Test.GetName("Yehia") );
console.log( Test.GetAge(66) );
Use of this methodology is for closures. Read this link for more about closures.
IIRC it allows you to create private properties and methods.
In javascript, when would you want to use this:
(function(){
//Bunch of code...
})();
over this:
//Bunch of code...
It's all about variable scoping. Variables declared in the self executing function are, by default, only available to code within the self executing function. This allows code to be written without concern of how variables are named in other blocks of JavaScript code.
For example, as mentioned in a comment by Alexander:
(function() {
var foo = 3;
console.log(foo);
})();
console.log(foo);
This will first log 3 and then throw an error on the next console.log because foo is not defined.
Simplistic. So very normal looking, its almost comforting:
var userName = "Sean";
console.log(name());
function name() {
return userName;
}
However, what if I include a really handy javascript library to my page that translates advanced characters into their base level representations?
Wait... what?
I mean, if someone types in a character with some kind of accent on it, but I only want 'English' characters A-Z in my program? Well... the Spanish 'ñ' and French 'é' characters can be translated into base characters of 'n' and 'e'.
So someone nice person has written a comprehensive character converter out there that I can include in my site... I include it.
One problem: it has a function in it called 'name' same as my function.
This is what's called a collision. We've got two functions declared in the same scope with the same name. We want to avoid this.
So we need to scope our code somehow.
The only way to scope code in javascript is to wrap it in a function:
function main() {
// We are now in our own sound-proofed room and the
// character-converter library's name() function can exist at the
// same time as ours.
var userName = "Sean";
console.log(name());
function name() {
return userName;
}
}
That might solve our problem. Everything is now enclosed and can only be accessed from within our opening and closing braces.
We have a function in a function... which is weird to look at, but totally legal.
Only one problem. Our code doesn't work.
Our userName variable is never echoed into the console!
We can solve this issue by adding a call to our function after our existing code block...
function main() {
// We are now in our own sound-proofed room and the
// character-converter libarary's name() function can exist at the
// same time as ours.
var userName = "Sean";
console.log(name());
function name() {
return userName;
}
}
main();
Or before!
main();
function main() {
// We are now in our own sound-proofed room and the
// character-converter libarary's name() function can exist at the
// same time as ours.
var userName = "Sean";
console.log(name());
function name() {
return userName;
}
}
A secondary concern: What are the chances that the name 'main' hasn't been used yet? ...so very, very slim.
We need MORE scoping. And some way to automatically execute our main() function.
Now we come to auto-execution functions (or self-executing, self-running, whatever).
((){})();
The syntax is awkward as sin. However, it works.
When you wrap a function definition in parentheses, and include a parameter list (another set or parentheses!) it acts as a function call.
So lets look at our code again, with some self-executing syntax:
(function main() {
var userName = "Sean";
console.log(name());
function name() {
return userName;
}
}
)();
So, in most tutorials you read, you will now be bombarded with the term 'anonymous self-executing' or something similar.
After many years of professional development, I strongly urge you to name every function you write for debugging purposes.
When something goes wrong (and it will), you will be checking the backtrace in your browser. It is always easier to narrow your code issues when the entries in the stack trace have names!
Self-invocation (also known as
auto-invocation) is when a function
executes immediately upon its
definition. This is a core pattern and
serves as the foundation for many
other patterns of JavaScript
development.
I am a great fan :) of it because:
It keeps code to a minimum
It enforces separation of behavior from presentation
It provides a closure which prevents naming conflicts
Enormously – (Why you should say its good?)
It’s about defining and executing a function all at once.
You could have that self-executing function return a value and pass the function as a param to another function.
It’s good for encapsulation.
It’s also good for block scoping.
Yeah, you can enclose all your .js files in a self-executing function and can prevent global namespace pollution. ;)
More here.
Namespacing. JavaScript's scopes are function-level.
I can't believe none of the answers mention implied globals.
The (function(){})() construct does not protect against implied globals, which to me is the bigger concern, see http://yuiblog.com/blog/2006/06/01/global-domination/
Basically the function block makes sure all the dependent "global vars" you defined are confined to your program, it does not protect you against defining implicit globals. JSHint or the like can provide recommendations on how to defend against this behavior.
The more concise var App = {} syntax provides a similar level of protection, and may be wrapped in the function block when on 'public' pages. (see Ember.js or SproutCore for real world examples of libraries that use this construct)
As far as private properties go, they are kind of overrated unless you are creating a public framework or library, but if you need to implement them, Douglas Crockford has some good ideas.
I've read all answers, something very important is missing here, I'll KISS. There are 2 main reasons, why I need Self-Executing Anonymous Functions, or better said "Immediately-Invoked Function Expression (IIFE)":
Better namespace management (Avoiding Namespace Pollution -> JS Module)
Closures (Simulating Private Class Members, as known from OOP)
The first one has been explained very well. For the second one, please study following example:
var MyClosureObject = (function (){
var MyName = 'Michael Jackson RIP';
return {
getMyName: function () { return MyName;},
setMyName: function (name) { MyName = name}
}
}());
Attention 1: We are not assigning a function to MyClosureObject, further more the result of invoking that function. Be aware of () in the last line.
Attention 2: What do you additionally have to know about functions in Javascript is that the inner functions get access to the parameters and variables of the functions, they are defined within.
Let us try some experiments:
I can get MyName using getMyName and it works:
console.log(MyClosureObject.getMyName());
// Michael Jackson RIP
The following ingenuous approach would not work:
console.log(MyClosureObject.MyName);
// undefined
But I can set an another name and get the expected result:
MyClosureObject.setMyName('George Michael RIP');
console.log(MyClosureObject.getMyName());
// George Michael RIP
Edit: In the example above MyClosureObject is designed to be used without the newprefix, therefore by convention it should not be capitalized.
Scope isolation, maybe. So that the variables inside the function declaration don't pollute the outer namespace.
Of course, on half the JS implementations out there, they will anyway.
Is there a parameter and the "Bunch of code" returns a function?
var a = function(x) { return function() { document.write(x); } }(something);
Closure. The value of something gets used by the function assigned to a. something could have some varying value (for loop) and every time a has a new function.
Here's a solid example of how a self invoking anonymous function could be useful.
for( var i = 0; i < 10; i++ ) {
setTimeout(function(){
console.log(i)
})
}
Output: 10, 10, 10, 10, 10...
for( var i = 0; i < 10; i++ ) {
(function(num){
setTimeout(function(){
console.log(num)
})
})(i)
}
Output: 0, 1, 2, 3, 4...
Short answer is : to prevent pollution of the Global (or higher) scope.
IIFE (Immediately Invoked Function Expressions) is the best practice for writing scripts as plug-ins, add-ons, user scripts or whatever scripts are expected to work with other people's scripts. This ensures that any variable you define does not give undesired effects on other scripts.
This is the other way to write IIFE expression. I personally prefer this following method:
void function() {
console.log('boo!');
// expected output: "boo!"
}();
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/void
From the example above it is very clear that IIFE can also affect efficiency and performance, because the function that is expected to be run only once will be executed once and then dumped into the void for good. This means that function or method declaration does not remain in memory.
One difference is that the variables that you declare in the function are local, so they go away when you exit the function and they don't conflict with other variables in other or same code.
First you must visit MDN IIFE , Now some points about this
this is Immediately Invoked Function Expression. So when your javascript file invoked from HTML this function called immediately.
This prevents accessing variables within the IIFE idiom as well as polluting the global scope.
Self executing function are used to manage the scope of a Variable.
The scope of a variable is the region of your program in which it is defined.
A global variable has global scope; it is defined everywhere in your JavaScript code and can be accessed from anywhere within the script, even in your functions. On the other hand, variables declared within a function are defined only within the body of the function.
They are local variables, have local scope and can only be accessed within that function. Function parameters also count as local variables and are defined only within the body of the function.
As shown below, you can access the global variables variable inside your function and also note that within the body of a function, a local variable takes precedence over a global variable with the same name.
var globalvar = "globalvar"; // this var can be accessed anywhere within the script
function scope() {
alert(globalvar);
var localvar = "localvar"; //can only be accessed within the function scope
}
scope();
So basically a self executing function allows code to be written without concern of how variables are named in other blocks of javascript code.
Since functions in Javascript are first-class object, by defining it that way, it effectively defines a "class" much like C++ or C#.
That function can define local variables, and have functions within it. The internal functions (effectively instance methods) will have access to the local variables (effectively instance variables), but they will be isolated from the rest of the script.
Self invoked function in javascript:
A self-invoking expression is invoked (started) automatically, without being called. A self-invoking expression is invoked right after its created. This is basically used for avoiding naming conflict as well as for achieving encapsulation. The variables or declared objects are not accessible outside this function. For avoiding the problems of minimization(filename.min) always use self executed function.
(function(){
var foo = {
name: 'bob'
};
console.log(foo.name); // bob
})();
console.log(foo.name); // Reference error
Actually, the above function will be treated as function expression without a name.
The main purpose of wrapping a function with close and open parenthesis is to avoid polluting the global space.
The variables and functions inside the function expression became private (i.e) they will not be available outside of the function.
Given your simple question: "In javascript, when would you want to use this:..."
I like #ken_browning and #sean_holding's answers, but here's another use-case that I don't see mentioned:
let red_tree = new Node(10);
(async function () {
for (let i = 0; i < 1000; i++) {
await red_tree.insert(i);
}
})();
console.log('----->red_tree.printInOrder():', red_tree.printInOrder());
where Node.insert is some asynchronous action.
I can't just call await without the async keyword at the declaration of my function, and i don't need a named function for later use, but need to await that insert call or i need some other richer features (who knows?).
It looks like this question has been answered all ready, but I'll post my input anyway.
I know when I like to use self-executing functions.
var myObject = {
childObject: new function(){
// bunch of code
},
objVar1: <value>,
objVar2: <value>
}
The function allows me to use some extra code to define the childObjects attributes and properties for cleaner code, such as setting commonly used variables or executing mathematic equations; Oh! or error checking. as opposed to being limited to nested object instantiation syntax of...
object: {
childObject: {
childObject: {<value>, <value>, <value>}
},
objVar1: <value>,
objVar2: <value>
}
Coding in general has a lot of obscure ways of doing a lot of the same things, making you wonder, "Why bother?" But new situations keep popping up where you can no longer rely on basic/core principals alone.
You can use this function to return values :
var Test = (function (){
const alternative = function(){ return 'Error Get Function '},
methods = {
GetName: alternative,
GetAge:alternative
}
// If the condition is not met, the default text will be returned
// replace to 55 < 44
if( 55 > 44){
// Function one
methods.GetName = function (name) {
return name;
};
// Function Two
methods.GetAge = function (age) {
return age;
};
}
return methods;
}());
// Call
console.log( Test.GetName("Yehia") );
console.log( Test.GetAge(66) );
Use of this methodology is for closures. Read this link for more about closures.
IIRC it allows you to create private properties and methods.
I was curious about how the node.js pattern of nested functions works with the garbage collector of v8.
here's a simple example
readfile("blah", function(str) {
var val = getvaluefromstr(str);
function restofprogram(val2) { ... } (val)
})
if restofprogram is long-running, doesn't that mean that str will never get garbage collected? My understanding is that with node you end up with nested functions a lot. Does this get garbage collected if restofprogram was declared outside, so str could not be in scope? Is this a recommended practice?
EDIT I didn't intend to make the problem complicated. That was just carelessness, so I've modified it.
Simple answer: if value of the str is not referenced from anywhere else (and str itself is not referenced from restofprogram) it will become unreachable as soon as the function (str) { ... } returns.
Details: V8 compiler distinguishes real local variables from so called context variables captured by a closure, shadowed by a with-statement or an eval invocation.
Local variables live on the stack and disappear as soon as function execution completes.
Context variables live in a heap allocated context structure. They disappear when the context structure dies. Important thing to note here is that context variables from the same scope live in the same structure. Let me illustrate it with an example code:
function outer () {
var x; // real local variable
var y; // context variable, referenced by inner1
var z; // context variable, referenced by inner2
function inner1 () {
// references context
use(y);
}
function inner2 () {
// references context
use(z);
}
function inner3 () { /* I am empty but I still capture context implicitly */ }
return [inner1, inner2, inner3];
}
In this example variable x will disappear as soon as outer returns but variables y and z will disappear only when both inner1, inner2 and inner3 die. This happens because y and z are allocated in the same context structure and all three closures implicitly reference this context structure (even inner3 which does not use it explicitly).
Situation gets even more complicated when you start using with-statement, try/catch-statement which on V8 contains an implicit with-statement inside catch clause or global eval.
function complication () {
var x; // context variable
function inner () { /* I am empty but I still capture context implicitly */ }
try { } catch (e) { /* contains implicit with-statement */ }
return inner;
}
In this example x will disappear only when inner dies. Because:
try/catch-contains implicit with-statement in catch clause
V8 assumes that any with-statement shadows all the locals
This forces x to become a context variable and inner captures the context so x exists until inner dies.
In general if you want to be sure that given variable does not retain some object for longer than really needed you can easily destroy this link by assigning null to that variable.
Actually your example is somewhat tricky. Was it on purpose? You seem to be masking the outer val variable with an inner lexically scoped restofprogram()'s val argument, instead of actually using it. But anyway, you're asking about str so let me ignore the trickiness of val in your example just for the sake of simplicity.
My guess would be that the str variable won't get collected before the restofprogram() function finishes, even if it doesn't use it. If the restofprogram() doesn't use str and it doesn't use eval() and new Function() then it could be safely collected but I doubt it would. This would be a tricky optimization for V8 probably not worth the trouble. If there was no eval and new Function() in the language then it would be much easier.
Now, it doesn't have to mean that it would never get collected because any event handler in a single-threaded event loop should finish almost instantly. Otherwise your whole process would be blocked and you'd have bigger problems than one useless variable in memory.
Now I wonder if you didn't mean something else than what you actually wrote in your example. The whole program in Node is just like in the browser – it just registers event callbacks that are fired asynchronously later after the main program body has already finished. Also none of the handlers are blocking so no function is actually taking any noticeable time to finish. I'm not sure if I understood what you actually meant in your question but I hope that what I've written will be helpful to understand how it all works.
Update:
After reading more info in the comments on how your program looks like I can say more.
If your program is something like:
readfile("blah", function (str) {
var val = getvaluefromstr(str);
// do something with val
Server.start(function (request) {
// do something
});
});
Then you can also write it like this:
readfile("blah", function (str) {
var val = getvaluefromstr(str);
// do something with val
Server.start(serverCallback);
});
function serverCallback(request) {
// do something
});
It will make the str go out of scope after Server.start() is called and will eventually get collected. Also, it will make your indentation more manageable which is not to be underestimated for more complex programs.
As for the val you might make it a global variable in this case which would greatly simplify your code. Of course you don't have to, you can wrestle with closures, but in this case making val global or making it live in an outer scope common for both the readfile callback and for the serverCallback function seems like the most straightforward solution.
Remember that everywhere when you can use an anonymous function you can also use a named function, and with those you can choose in which scope do you want them to live.
My guess is that str will NOT be garbage collected because it can be used by restofprogram().
Yes, and str should get GCed if restofprogram was declared outside, except, if you do something like this:
function restofprogram(val) { ... }
readfile("blah", function(str) {
var val = getvaluefromstr(str);
restofprogram(val, str);
});
Or if getvaluefromstr is declared as something like this:
function getvaluefromstr(str) {
return {
orig: str,
some_funky_stuff: 23
};
}
Follow-up-question: Does v8 do just plain'ol GC or does it do a combination of GC and ref. counting (like python?)