What is the purpose of wrapping whole Javascript files in anonymous functions like function

Question

I have been reading a lot of Javascript lately and I have been noticing that the whole file is wrapped like the following in the  js files to be imported    function                  code                 What is the reason for doing this rather than a simple set of constructor functions

User · Answer

Javascript in a browser only really has a couple of effective scopes: function scope and global scope.

If a variable isn't in function scope, it's in global scope. And global variables are generally bad, so this is a construct to keep a library's variables to itself.

User · Answer

To avoid clash with other methods libraries in the same window  Avoid Global scope  make it local scope  To make debugging faster  local scope   JavaScript has function scope only  so it will help in compilation of codes as well

User · Answer

That s called a closure  It basically seals the code inside the function so that other libraries don t interfere with it  It s similar to creating a namespace in compiled languages   Example  Suppose I write    function          var x   2          do stuff with x          Now other libraries cannot access the variable x I created to use in my library

User · Answer

We should also use  use strict  in the scope function to make sure that the code should be executed in  strict mode   Sample code shown below   function          use strict          Your code from here

User · Answer

You can use function closures as data in larger expressions as well  as in this method of determining browser support for some of the html5 objects      navigator html5        canvas   function          var dc  document createElement  canvas          if  dc getContext  return 0        var c  dc getContext  2d          return typeof c fillText    function   2  1                  localStorage   function          return   window localStorage                  webworkers   function          return   window Worker                  offline   function          return   window applicationCache

User · Answer

In short  Summary  In its simplest form  this technique aims to wrap code inside a function scope   It helps decreases chances of    clashing with other applications libraries polluting superior  global most likely  scope   It does not detect when the document is ready - it is not some kind of document onload nor window onload  It is commonly known as an Immediately Invoked Function Expression  IIFE  or Self Executing Anonymous Function   Code Explained  var someFunction   function    console log  wagwan          function                          function scope starts here      console log  start of IIFE       var myNumber   4                 number variable declaration      var myFunction   function        function variable declaration        console log  formidable             var myObject                     object variable declaration        anotherNumber   1001       anotherFunc   function    console log  formidable              console log  end of IIFE                                       function scope ends     someFunction                  reachable  hence works  see in the console myFunction                    unreachable  will throw an error  see in the console myObject anotherFunc          unreachable  will throw an error  see in the console   In the example above  any variable defined in the function  i e  declared using var  will be  private  and accessible within the function scope ONLY  as Vivin Paliath puts it   In other words  these variables are not visible reachable outside the function  See live demo   Javascript has function scoping   Parameters and variables defined in a function are not visible outside of the function  and that a variable defined anywhere within a function is visible everywhere within the function    from  Javascript  The Good Parts       More details  Alternative Code  In the end  the code posted before could also be done as follows   var someFunction   function    console log  wagwan         var myMainFunction   function       console log  start of IIFE       var myNumber   4    var myFunction   function    console log  formidable          var myObject          anotherNumber   1001       anotherFunc   function    console log  formidable              console log  end of IIFE        myMainFunction                I CALL  myMainFunction  FUNCTION HERE someFunction                  reachable  hence works  see in the console myFunction                    unreachable  will throw an error  see in the console myObject anotherFunc          unreachable  will throw an error  see in the console   See live demo     The Roots  Iteration 1  One day  someone probably thought  there must be a way to avoid naming  myMainFunction   since all we want is to execute it immediately    If you go back to the basics  you find out that    expression  something evaluating to a value  i e  3 11 x statement  line s  of code doing something BUT it does not evaluate to a value  i e  if       Similarly  function expressions evaluate to a value  And one consequence  I assume   is that they can be immediately invoked    var italianSayinSomething   function    console log  mamamia            So our more complex example becomes   var someFunction   function    console log  wagwan         var myMainFunction   function       console log  start of IIFE       var myNumber   4    var myFunction   function    console log  formidable          var myObject          anotherNumber   1001       anotherFunc   function    console log  formidable              console log  end of IIFE          someFunction                  reachable  hence works  see in the console myFunction                    unreachable  will throw an error  see in the console myObject anotherFunc          unreachable  will throw an error  see in the console   See live demo   Iteration 2  The next step is the thought  why have var myMainFunction   if we don t even use it      The answer is simple  try removing this  such as below    function    console log  mamamia            See live demo   It won t work because  function declarations are not invokable    The trick is that by removing var myMainFunction   we transformed the function expression into a function declaration  See the links in  Resources  for more details on this   The next question is  why can t I keep it as a function expression with something other than var myMainFunction     The answer is  you can   and there are actually many ways you could do this  adding a    a    a -  or maybe wrapping in a pair of parenthesis  as it s now done by convention   and more I believe   As example     function    console log  mamamia              live demo  jsbin com zokuwodoco 1 edit js console    or    function    console log  mamamia             live demo  jsbin com wuwipiyazi 1 edit js console   or   -function    console log  mamamia             live demo  jsbin com wejupaheva 1 edit js console    What does the exclamation mark do before the function  JavaScript plus sign in front of function name   So once the relevant modification is added to what was once our  Alternative Code   we return to the exact same code as the one used in the  Code Explained  example  var someFunction   function    console log  wagwan          function       console log  start of IIFE       var myNumber   4    var myFunction   function    console log  formidable          var myObject          anotherNumber   1001       anotherFunc   function    console log  formidable              console log  end of IIFE           someFunction                  reachable  hence works  see in the console myFunction                    unreachable  will throw an error  see in the console myObject anotherFunc          unreachable  will throw an error  see in the console   Read more about Expressions vs Statements    developer mozilla org en-US docs Web JavaScript Guide Expressions and Operators developer mozilla org en-US docs Web JavaScript Reference Functions Function constructor vs  function declaration vs  function expression Javascript  difference between a statement and an expression  Expression Versus Statement     Demystifying Scopes  One thing one might wonder is  what happens when you do NOT define the variable  properly  inside the function -- i e  do a simple assignment instead     function       var myNumber   4                 number variable declaration      var myFunction   function        function variable declaration        console log  formidable             var myObject                     object variable declaration        anotherNumber   1001       anotherFunc   function    console log  formidable              myOtherFunction   function        oops  an assignment instead of a declaration        console log  haha  got ya                myOtherFunction               reachable  hence works  see in the console window myOtherFunction        works in the browser  myOtherFunction is then in the global scope myFunction                    unreachable  will throw an error  see in the console   See live demo   Basically  if a variable that was not declared in its current scope is assigned a value  then  a look up the scope chain occurs until it finds the variable or hits the global scope  at which point it will create it     When in a browser environment  vs a server environment like nodejs  the global scope is defined by the window object  Hence we can do window myOtherFunction     My  Good practices  tip on this topic is to always use var when defining anything  whether it s a number  object or function   amp  even when in the global scope  This makes the code much simpler   Note    javascript does not have block scope  Update  block scope local variables added in ES6   javascript has only function scope  amp  global scope  window scope in a browser environment    Read more about Javascript Scopes    What is the purpose of the var keyword and when to use it  or omit it   What is the scope of variables in JavaScript      Resources   youtu be i qE1iAmjFg t 2m15s - Paul Irish presents the IIFE at min 2 15  do watch this  developer mozilla org en-US docs Web JavaScript Reference Functions Book  Javascript  the good parts - highly recommended youtu be i qE1iAmjFg t 4m36s - Paul Irish presents the module pattern at 4 36     Next Steps  Once you get this IIFE concept  it leads to the module pattern  which is commonly done by leveraging this IIFE pattern  Have fun

User · Answer

In addition to keeping the variables local  one very handy use is when writing a library using a global variable  you can give it a shorter variable name to use within the library  It s often used in writing jQuery plugins  since jQuery allows you to disable the   variable pointing to jQuery  using jQuery noConflict    In case it is disabled  your code can still use   and not break if you just do    function         code      jQuery

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It s usually to namespace  see later  and control the visibility of member functions and or variables  Think of it like an object definition  The technical name for it is an Immediately Invoked Function Expression  IIFE   jQuery plugins are usually written like this   In Javascript  you can nest functions  So  the following is legal   function outerFunction        function innerFunction              code          Now you can call outerFunction    but the visiblity of innerFunction   is limited to the scope of outerFunction    meaning it is private to outerFunction    It basically follows the same principle as variables in Javascript   var globalVariable   function someFunction        var localVariable      Correspondingly   function globalFunction         var localFunction1   function              I m anonymous  But localFunction1 is a reference to me            function localFunction2             I m named           In the above scenario  you can call globalFunction   from anywhere  but you cannot call localFunction1 or localFunction2   What you re doing when you write  function               is you re making the code inside the first set of parentheses a function literal  meaning the whole  object  is actually a function   After that  you re self-invoking the function  the final     that you just defined  So the major advantage of this as I mentioned before  is that you can have private methods functions and properties    function        var private var      function private function            code              In the first example  you would explicitly invoke globalFunction by name to run it  That is  you would just do globalFunction   to run it  But in the above example  you re not just defining a function  you re defining and invoking it in one go  This means that when the your JavaScript file is loaded  it is immediately executed  Of course  you could do   function globalFunction            code   globalFunction      The behavior would largely be the same except for one significant difference  you avoid polluting the global scope when you use an IIFE  as a consequence it also means that you cannot invoke the function multiple times since it doesn t have a name  but since this function is only meant to be executed once it really isn t an issue    The neat thing with IIFEs is that you can also define things inside and only expose the parts that you want to the outside world so  an example of namespacing so you can basically create your own library plugin    var myPlugin    function      var private var    function private function          return       public function1  function                public function2  function                     Now you can call myPlugin public function1    but you cannot access private function    So pretty similar to a class definition  To understand this better  I recommend the following links for some further reading    Namespacing your Javascript Private members in Javascript  by Douglas Crockford    EDIT  I forgot to mention  In that final     you can pass anything you want inside  For example  when you create jQuery plugins  you pass in jQuery or   like so     function jQ        code        jQuery     So what you re doing here is defining a function that takes in one parameter  called jQ  a local variable  and known only to that function   Then you re self-invoking the function and passing in a parameter  also called jQuery  but this one is from the outside world and a reference to the actual jQuery itself    There is no pressing need to do this  but there are some advantages    You can redefine a global parameter and give it a name that makes sense in the local scope  There is a slight performance advantage since it is faster to look things up in the local scope instead of having to walk up the scope chain into the global scope  There are benefits for compression  minification     Earlier I described how these functions run automatically at startup  but if they run automatically who is passing in the arguments   This technique assumes that all the parameters you need are already defined as global variables  So if jQuery wasn t already defined as a global variable this example would not work  As you might guess  one things jquery js does during its initialization is define a  jQuery  global variable  as well as its more famous     global variable  which allows this code to work after jQuery has been included

[javascript] What is the purpose of wrapping whole Javascript files in anonymous functions like “(function(){ … })()”?

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