What is lexical scope

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What is a brief introduction to lexical scoping

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Lexical scoping  Variables declared outside of a function are global variables and are visible everywhere in a JavaScript program  Variables declared inside a function have function scope and are visible only to code that appears inside that function

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Here s a different angle on this question that we can get by taking a step back and looking at the role of scoping in the larger framework of interpretation  running a program   In other words  imagine that you were building an interpreter  or compiler  for a language and were responsible for computing the output  given a program and some input to it   Interpretation involves keeping track of three things    State - namely  variables and referenced memory locations on the heap and stack  Operations on that state - namely  every line of code in your program The environment in which a given operation runs - namely  the projection of state on an operation    An interpreter starts at the first line of code in a program  computes its environment  runs the line in that environment and captures its effect on the program s state  It then follows the program s control flow to execute the next line of code  and repeats the process till the program ends   The way you compute the environment for any operation is through a formal set of rules defined by the programming language  The term  binding  is frequently used to describe the mapping of the overall state of the program to a value in the environment  Note that by  overall state  we do not mean global state  but rather the sum total of every reachable definition  at any point in the execution    This is the framework in which the scoping problem is defined  Now to the next part of what our options are    As the implementor of the interpreter  you could simplify your task by making the environment as close as possible to the program s state  Accordingly  the environment of a line of code would simply be defined by environment of the previous line of code with the effects of that operation applied to it  regardless of whether the previous line was an assignment  a function call  return from a function  or a control structure such as a while loop    This is the gist of dynamic scoping  wherein the environment that any code runs in is bound to the state of the program as defined by its execution context    Or  you could think of a programmer using your language and simplify his or her task of keeping track of the values a variable can take  There are way too many paths and too much complexity involved in reasoning about the outcome the totality of past execution  Lexical Scoping helps do this by restricting the current environment to the portion of state defined in the current block  function or other unit of scope  and its parent  i e  the block enclosing the current clock  or the function that called the present function     In other words  with lexical scope the environment that any code sees is bound to state associated with a scope defined explicitly in the language  such as a block or a function

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I love the fully featured  language-agnostic answers from folks like  Arak  Since this question was tagged JavaScript though  I d like to chip in some notes very specific to this language   In JavaScript our choices for scoping are    as-is  no scope adjustment  lexical var  this   this  function callback    console log  this     bound callback bind this    It s worth noting  I think  that JavaScript doesn t really have dynamic scoping   bind adjusts the this keyword  and that s close  but not technically the same   Here is an example demonstrating both approaches  You do this every time you make a decision about how to scope callbacks so this applies to promises  event handlers  and more   Lexical  Here is what you might term Lexical Scoping of callbacks in JavaScript   var downloadManager       initialize  function         var  this   this     Set up   this  for lexical access         downloadLink   on  click   function             this startDownload                   startDownload  function        this thinking   true         Request the file from the server and bind more callbacks for when it returns success or failure                  Bound  Another way to scope is to use Function prototype bind   var downloadManager       initialize  function             downloadLink   on  click   function            this startDownload          bind this       Create a function object bound to  this              These methods are  as far as I know  behaviorally equivalent

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var scope    I am global   function whatismyscope       var scope    I am just a local      function func    return scope      return func     whatismyscope       The above code will return  I am just a local   It will not return  I am a global   Because the function func   counts where is was originally defined which is under the scope of function whatismyscope   It will not bother from whatever it is being called the global scope from within another function even   that s why global scope value I am global will not be printed   This is called lexical scoping where  functions are executed using the scope chain that was in effect when they were defined  - according to JavaScript Definition Guide   Lexical scope is a very very powerful concept    Hope this helps

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Lets try the shortest possible definition   Lexical Scoping defines how variable names are resolved in nested functions  inner functions contain the scope of parent functions even if the parent function has returned   That is all there is to it

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Scope defines the area  where functions  variables and such are available  The availability of a variable for example is defined within its  the context  let s say the function  file  or object  they are defined in  We usually call these local variables   The lexical part means that you can derive the scope from reading the source code    Lexical scope is also known as static scope   Dynamic scope defines global variables that can be called or referenced from anywhere after being defined  Sometimes they are called global variables  even though global variables in most programmin languages are of lexical scope  This means  it can be derived from reading the code that the variable is available in this context  Maybe one has to follow a uses or includes clause to find the instatiation or definition  but the code compiler knows about the variable in this place   In dynamic scoping  by contrast  you search in the local function first  then you search in the function that called the local function  then you search in the function that called that function  and so on  up the call stack   Dynamic  refers to change  in that the call stack can be different every time a given function is called  and so the function might hit different variables depending on where it is called from   see here   To see an interesting example for dynamic scope see here   For further details see here and here   Some examples in Delphi Object Pascal  Delphi has lexical scope   unit Main  uses aUnit      makes available all variables in interface section of aUnit  interface    var aGlobal  string     global in the scope of all units that use Main    type      TmyClass   class       strict private aPrivateVar  Integer     only known by objects of this class type                                        lexical  within class definition                                          reserved word private          public aPublicVar  double        known to everyboday that has access to a                                         object of this class type     end   implementation    var aLocalGlobal  string     known to all functions following                                 the definition in this unit      end    The closest Delphi gets to dynamic scope is the RegisterClass   GetClass   function pair  For its use see here   Let s say that the time RegisterClass  TmyClass   is called to register a certain class cannot be predicted by reading the code  it gets called in a button click method called by the user   code calling GetClass  TmyClass   will get a result or not  The call to RegisterClass   does not have to be in the lexical scope of the unit using GetClass     Another possibility for dynamic scope are anonymous methods  closures  in Delphi 2009  as they know the variables of their calling function  It does not follow the calling path from there recursively and therefore is not fully dynamic

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I understand them through examples      First  lexical scope  also called static scope   in C-like syntax   void fun         int x   5       void fun2                 printf   d   x             Every inner level can access its outer levels   There is another way  called dynamic scope used by the first implementation of Lisp  again in a C-like syntax   void fun         printf   d   x      void dummy1         int x   5       fun       void dummy2         int x   10       fun        Here fun can either access x in dummy1 or dummy2  or any x in any function that call fun with x declared in it   dummy1      will print 5   dummy2      will print 10   The first one is called static because it can be deduced at compile-time  and the second is called dynamic because the outer scope is dynamic and depends on the chain call of the functions   I find static scoping easier for the eye  Most languages went this way eventually  even Lisp  can do both  right    Dynamic scoping is like passing references of all variables to the called function   As an example of why the compiler can not deduce the outer dynamic scope of a function  consider our last example  If we write something like this   if    some condition         dummy1    else     dummy2      The call chain depends on a run time condition  If it is true  then the call chain looks like   dummy1 -- gt  fun     If the condition is false   dummy2 -- gt  fun     The outer scope of fun in both cases is the caller plus the caller of the caller and so on   Just to mention that the C language does not allow nested functions nor dynamic scoping

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Ancient question  but here is my take on it   Lexical  static  scope refers to the scope of a variable in the source code   In a language like JavaScript  where functions can be passed around and attached and re-attached to miscellaneous objects  you might have though that scope would depend on who   s calling the function at the time  but it doesn   t  Changing the scope that way would be dynamic scope  and JavaScript doesn   t do that  except possibly with the this object reference   To illustrate the point    x000D   x000D  var a  apple   x000D   x000D  function doit     x000D      var a  aardvark   x000D      return function     x000D          alert a   x000D        x000D    x000D   x000D  var test doit    x000D  test    x000D   x000D   x000D    In the example  the variable a is defined globally  but shadowed in the doit   function  This function returns another function which  as you see  relies on the a variable outside of its own scope   If you run this  you will find that the value used is aardvark  not apple which  though it is in the scope of the test   function  is not in the lexical scope of the original function  That is  the scope used is the scope as it appears in the source code  not the scope where the function is actually used   This fact can have annoying consequences  For example  you might decide that it   s easier to organise your functions separately  and then use them when the time comes  such as in an event handler    x000D   x000D  var a  apple  b  banana   x000D   x000D  function init     x000D    var a  aardvark  b  bandicoot   x000D    document querySelector  button a   onclick function event    x000D      alert a   x000D      x000D    document querySelector  button b   onclick doB  x000D    x000D   x000D  function doB event    x000D    alert b   x000D    x000D   x000D  init    x000D   lt button id  a  gt A lt  button gt  x000D   lt button id  b  gt B lt  button gt  x000D   x000D   x000D    This code sample does one of each  You can see that because of lexical scoping  button A uses the inner variable  while button B doesn   t  You may end up nesting functions more than you would have liked   By the way  in both examples  you will also notice that the inner lexically scoped variables persist even though the containing function function has run its course  This is called closure  and refers to a nested function   s access to outer variables  even if the outer function has finished  JavaScript needs to be smart enough to determine whether those variables are no longer needed  and if not  can garbage collect them

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This topic is strongly related with the built-in bind function and introduced in ECMAScript nbsp 6 Arrow Functions  It was really annoying  because for every new  class   function actually  method we wanted to use  we had to bind this in order to have access to the scope   JavaScript by default doesn t set its scope of this on functions  it doesn t set the context on this   By default you have to explicitly say which context you want to have   The arrow functions automatically gets so-called lexical scope  have access to variable s definition in its containing block   When using arrow functions it automatically binds this to the place where the arrow function was defined in the first place  and the context of this arrow functions is its containing block   See how it works in practice on the simplest examples below   Before Arrow Functions  no lexical scope by default    const programming       language   JavaScript     getLanguage  function         return this language         const globalScope   programming getLanguage  console log globalScope        Output  undefined  const localScope   programming getLanguage bind programming   console log localScope        Output   JavaScript    With arrow functions  lexical scope by default    const programming       language   JavaScript     getLanguage  function         return this language         const arrowFunction        gt        console log programming getLanguage        arrowFunction       Output   JavaScript

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In simple language  lexical scope is a variable defined outside your scope or upper scope is automatically available inside your scope which means you don t need to pass it there   Example   let str  JavaScript    const myFun        gt        console log str      myFun         Output  JavaScript

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I normally learn by example  and here s a little something   const lives   0   function catCircus          this lives   1      const lives   2       const cat1             lives  5          jumps       gt                console log this lives                        cat1 jumps       1     console log cat1        lives  5  jumps   Function  jumps         const cat2             lives  5          jumps       gt                console log lives                        cat2 jumps       2     console log cat2        lives  5  jumps   Function  jumps         const cat3             lives  5          jumps       gt                const lives   3              console log lives                        cat3 jumps       3     console log cat3        lives  5  jumps   Function  jumps         const cat4             lives  5          jumps  function                  console log lives                        cat4 jumps       2     console log cat4        lives  5  jumps   Function  jumps         const cat5             lives  5          jumps  function                  var lives   4              console log lives                        cat5 jumps       4     console log cat5        lives  5  jumps   Function  jumps         const cat6             lives  5          jumps  function                  console log this lives                        cat6 jumps       5     console log cat6        lives  5  jumps   Function  jumps         const cat7             lives  5          jumps  function thrownOutOfWindow                  console log this lives                        cat7 jumps       5     console log cat7        lives  5  jumps   Function  thrownOutOfWindow       catCircus

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Lexical scope refers to the lexicon of identifiers  e g   variables  functions  etc   visible from the current position in the execution stack   - global execution context     - foo     - bar     - function1 execution context         - foo2         - bar2         - function2 execution context             - foo3             - bar3   foo and bar are always within the lexicon of available identifiers because they are global    When function1 is executed  it has access to a lexicon of foo2  bar2  foo  and bar    When function2 is executed  it has access to a lexicon of foo3  bar3  foo2  bar2  foo  and bar   The reason global and or outer functions do not have access to an inner functions identifiers is because the execution of that function has not occurred yet and therefore  none of its identifiers have been allocated to memory  What   s more  once that inner context executes  it is removed from the execution stack  meaning that all of it   s identifiers have been garbage collected and are no longer available   Finally  this is why a nested execution context can ALWAYS access it   s ancestors execution context and thus why it has access to a greater lexicon of identifiers   See    https   tylermcginnis com ultimate-guide-to-execution-contexts-hoisting-scopes-and-closures-in-javascript  https   developer mozilla org en-US docs Glossary Identifier   Special thanks to  robr3rd for help simplifying the above definition

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IBM defines it as      The portion of a program or segment unit in which a declaration   applies  An identifier declared in a routine is known within that   routine and within all nested routines  If a nested routine declares   an item with the same name  the outer item is not available in the   nested routine    Example 1   function x                Variable  a  is only available to function  x  and function  y       In other words the area defined by  x  is the lexical scope of     variable  a             var a    I am a        function y             console log  a             y          outputs  I am a  x      Example 2   function x          var a    I am a        function y                          If a nested routine declares an item with the same name           the outer item is not available in the nested routine                      var a    I am inner a           console log  a             y          outputs  I am inner a  x

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Lexical  AKA static  scoping refers to determining a variable s scope based solely on its position within the textual corpus of code  A variable always refers to its top-level environment  It s good to understand it in relation to dynamic scope

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Lexical scope means that in a nested group of functions  the inner functions have access to the variables and other resources of their parent scope  This means that the child functions are lexically bound to the execution context of their parents  Lexical scope is sometimes also referred to as static scope  function grandfather         var name    Hammad           likes  is not accessible here     function parent                 name  is accessible here             likes  is not accessible here         function child                    Innermost level of the scope chain                 name  is also accessible here             var likes    Coding                      The thing you will notice about lexical scope is that it works forward  meaning name can be accessed by its children s execution contexts  But it doesn t work backward to its parents  meaning that the variable likes cannot be accessed by its parents  This also tells us that variables having the same name in different execution contexts gain precedence from top to bottom of the execution stack  A variable  having a name similar to another variable  in the innermost function  topmost context of the execution stack  will have higher precedence  Source

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There is an important part of the conversation surrounding lexical and dynamic scoping that is missing  a plain explanation of the lifetime of the scoped variable - or when the variable can be accessed   Dynamic scoping only very loosely corresponds to  global  scoping in the way that we traditionally think about it  the reason I bring up the comparison between the two is that it has already been mentioned - and I don t particularly like the linked article s explanation   it is probably best we don t make the comparison between global and dynamic - though supposedly  according to the linked article       it  is useful as a substitute for globally scoped variables    So  in plain English  what s the important distinction between the two scoping mechanisms   Lexical scoping has been defined very well throughout the answers above  lexically scoped variables are available - or  accessible - at the local level of the function in which it was defined   However - as it is not the focus of the OP - dynamic scoping has not received a great deal of attention and the attention it has received means it probably needs a bit more  that s not a criticism of other answers  but rather a  oh  that answer made we wish there was a bit more    So  here s a little bit more   Dynamic scoping means that a variable is accessible to the larger program during the lifetime of the function call - or  while the function is executing  Really  Wikipedia actually does a nice job with the explanation of the difference between the two  So as not to obfuscate it  here is the text that describes dynamic scoping          I n dynamic scoping  or dynamic scope   if a variable name s scope is a   certain function  then its scope is the time-period during which the   function is executing  while the function is running  the variable   name exists  and is bound to its variable  but after the function   returns  the variable name does not exist

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A lexical scope in JavaScript means that a variable defined outside a function can be accessible inside another function defined after the variable declaration  But the opposite is not true  the variables defined inside a function will not be accessible outside that function   This concept is heavily used in closures in JavaScript   Let s say we have the below code   var x   2  var add   function         var y   1      return x   y       Now  when you call add   --  this will print 3   So  the add   function is accessing the global variable x which is defined before method function add  This is called due to lexical scoping in JavaScript

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Lexical scope means that a function looks up variables in the context where it was defined  and not in the scope immediately around it   Look at how lexical scope works in Lisp if you want more detail  The selected answer by Kyle Cronin in Dynamic and Lexical variables in Common Lisp is a lot clearer than the answers here   Coincidentally I only learned about this in a Lisp class  and it happens to apply in JavaScript as well   I ran this code in Chrome s console      JavaScript               Equivalent Lisp var x   5                    setf x 5  console debug x              print x  function print x             defun print-x        console debug x              print x                                 function                    let     var x   10                    x 10       console debug x              print x      print x                      print-x                                  Output   5 10 5

[javascript] What is lexical scope?

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