What is the purpose of a self executing function in javascript

Question

In javascript  when would you want to use this    function          Bunch of code            over this     Bunch of code

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Namespacing  JavaScript s scopes are function-level

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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   lt value gt       objVar2   lt value gt      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    lt value gt    lt value gt    lt value gt               objVar1   lt value gt       objVar2   lt value gt      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

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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

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Here s a solid example of how a self invoking anonymous function could be useful   for  var i   0  i  lt  10  i         setTimeout function        console log i           Output  10  10  10  10  10     for  var i   0  i  lt  10  i          function num       setTimeout function          console log num              i      Output  0  1  2  3  4

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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    quot globalvar quot      this var can be accessed anywhere within the script  function scope         alert globalvar       var localvar    quot localvar quot     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

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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

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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

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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

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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    x000D   x000D   function     x000D    var foo   3  x000D    console log foo   x000D        x000D   x000D  console log foo   x000D   x000D   x000D    This will first log 3 and then throw an error on the next console log because foo is not defined

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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

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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

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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

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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

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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

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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

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IIRC it allows you to create private properties and methods

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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      and French      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 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 libarary 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 bombard 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   Hugely long-winded and I hope it helps

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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

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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  lt  1000  i              await red tree insert i                console log  ----- gt 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

[javascript] What is the purpose of a self executing function in javascript?

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