What is a Closure

Question

I asked a question about Currying and closures were mentioned  What is a closure  How does it relate to currying

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To help facilitate understanding of closures it might be useful to examine how they might be implemented in a procedural language. This explanation will follow a simplistic implementation of closures in Scheme.

To start, I must introduce the concept of a namespace. When you enter a command into a Scheme interpreter, it must evaluate the various symbols in the expression and obtain their value. Example:

(define x 3)

(define y 4)

(+ x y) returns 7

The define expressions store the value 3 in the spot for x and the value 4 in the spot for y. Then when we call (+ x y), the interpreter looks up the values in the namespace and is able to perform the operation and return 7.

However, in Scheme there are expressions that allow you to temporarily override the value of a symbol. Here's an example:

(define x 3)

(define y 4)

(let ((x 5))
   (+ x y)) returns 9

x returns 3

What the let keyword does is introduces a new namespace with x as the value 5. You will notice that it's still able to see that y is 4, making the sum returned to be 9. You can also see that once the expression has ended x is back to being 3. In this sense, x has been temporarily masked by the local value.

Procedural and object-oriented languages have a similar concept. Whenever you declare a variable in a function that has the same name as a global variable you get the same effect.

How would we implement this? A simple way is with a linked list - the head contains the new value and the tail contains the old namespace. When you need to look up a symbol, you start at the head and work your way down the tail.

Now let's skip to the implementation of first-class functions for the moment. More or less, a function is a set of instructions to execute when the function is called culminating in the return value. When we read in a function, we can store these instructions behind the scenes and run them when the function is called.

(define x 3)

(define (plus-x y)
  (+ x y))

(let ((x 5))
  (plus-x 4)) returns ?

We define x to be 3 and plus-x to be its parameter, y, plus the value of x. Finally we call plus-x in an environment where x has been masked by a new x, this one valued 5. If we merely store the operation, (+ x y), for the function plus-x, since we're in the context of x being 5 the result returned would be 9. This is what's called dynamic scoping.

However, Scheme, Common Lisp, and many other languages have what's called lexical scoping - in addition to storing the operation (+ x y) we also store the namespace at that particular point. That way, when we're looking up the values we can see that x, in this context, is really 3. This is a closure.

(define x 3)

(define (plus-x y)
  (+ x y))

(let ((x 5))
  (plus-x 4)) returns 7

In summary, we can use a linked list to store the state of the namespace at the time of function definition, allowing us to access variables from enclosing scopes, as well as providing us the ability to locally mask a variable without affecting the rest of the program.

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From Lua org      When a function is written enclosed in another function  it has full access to local variables from the enclosing function  this feature is called lexical scoping  Although that may sound obvious  it is not  Lexical scoping  plus first-class functions  is a powerful concept in a programming language  but few languages support that concept

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Kyle s answer is pretty good  I think the only additional clarification is that the closure is basically a snapshot of the stack at the point that the lambda function is created  Then when the function is re-executed the stack is restored to that state before executing the function  Thus as Kyle mentions  that hidden value  count  is available when the lambda function executes

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Closure is very easy  We can consider it as follows   Closure   function   its lexical environment  Consider the following function   function init         var name      Mozilla         What will be the closure in the above case    Function init   and variables in its lexical environment ie name  Closure   init     name  Consider another function    function init         var name      Mozilla         function displayName            alert name     displayName        What will be the closures here   Inner function can access variables of outer function  displayName   can access the variable name declared in the parent function  init    However  the same local variables in displayName   will be used if they exists   Closure 1   init function      name variable   displayName   function  --  lexical scope  Closure 2   displayName function      name variable   --  lexical scope

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A closure is a function that can reference state in another function  For example  in Python  this uses the closure  inner    def outer  a       b    variable in outer        def inner  c           print a  b  c     return inner    Now the return value from outer   can be saved for later func   outer   test   func  1    prints  test variable in outer   1

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Functions containing no free variables are called pure functions  Functions containing one or more free variables are called closures  var pure   function pure x     return x       only own environment is used    var foo    quot bar quot   var closure   function closure      return foo       foo is a free variable from the outer environment    src  https   leanpub com javascriptallongesix read leanpub-auto-if-functions-without-free-variables-are-pure-are-closures-impure

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A closure is a stateful function that is returned by another function  It acts as a container to remember variables and parameters from its parent scope even if the parent function has finished executing  Consider this simple example    function sayHello       const greeting    quot Hello World quot      return function        anonymous function nameless function     console log greeting          const hello   sayHello       hello holds the returned function hello       - gt  Hello World  Look  we have a function that returns a function  The returned function gets saved to a variable and invoked the line below

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First of all  contrary to what most of the people here tell you  closure is not a function  So what is it  It is a set of symbols defined in a function s  quot surrounding context quot   known as its environment  which make it a CLOSED expression  that is  an expression in which every symbol is defined and has a value  so it can be evaluated   For example  when you have a JavaScript function  function closed x      return x   3     it is a closed expression because all the symbols occurring in it are defined in it  their meanings are clear   so you can evaluate it  In other words  it is self-contained  But if you have a function like this  function open x      return x y   3     it is an open expression because there are symbols in it which have not been defined in it  Namely  y  When looking at this function  we can t tell what y is and what does it mean  we don t know its value  so we cannot evaluate this expression  I e  we cannot call this function until we tell what y is supposed to mean in it  This y is called a free variable  This y begs for a definition  but this definition is not part of the function     it is defined somewhere else  in its  quot surrounding context quot   also known as the environment   At least that s what we hope for  P For example  it could be defined globally  var y   7   function open x      return x y   3     Or it could be defined in a function which wraps it  var global   2   function wrapper y      var w    quot unused quot      return function x        return x y   3         The part of the environment which gives the free variables in an expression their meanings  is the closure  It is called this way  because it turns an open expression into a closed one  by supplying these missing definitions for all of its free variables  so that we could evaluate it  In the example above  the inner function  which we didn t give a name because we didn t need it  is an open expression because the variable y in it is free     its definition is outside the function  in the function which wraps it  The environment for that anonymous function is the set of variables      global  2    w   quot unused quot     y   whatever has been passed to that wrapper function as its parameter  y      Now  the closure is that part of this environment which closes the inner function by supplying the definitions for all its free variables  In our case  the only free variable in the inner function was y  so the closure of that function is this subset of its environment      y   whatever has been passed to that wrapper function as its parameter  y      The other two symbols defined in the environment are not part of the closure of that function  because it doesn t require them to run  They are not needed to close it  More on the theory behind that here  https   stackoverflow com a 36878651 434562 It s worth to note that in the example above  the wrapper function returns its inner function as a value  The moment we call this function can be remote in time from the moment the function has been defined  or created   In particular  its wrapping function is no longer running  and its parameters which has been on the call stack are no longer there  P This makes a problem  because the inner function needs y to be there when it is called  In other words  it requires the variables from its closure to somehow outlive the wrapper function and be there when needed  Therefore  the inner function has to make a snapshot of these variables which make its closure and store them somewhere safe for later use   Somewhere outside the call stack   And this is why people often confuse the term closure to be that special type of function which can do such snapshots of the external variables they use  or the data structure used to store these variables for later  But I hope you understand now that they are not the closure itself     they re just ways to implement closures in a programming language  or language mechanisms which allows the variables from the function s closure to be there when needed  There s a lot of misconceptions around closures which  unnecessarily  make this subject much more confusing and complicated than it actually is

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A simple example in Groovy for your reference   def outer         def x   1     return   - gt  println x      inner   def innerObj   outer   innerObj      prints 1

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If you are from the Java world  you can compare a closure with a member function of a class  Look at this example  var f function      var a 7    var g function        return a        return g      The function g is a closure  g closes a in  So g can be compared with a member function  a can be compared with a class field  and the function f with a class

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I ll give an example  in JavaScript    function makeCounter        var count   0    return function          count    1      return count         var x   makeCounter     x    returns 1  x    returns 2     etc      What this function  makeCounter  does is it returns a function  which we ve called x  that will count up by one each time its called  Since we re not providing any parameters to x it must somehow remember the count  It knows where to find it based on what s called lexical scoping - it must look to the spot where it s defined to find the value  This  hidden  value is what is called a closure   Here is my currying example again   function add  a      return function  b        return a   b         var add3   add 3    add3 4   returns 7   What you can see is that when you call add with the parameter a  which is 3   that value is contained in the closure of the returned function that we re defining to be add3  That way  when we call add3 it knows where to find the a value to perform the addition

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Here is an example illustrating a closure in the Scheme programming language  First we define a function defining a local variable  not visible outside the function    Function using a local variable  define  function     define a 1     display a    prints 1  when calling  function       function    prints 1  display a    fails  a undefined  Here is the same example  but now the function uses a global variable  defined outside the function    Function using a global variable  define b 2   define  function     display b    prints 2  when calling  function       function    prints 2  display 2    prints 2  And finally  here is an example of a function carrying its own closure    Function with closure  define  outer     define c 3     define  inner       display c     inner   outer function returns the inner function as result      define function  outer    function    prints 3

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In a normal situation  variables are bound by scoping rule  Local variables work only within the defined function  Closure is a way of breaking this rule temporarily for convenience   def n times a thing    return lambda  n  a thing   n  end   in the above code  lambda  n  a thing   n  is the closure because a thing is referred by the lambda  an anonymous function creator    Now  if you put the resulting anonymous function in a function variable   foo   n times 4    foo will break the normal scoping rule and start using 4 internally   foo call 3    returns 12

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Here s a real world example of why Closures kick ass    This is straight out of my Javascript code  Let me illustrate   Function prototype delay   function ms      arg            var fn   this        args   Array prototype slice call arguments  1      return window setTimeout function           return fn apply fn  args        ms        And here s how you would use it   var startPlayback   function track      Player play track        startPlayback someTrack     Now imagine you want the playback to start delayed  like for example 5 seconds later after this code snippet runs  Well that s easy with delay and it s closure   startPlayback delay 5000  someTrack      Keep going  do other things   When you call delay with 5000ms  the first snippet runs  and stores the passed in arguments in it s closure  Then 5 seconds later  when the setTimeout callback happens  the closure still maintains those variables  so it can call the original function with the original parameters  This is a type of currying  or function decoration   Without closures  you would have to somehow maintain those variables state outside the function  thus littering code outside the function with something that logically belongs inside it  Using closures can greatly improve the quality and readability of your code

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Variable scope  When you declare a local variable  that variable has a scope  Generally  local variables exist only within the block or function in which you declare them   function       var a   1    console log a      works       console log a      fails   If I try to access a local variable  most languages will look for it in the current scope  then up through the parent scopes until they reach the root scope   var a   1  function       console log a      works       console log a      works   When a block or function is done with  its local variables are no longer needed and are usually blown out of memory   This is how we normally expect things to work   A closure is a persistent local variable scope  A closure is a persistent scope which holds on to local variables even after the code execution has moved out of that block  Languages which support closure  such as JavaScript  Swift  and Ruby  will allow you to keep a reference to a scope  including its parent scopes   even after the block in which those variables were declared has finished executing  provided you keep a reference to that block or function somewhere   The scope object and all its local variables are tied to the function and will persist as long as that function persists   This gives us function portability  We can expect any variables that were in scope when the function was first defined to still be in scope when we later call the function  even if we call the function in a completely different context   For example  Here s a really simple example in JavaScript that illustrates the point   outer   function       var a   1    var inner   function         console log a         return inner     this returns a function    var fnc   outer       execute outer to get inner  fnc      Here I have defined a function within a function  The inner function gains access to all the outer function s local variables  including a  The variable a is in scope for the inner function   Normally when a function exits  all its local variables are blown away  However  if we return the inner function and assign it to a variable fnc so that it persists after outer has exited  all of the variables that were in scope when inner was defined also persist  The variable a has been closed over -- it is within a closure   Note that the variable a is totally private to fnc  This is a way of creating private variables in a functional programming language such as JavaScript   As you might be able to guess  when I call fnc   it prints the value of a  which is  1     In a language without closure  the variable a would have been garbage collected and thrown away when the function outer exited  Calling fnc would have thrown an error because a no longer exists   In JavaScript  the variable a persists because the variable scope is created when the function is first declared and persists for as long as the function continues to exist   a belongs to the scope of outer  The scope of inner has a parent pointer to the scope of outer  fnc is a variable which points to inner  a persists as long as fnc persists  a is within the closure

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In short  function pointer is just a pointer to a location in the program code base  like program counter   Whereas Closure   Function pointer   Stack frame

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tl dr A closure is a function and its scope assigned to  or used as  a variable  Thus  the name closure  the scope and the function is enclosed and used just like any other entity  In depth Wikipedia style explanation According to Wikipedia  a closure is   Techniques for implementing lexically scoped name binding in languages with first-class functions   What does that mean  Lets look into some definitions  I will explain closures and other related definitions by using this example   x000D   x000D  function startAt x        return function  y            return x   y           var closure1   startAt 1   var closure2   startAt 5    console log closure1 3       4  x    1  y    3  console log closure2 3       8  x    5  y    3  x000D   x000D   x000D   First-class functions Basically that means we can use functions just like any other entity  We can modify them  pass them as arguments  return them from functions or assign them for variables  Technically speaking  they are first-class citizens  hence the name  first-class functions  In the example above  startAt returns an  anonymous  function which function get assigned to closure1 and closure2  So as you see JavaScript treats functions just like any other entities  first-class citizens   Name binding Name binding is about finding out what data a variable  identifier  references  The scope is really important here  as that is the thing that will determine how a binding is resolved  In the example above   In the inner anonymous function s scope  y is bound to 3  In startAt s scope  x is bound to 1 or 5  depending on the closure    Inside the anonymous function s scope  x is not bound to any value  so it needs to be resolved in an upper  startAt s  scope  Lexical scoping As Wikipedia says  the scope   Is the region of a computer program where the binding is valid  where the name can be used to refer to the entity   There are two techniques   Lexical  static  scoping  A variable s definition is resolved by searching its containing block or function  then if that fails searching the outer containing block  and so on  Dynamic scoping  Calling function is searched  then the function which called that calling function  and so on  progressing up the call stack   For more explanation  check out this question and take a look at Wikipedia  In the example above  we can see that JavaScript is lexically scoped  because when x is resolved  the binding is searched in the upper  startAt s  scope  based on the source code  the anonymous function that looks for x is defined inside startAt  and not based on the call stack  the way  the scope where  the function was called  Wrapping  closuring  up In our example  when we call startAt  it will return a  first-class  function that will be assigned to closure1 and closure2 thus a closure is created  because the passed variables 1 and 5 will be saved within startAt s scope  that will be enclosed with the returned anonymous function  When we call this anonymous function via closure1 and closure2 with the same argument  3   the value of y will be found immediately  as that is the parameter of that function   but x is not bound in the scope of the anonymous function  so the resolution continues in the  lexically  upper function scope  that was saved in the closure  where x is found to be bound to either 1 or 5  Now we know everything for the summation so the result can be returned  then printed  Now you should understand closures and how they behave  which is a fundamental part of JavaScript  Currying Oh  and you also learned what currying is about  you use functions  closures  to pass each argument of an operation instead of using one functions with multiple parameters

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Closures provide JavaScript with state   State in programming simply means remembering things   Example  var a   0   a   a   1       gt  1 a   a   1       gt  2 a   a   1       gt  3   In the case above  state is stored in the variable  a   We follow by adding 1 to  a  several times  We can only do that because we are able to  remember  the value  The state holder   a   holds that value in memory   Often  in programming languages  you want to keep track of things  remember information and access it at a later time   This  in other languages  is commonly accomplished through the use of classes  A class  just like variables  keeps track of its state  And instances of that class  in turns  also have state within them  State simply means information that you can store and retrieve later   Example  class Bread     constructor  weight        this weight   weight         render          return  My weight is   this weight             How can we access  weight  from within the  render  method  Well  thanks to state  Each instance of the class Bread can render its own weight by reading it from the  state   a place in memory where we could store that information   Now  JavaScript is a very unique language which historically does not have classes  it now does  but under the hood there s only functions and variables  so Closures provide a way for JavaScript to remember things and access them later   Example  var n   0  var count   function        n   n   1    return n      count         1 count         2 count         3   The example above achieved the goal of  keeping state  with a variable  This is great  However  this has the disadvantage that the variable  the  state  holder  is now exposed  We can do better  We can use Closures   Example  var countGenerator   function        var n   0    var count   function          n   n   1      return n          return count      var count   countGenerator    count         1 count         2 count         3   This is fantastic   Now our  count  function can count  It is only able to do so because it can  hold  state  The state in this case is the variable  n   This variable is now closed  Closed in time and space  In time because you won t ever be able to recover it  change it  assign it a value or interact directly with it  In space because it s geographically nested within the  countGenerator  function   Why is this fantastic  Because without involving any other sophisticated and complicated tool  e g  classes  methods  instances  etc  we are able to 1  conceal 2  control from a distance  We conceal the state  the variable  n   which makes it a private variable  We also have created an API that can control this variable in a pre-defined way  In particular  we can call the API like so  count    and that adds 1 to  n  from a  distance   In no way  shape or form anyone will ever be able to access  n  except through the API   JavaScript is truly amazing in its simplicity   Closures are a big part of why this is

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Closures Whenever we have a function defined inside another function  the inner function has access to the variables declared in the outer function  Closures are best explained with examples  In Listing 2-18  you can see that the inner function has access to a variable  variableInOuterFunction  from the outer scope  The variables in the outer function have been closed by  or bound in  the inner function  Hence the term closure  The concept in itself is simple enough and fairly intuitive   Listing 2-18      function outerFunction arg         var variableInOuterFunction   arg        function bar                  console log variableInOuterFunction      Access a variable from the outer scope                Call the local function to demonstrate that it has access to arg      bar               outerFunction  hello closure        logs hello closure    source  http   index-of es Varios Basarat 20Ali 20Syed 20 auth  -Beginning 20Node js-Apress 20 2014  pdf

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Closure is a feature in JavaScript where a function has access to its own scope variables  access to the outer function variables and access to the global variables   Closure has access to its outer function scope even after the outer function has returned  This means a closure can remember and access variables and arguments of its outer function even after the function has finished   The inner function can access the variables defined in its own scope  the outer function   s scope  and the global scope  And the outer function can access the variable defined in its own scope and the global scope   Example of Closure   var globalValue   5   function functOuter       var outerFunctionValue   10       Inner function has access to the outer function value     and the global variables   function functInner         var innerFunctionValue   5      alert globalValue   outerFunctionValue   innerFunctionValue         functInner      functOuter        Output will be 20 which sum of its inner function own variable  outer function variable and global variable value

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Here is another real life example  and using a scripting language popular in games - Lua   I needed to slightly change the way a library function worked to avoid a problem with stdin not being available   local old dofile   dofile  function dofile  filename     if filename    nil then     error   Can not use default of stdin       end    old dofile  filename   end   The value of old dofile disappears when this block of code finishes it s scope  because it s local   however the value has been enclosed in a closure  so the new redefined dofile function CAN access it  or rather a copy stored along with the function as an  upvalue

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A closure is a subprogram and the referencing environment where it was defined      The referencing environment is needed if the subprogram can be called from any arbitrary place in the program      A static-scoped language that does not permit nested subprograms doesn   t need closures      Closures are only needed if a subprogram can access variables in nesting scopes and it can be called from anywhere      To support closures  an implementation may need to provide unlimited extent to some variables  because a subprogram may access a nonlocal variable that is normally no longer alive   Example  function makeAdder x    return function y   return x   y     var add10   makeAdder 10   var add5   makeAdder 5   document write  add 10 to 20      add10 20      lt br   gt     document write  add 5 to 20      add5 20      lt br   gt

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Please have a look below code to understand closure in more deep           for var i 0  i lt  5  i                             setTimeout function                    console log i                  1000                                       Here what will be output  0 1 2 3 4 not that will be 5 5 5 5 5 because of closure  So how it will solve  Answer is below          for var i 0  i lt  5  i                 function j         using IIFE                            setTimeout function                                   console log j                                1000                  i                         Let me simple explain  when a function created nothing happen until it called so for loop in 1st code called 5 times but not called immediately so when it called i e after 1 second and also this is asynchronous so before this for loop finished and store value 5 in var i and finally execute setTimeout function five time and print 5 5 5 5 5  Here how it solve using IIFE i e Immediate Invoking Function Expression          function j      i is passed here                        setTimeout function                               console log j                            1000              i      look here it called immediate that is store i 0 for 1st loop  i 1 for 2nd loop  and so on and print 0 1 2 3 4   For more  please understand execution context to understand closure    There is one more solution to solve this using let  ES6 feature  but under the hood above function is worked   for let i 0  i lt  5  i                     setTimeout function                        console log i                     1000                               Output  0 1 2 3 4       More explanation   In memory  when for loop execute picture make like below   Loop 1        setTimeout function                        console log i                     1000       Loop 2        setTimeout function                        console log i                     1000      Loop 3        setTimeout function                        console log i                     1000      Loop 4        setTimeout function                        console log i                     1000      Loop 5        setTimeout function                        console log i                     1000       Here i is not executed and then after complete loop  var i stored value 5 in memory but it s scope is always visible in it s children function so when function execute inside setTimeout out five time it prints 5 5 5 5 5  so to resolve this use IIFE as explain above

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Currying   It allows you to partially evaluate a function by only passing in a subset of its arguments  Consider this   function multiply  x  y      return x   y     const double   multiply bind null  2    const eight   double 4    eight    8    Closure  A closure is nothing more than accessing a variable outside of a function s scope  It is important to remember that a function inside a function or a nested function isn t a closure  Closures are always used when need to access the variables outside the function scope   function apple x      function google y z        console log x y           google 7 2      apple 3       the answer here will be 21

[functional-programming] What is a 'Closure'?

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