JavaScript Nested function

Question

I got a piece of code for javascript which I just do not understand   function dmy d        function pad2 n            return  n  lt  10     0    n   n             return pad2 d getUTCDate                   pad2 d getUTCMonth     1                 d getUTCFullYear       function outerFunc base        var punc               inner function     function returnString ext           return base   ext   punc             return returnString      How can a function be defined within another function  Can we call pad2   from outside of my   function    Please put some light on it  Thanks

User · Answer

function x        is equivalent  or very similar  to  var x   function        unless I m mistaken   So there is nothing funny going on

User · Answer

It is called closure   Basically  the function defined within other function is accessible only within this function  But may be passed as a result and then this result may be called   It is a very powerful feature  You can see more explanation here   javascript closures for dummies html mirror on Archive org

User · Answer

Functions are another type of variable in JavaScript  with some nuances of course   Creating a function within another function changes the scope of the function in the same way it would change the scope of a variable  This is especially important for use with closures to reduce total global namespace pollution   The functions defined within another function won t be accessible outside the function unless they have been attached to an object that is accessible outside the function   function foo doBar      function bar           console log   bar            function baz           console log   baz            window baz   baz    if   doBar   bar        In this example  the baz function will be available for use after the foo function has been run  as it s overridden window baz  The bar function will not be available to any context other than scopes contained within the foo function   as a different example   function Fizz qux      this buzz   function        console log  qux             The Fizz function is designed as a constructor so that  when run  it assigns a buzz function to the newly created object

User · Answer

When you declare a function within a function  the inner functions are only available in the scope in which they are declared  or in your case  the pad2 can only be called in the dmy scope   All the variables existing in dmy are visible in pad2  but it doesn t happen the other way around  D

User · Answer

x000D   x000D  function foo     x000D    function bar     x000D      return 1  x000D      x000D    x000D  bar    x000D   x000D   x000D   Will throw an error  Since bar is defined inside foo  bar will only be accessible inside foo  To use bar you need to run it inside foo    x000D   x000D  function foo     x000D    function bar     x000D      return 1  x000D      x000D    bar    x000D    x000D   x000D   x000D

User · Answer

Function-instantiation is allowed inside and outside of functions  Inside those functions  just like variables  the nested functions are local and therefore cannot be obtained from the outside scope   function foo         function bar             return 1            return bar        foo manipulates bar within itself  bar cannot be touched from the outer scope unless it is defined in the outer scope   So this will not work   function foo         function bar             return 1           bar       throws error  bar is not defined

User · Answer

It s perfectly normal in Javascript  and many languages  to have functions inside functions   Take the time to learn the language  don t use it on the basis that it s similar to what you already know  I d suggest watching Douglas Crockford s series of YUI presentations on Javascript  with special focus on Act III  Function the Ultimate  link to video download  slides  and transcript

[javascript] JavaScript Nested function

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