Functions that return a function

Question

I m stuck with this concept of  Functions that return functions   I m referring the book  Object Oriented Javascript  by Stoyan Stefanov   Snippet One      x000D   x000D      function a     x000D         x000D          alert  A     x000D       x000D          function b    x000D              alert  B      x000D            x000D       x000D          return b    x000D        x000D       x000D      var s   a    x000D      alert  break    x000D      s    x000D   x000D   x000D    Output   A  B  break   Snippet Two   x000D   x000D  function a     x000D     x000D      alert  A     x000D   x000D      function b    x000D          alert  B      x000D        x000D   x000D      return b  x000D    x000D   x000D  var s   a    x000D  alert  break    x000D  s    x000D   x000D   x000D   Output   A  break B    Can someone please tell me the difference between returning b and b   in the above snippets

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Returning b is returning a function object   In Javascript  functions are just objects  like any other object   If you find that not helpful  just replace the word  object  with  thing    You can return any object from a function   You can return a true false value  An integer  1 2 3 4       You can return  a string   You can return a complex object with multiple properties   And you can return a function   a function is just a thing    In your case  returning b returns the thing  the thing is a callable function  Returning b   returns the value returned by the callable function     Consider this code   function b        return 42      Using the above definition  return b    returns the value 42   On the other hand return b  returns a function  that itself returns the value of 42   They are two different things

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Imagine the function as a type  like an int  You can return ints in a function  You can return functions too  they are object of type  function    Now the syntax problem  because functions returns values  how can you return a function and not it s returning value   by omitting brackets  Because without brackets  the function won t be executed  So   return b    Will return the  function   imagine it like if you are returning a number   while   return b      First executes the function then return the value obtained by executing it  it s a big difference

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Create a variable   var thing1   undefined    Declare a Function   function something1          return  Hi there  I m number 1        Alert the value of thing1  our first variable    alert thing1      Outputs   undefined     Now  if we wanted thing1 to be a reference to the function something1  meaning it would be the same thing as our created function  we would do   thing1   something1    However  if we wanted the return value of the function then we must assign it the return value of the executed function  You execute the function by using parenthesis   thing1   something1       Value of thing1   Hi there  I m number 1

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Snippet one  function a            alert  A          function b            alert  B                 return b      return nothing here as b not defined a return value    var s   a      s got nothing assigned as b   and thus a   return nothing  alert  break    s       s equals nothing so nothing will be executed  JavaScript interpreter will complain  the statement  b    means to execute the function named  b  which shows a dialog box with text  B   the statement  return b     means to execute a function named  b  and then return what function  b  return  but  b  returns nothing  then this statement  return b    returns nothing either  If b   return a number  then    return b      is a number too  Now    s    is assigned the value of what  a    return  which returns  b     which is nothing  so  s  is nothing  in JavaScript it   s a thing actually  it s an  undefined   So when you ask JavaScript to interpret what data type the  s  is  JavaScript interpreter will tell you  s  is an undefined   As  s  is an undefined  when you ask JavaScript to execute this statement  s     you re asking JavaScript to execute a function named as  s   but  s  here is an  undefined   not a function  so JavaScript will complain   quot hey  s is not a function  I don t know how to do with this s quot   then a  quot Uncaught TypeError  s is not a function quot  error message will be shown by JavaScript  tested in Firefox and Chrome   Snippet Two function a            alert  A          function b            alert  B                 return b    return pointer to function b here    var s   a       s get the value of pointer to b alert  break    s       b   function is executed  now  function  a  returning a pointer alias to a function named  b   so when execute  s a      s  will get a value pointing to b  i e   s  is an alias of  b  now  calling  s  equals calling  b   i e   s  is a function now   Execute  s    means to run function  b   same as executing  b      a dialog box showing  B   will appeared  i e  running the  alert  B     statement in the function  b

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Assigning a variable to a function  without the parenthesis  copies the reference to the function  Putting the parenthesis at the end of a function name  calls the function  returning the functions return value   Demo  function a         alert  A        alerts  A   returns undefined  function b         alert  B        return a      alerts  B   returns function a  function c         alert  C        return a        alerts  C   alerts  A   returns undefined  alert  Function  a  returns     a     alert  Function  b  returns     b     alert  Function  c  returns     c       In your example  you are also defining functions within a function  Such as   function d         function e             alert  E              return e    d        alerts  E    The function is still callable  It still exists  This is used in JavaScript all the time  Functions can be passed around just like other values  Consider the following   function counter         var count   0      return function             alert count             var count   counter    count    count    count      The function count can keep the variables that were defined outside of it  This is called a closure  It s also used a lot in JavaScript

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return b    calls the function b    and returns its result   return b  returns a reference to the function b  which you can store in a variable to call later

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This is super useful in real life   Working with Express js  So your regular express route looks like this     function itWorksHandler  req  res  next       res send  It works        router get   check works   itWorksHandler      But what if you need to add some wrapper  error handler or smth   Then you invoke your function off a wrapper     function loggingWrapper  req  res  next  yourFunction       try       yourFunction  req  res        catch   err         console error  err        next  err           router get   check works   function  req  res  next       loggingWrapper  req  res  next  itWorksHandler          Looks complicated  Well  how about this     function function loggingWrapper  yourFunction     gt    req  res  next       try       yourFunction  req  res  next        catch   err         console error  err        next  err           router get   check works   loggingWrapper  itWorksHandler        See at the end you re passing a function loggingWrapper having one argument as another function itWorksHandler  and your loggingWrapper returns a new function which takes req  res  next as arguments

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When you return b  it is just a reference to function b  but not being executed at this time   When you return b    you re executing the function and returning its value   Try alerting typeof s  in your examples  Snippet b will give you  function   What will snippet a give you

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Returning the function name without    returns a reference to the function  which can be assigned as you ve done with var s   a    s now contains a reference to the function b    and calling s   is functionally equivalent to calling b        Return a reference to the function b       In your example  the reference is assigned to var s return b    Calling the function with    in a return statement executes the function  and returns whatever value was returned by the function   It is similar to calling var x   b     but instead of assigning the return value of b   you are returning it from the calling function a     If the function b   itself does not return a value  the call returns undefined after whatever other work is done by b        Execute function b   and return its value return b       If b   has no return value  this is equivalent to calling b    followed by    return undefined

[javascript] Functions that return a function

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