Are there constants in JavaScript

Question

Is there a way to use constants in JavaScript   If not  what s the common practice for specifying variables that are used as constants

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If it is worth mentioning  you can define constants in angular using  provide constant    angularApp constant  YOUR CONSTANT    value

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Checkout https   www npmjs com package constjs  which provides three functions to create enum  string const and bitmap  The returned result is either frozen or sealed thus you can t change delete the properties after they are created  you can neither add new properties to the returned result  create Enum   var ConstJs   require  constjs     var Colors   ConstJs enum  blue red     var myColor   Colors blue   console log myColor isBlue        output true  console log myColor is  blue        output true  console log myColor is  BLUE        output true  console log myColor is 0       output true  console log myColor is Colors blue       output true   console log myColor isRed        output false  console log myColor is  red        output false   console log myColor  id      output blue  console log myColor name        output blue  console log myColor toString        output blue      See how CamelCase is used to generate the isXxx   functions  var AppMode   ConstJs enum  SIGN UP  LOG IN  FORGOT PASSWORD    var curMode   AppMode LOG IN   console log curMode isLogIn        output true  console log curMode isSignUp        output false  console log curMode isForgotPassword        output false    Create String const   var ConstJs   require  constjs     var Weekdays   ConstJs const  Mon  Tue  Wed    console log Weekdays      output  Mon   Mon   Tue   Tue   Wed   Wed     var today   Weekdays Wed  console log today      output   Wed      Create Bitmap   var ConstJs   require  constjs     var ColorFlags   ConstJs bitmap  blue red    console log ColorFlags blue      output false   var StyleFlags   ConstJs bitmap true   rustic model minimalist    console log StyleFlags rustic      output true   var CityFlags   ConstJs bitmap  Chengdu  true  Sydney  false    console log CityFlags Chengdu     output true  console log CityFlags Sydney      output false   var DayFlags   ConstJs bitmap true   Mon  false  Tue  true    console log DayFlags Mon      output false  Default val wont override specified val if the type is boolean     For more information please checkout    https   www npmjs com package constjs or https   github com greenlaw110 constjs   Disclaim  I am the author if this tool

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Yet there is no exact cross browser predefined way to do it   you can achieve it by controlling the scope of variables as showed on other answers   But i will suggest to use name space to distinguish from other variables  this will reduce the chance of collision to minimum from other variables   Proper namespacing like  var iw constant        name  sudhanshu        age  23         all varibale come like this     so while using it will be iw constant name or iw constant age  You can also block adding any new key or changing any key inside iw constant using Object freeze method  However its not supported on legacy browser   ex    Object freeze iw constant     For older browser you can use polyfill for freeze method     If you are ok with calling function following is best cross browser way to define constant  Scoping your object within a self executing function and returning a get function for your constants ex   var iw constant   function           var allConstant                name  sudhanshu                age  23                 all varibale come like this                    return function key             allConstant key                       to get the value use iw constant  name   or iw constant  age         In both example you have to be very careful on name spacing so that your object or function shouldn t be replaced through other library  If object or function itself wil be replaced your whole constant will go

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My opinion  works only with objects    var constants    function      var a   9       GLOBAL CONSTANT  through  return     window   defineGetter    GCONST   function        return a             LOCAL CONSTANT   return       get CONST          return a                   constants CONST   8    9 alert constants CONST     9   Try  But understand - this is object  but not simple variable   Try also just   const a   9

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Since ES2015  JavaScript has a notion of const   const MY CONSTANT    some-value     This will work in pretty much all browsers except IE 8  9 and 10  Some may also need strict mode enabled   You can use var with conventions like ALL CAPS to show that certain values should not be modified if you need to support older browsers or are working with legacy code   var MY CONSTANT    some-value

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You can easily equip your script with a mechanism for constants that can be set but not altered   An attempt to alter them will generate an error        author Keith Evetts 2009 License  LGPL   anonymous function sets up    global function SETCONST  String name  mixed value    global function CONST  String name    constants once set may not be altered - console error is generated   they are retrieved as CONST name    the object holding the constants is private and cannot be accessed from the outer script directly  only through the setter and getter provided        function        var constants           self SETCONST   function name value            if  typeof name      string     throw new Error  constant name is not a string              if   value    throw new Error   no value supplied for constant     name             else if   name in constants      throw new Error  constant     name     is already defined               else                constants name    value               return true                       self CONST   function name            if  typeof name      string     throw new Error  constant name is not a string              if   name in constants     return constants name               else   throw new Error  constant     name     has not been defined                           -------------  demo ----------------------------   SETCONST   VAT   0 175      alert  CONST  VAT          try to alter the value of VAT   try      SETCONST   VAT   0 22        catch   exc           alert  exc message           check old value of VAT remains   alert  CONST  VAT             try to get at constants object directly   constants  DODO      dead bird       error

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Okay  this is ugly  but it gives me a constant in Firefox and Chromium  an inconstant constant  WTF   in Safari and Opera  and a variable in IE   Of course eval   is evil  but without it  IE throws an error  preventing scripts from running   Safari and Opera support the const keyword  but you can change the const s value   In this example  server-side code is writing JavaScript to the page  replacing  0  with a value   try         i can haz const      eval  const FOO   0              for reals      var original FOO      try          FOO   NO         catch err1              no err from Firefox Chrome - fails silently         alert  err1   err1             alert  const   FOO       if FOO    NO                changed in Sf Op - set back to original value         FOO original         catch err2          IE fail     alert  err2   err2          set var  no var keyword - Chrome Firefox complain about redefining const      FOO   0        alert  var   FOO     alert  FOO   FOO     What is this good for   Not much  since it s not cross-browser   At best  maybe a little peace of mind that at least some browsers won t let bookmarklets or third-party script modify the value   Tested with Firefox 2  3  3 6  4  Iron 8  Chrome 10  12  Opera 11  Safari 5  IE 6  9

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The keyword  const  was proposed earlier and now it has been officially included in ES6  By using the const keyword  you can pass a value string that will act as an immutable string

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Another alternative is something like   var constants           MY CONSTANT    myconstant         SOMETHING ELSE   123           constantMap   new function ConstantMap        for var c in constants       function cKey        Object defineProperty constantMap  cKey          enumerable   true        get   function name    return constants cKey                c       Then simply  var foo   constantMap MY CONSTANT  If you were to constantMap MY CONSTANT    bar  it would have no effect as we re trying to use an assignment operator with a getter  hence constantMap MY CONSTANT      myconstant  would remain true

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use strict    var constants   Object freeze        p   3 141592653589793        e   2 718281828459045        i   Math sqrt -1       constants p            - gt  3 141592653589793 constants p   3        - gt  TypeError  Cannot assign to read only property  p      constants p            - gt  3 141592653589793  delete constants p     - gt  TypeError  Unable to delete property  constants p            - gt  3 141592653589793   See Object freeze  You can use const if you want to make the constants reference read-only as well

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Rhino js implements const in addition to what was mentioned above

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ECMAScript 5 does introduce Object defineProperty   Object defineProperty  window  CONSTANT    value   5  writable  false       It s supported in every modern browser  as well as IE   9    See also  Object defineProperty in ES5

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Group constants into structures where possible     Example  in my current game project  I have used below   var CONST WILD TYPES         REGULAR   REGULAR       EXPANDING   EXPANDING       STICKY   STICKY       SHIFTING   SHIFTING       Assignment   var wildType   CONST WILD TYPES REGULAR    Comparision   if  wildType     CONST WILD TYPES REGULAR           do something here     More recently I am using  for comparision   switch  wildType        case CONST WILD TYPES REGULAR             do something here         break      case CONST WILD TYPES EXPANDING             do something here         break      IE11 is with new ES6 standard that has  const  declaration  Above works in earlier browsers like IE8  IE9  amp  IE10

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Declare a readonly named constatnt        Variables declared via const cannot be re-declared or re-assigned       Constants can be declared with uppercase or lowercase  but a common   convention is to use all-uppercase letters       const c     c   9      intialization and declearation at same place const c   9     const c   9    re-declare and initialization is not possible console log c    9

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JavaScript ES6  re- introduced the const keyword which is supported in all major browsers      Variables declared via const cannot be re-declared or re-assigned    Apart from that  const behaves similar to let   It behaves as expected for primitive datatypes  Boolean  Null  Undefined  Number  String  Symbol    const x   1  x   2  console log x      1    as expected  re-assigning fails   Attention  Be aware of the pitfalls regarding objects   const o    x  1   o    x  2   console log o       x  1     as expected  re-assigning fails  o x   2  console log o       x  2      const does not make objects immutable   const a       a    1   console log a      1    as expected  re-assigning fails  a push 1   console log a       1      const does not make objects immutable   If you really need an immutable and absolutely constant object  Just use const ALL CAPS to make your intention clear  It is a good convention to follow for all const declarations anyway  so just rely on it

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Forget IE and use the const keyword

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Introducing constants into JavaScript is at best a hack   A nice way of making persistent and globally accessible values in JavaScript  would be declaring an object literal with some  read-only  properties like this               my  get constant1   return  constant 1                    get constant2   return  constant 2                    get constant3   return  constant 3                    get constantN   return  constant N                       you ll have all your constants grouped in one single  my  accessory object where you can look for your stored values or anything else you may have decided to put there for that matter  Now let s test if it works               my constant1   gt  gt   constant 1              my constant1    new constant 1              my constant1   gt  gt   constant 1     As we can see  the  my constant1  property has preserved its original value  You ve made yourself some nice  green  temporary constants     But of course this will only guard you from accidentally modifying  altering  nullifying  or emptying your property constant value with a direct access as in the given example    Otherwise I still think that constants are for dummies   And I still think that exchanging your great freedom for a small corner of deceptive security is the worst trade possible

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const keyword available in javscript language but it does not support IE browser  Rest all browser supported

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In JavaScript  my preference is to use functions to return constant values     function MY CONSTANT        return  some-value       alert MY CONSTANT

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IE does support constants  sort of  e g     lt script language  VBScript  gt   Const IE CONST   True  lt  script gt   lt script type  text javascript  gt   if  typeof TEST CONST     undefined         const IE CONST   false      alert IE CONST    lt  script gt

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in Javascript already exists constants  You define a constant like this   const name1   value    This cannot change through reassignment

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Are you trying to protect the variables against modification   If so  then you can use a module pattern   var CONFIG    function          var private               MY CONST    1             ANOTHER CONST    2                return           get  function name    return private name                   alert  MY CONST      CONFIG get  MY CONST         1  CONFIG MY CONST    2   alert  MY CONST      CONFIG get  MY CONST         1  CONFIG private MY CONST    2                      error alert  MY CONST      CONFIG get  MY CONST         1   Using this approach  the values cannot be modified   But  you have to use the get   method on CONFIG      If you don t need to strictly protect the variables value  then just do as suggested and use a convention of ALL CAPS

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An improved version of Burke s answer that lets you do CONFIG MY CONST instead of CONFIG get  MY CONST     It requires IE9  or a real web browser   var CONFIG    function         var constants              MY CONST   1           ANOTHER CONST   2             var result           for  var n in constants          if  constants hasOwnProperty n               Object defineProperty result  n    value  constants n           return result            The properties are read-only  only if the initial values are immutable

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I too have had a problem with this  And after quite a while searching for the answer and looking at all the responses by everybody  I think I ve come up with a viable solution to this   It seems that most of the answers that I ve come across is using functions to hold the constants  As many of the users of the MANY forums post about  the functions can be easily over written by users on the client side  I was intrigued by Keith Evetts  answer that the constants object can not be accessed by the outside  but only from the functions on the inside   So I came up with this solution   Put everything inside an anonymous function so that way  the variables  objects  etc  cannot be changed by the client side  Also hide the  real  functions by having other functions call the  real  functions from the inside  I also thought of using functions to check if a function has been changed by a user on the client side  If the functions have been changed  change them back using variables that are  protected  on the inside and cannot be changed     Tested in  IE 9 0 8  Firefox 14 0 1  Chrome 20 0 1180 60 m  Not Tested in Safari     function        The two functions  define and  access are from Keith Evetts 2009 License  LGPL  SETCONST and CONST       They re the same just as he did them  the only things I changed are the variable names and the text     of the error messages            object literal to hold the constants   var j            Global function  define String h  mixed m   I named it define to mimic the way PHP  defines  constants      The argument  h  is the name of the const and has to be a string   m  is the value of the const and has     to exist  If there is already a property with the same name in the object holder  then we throw an error      If not  we add the property and set the value to it  This is a  hidden  function and the user doesn t     see any of your coding call this function  You call the  makeDef   in your code and that function calls     this function     -    You can change the error messages to whatever you want them to say         self  define   function h m          if  typeof h      string     throw new Error  I don  t know what to do             if   m    throw new Error  I don  t know what to do             else if   h in j      throw new Error  We have a problem             else             j h    m            return true                  Global function  makeDef String t  mixed y   I named it makeDef because we  make the define  with this     function  The argument  t  is the name of the const and doesn t need to be all caps because I set it     to upper case within the function   y  is the value of the value of the const and has to exist  I     make different variables to make it harder for a user to figure out whats going on  We then call the      define function with the two new variables  You call this function in your code to set the constant      You can change the error message to whatever you want it to say         self  makeDef   function t  y          if  y    throw new Error  I don  t know what to do     return false          q   t toUpperCase          w   y         define q  w             Global function  getDef String s   I named it getDef because we  get the define  with this function  The     argument  s  is the name of the const and doesn t need to be all capse because I set it to upper case     within the function  I make a different variable to make it harder for a user to figure out whats going     on  The function returns the  access function call  I pass the new variable and the original string     along to the  access function  I do this because if a user is trying to get the value of something  if     there is an error the argument doesn t get displayed with upper case in the error message  You call this     function in your code to get the constant         self  getDef   function s          z   s toUpperCase          return  access z  s             Global function  access String g  String f   I named it access because we  access  the constant through     this function  The argument  g  is the name of the const and its all upper case   f  is also the name     of the const  but its the original string that was passed to the  getDef   function  If there is an     error  the original string   f   is displayed  This makes it harder for a user to figure out how the     constants are being stored  If there is a property with the same name in the object holder  we return     the constant value  If not  we check if the  f  variable exists  if not  set it to the value of  g  and     throw an error  This is a  hidden  function and the user doesn t see any of your coding call this     function  You call the  getDef   function in your code and that function calls this function      You can change the error messages to whatever you want them to say         self  access   function g  f          if  typeof g      string     throw new Error  I don  t know what to do             if   g in j     return j g           else   if  f    f   g    throw new Error  I don  t know what to do  I have no idea what     f     is                 The four variables below are private and cannot be accessed from the outside script except for the     functions inside this anonymous function  These variables are strings of the four above functions and     will be used by the all-dreaded eval   function to set them back to their original if any of them should     be changed by a user trying to hack your code         var  define func string    function h m            if  typeof h      string     throw new Error  I don   t know what to do               if   m    throw new Error  I don   t know what to do               else if   h in j      throw new Error  We have a problem               else               j h    m              return true                    var  makeDef func string    function t  y            if  y    throw new Error  I don   t know what to do     return false            q   t toUpperCase            w   y           define q  w             var  getDef func string    function s            z   s toUpperCase            return  access z  s             var  access func string    function g  f            if  typeof g      string     throw new Error  I don   t know what to do               if   g in j     return j g             else   if  f    f   g    throw new Error  I don   t know what to do  I have no idea what      f      is                    Global function  doFunctionCheck String u   I named it doFunctionCheck because we re  checking the functions      The argument  u  is the name of any of the four above function names you want to check  This function will     check if a specific line of code is inside a given function  If it is  then we do nothing  if not  then     we use the eval   function to set the function back to its original coding using the function string     variables above  This function will also throw an error depending upon the doError variable being set to true     This is a  hidden  function and the user doesn t see any of your coding call this function  You call the     doCodeCheck   function and that function calls this function     -    You can change the error messages to     whatever you want them to say         self  doFunctionCheck   function u          var errMsg    We have a BIG problem  You  ve changed my code          var doError   true        d   u        switch d toLowerCase                       case   getdef                  if  getDef toString   indexOf  z   s toUpperCase         -1      do nothing                    else   eval   getDef      getDef func string   if doError     true    throw new Error errMsg                      break             case   makedef                  if  makeDef toString   indexOf  q   t toUpperCase         -1      do nothing                    else   eval   makeDef      makeDef func string   if doError     true    throw new Error errMsg                      break             case   define                  if  define toString   indexOf  else if  h in j           -1      do nothing                    else   eval   define      define func string   if doError     true    throw new Error errMsg                      break             case   access                  if  access toString   indexOf  else   if  f    f   g         -1      do nothing                    else   eval   access      access func string   if doError     true    throw new Error errMsg                      break             default                  if doError     true    throw new Error  I don  t know what to do                         Global function  doCodeCheck String v   I named it doCodeCheck because we re  doing a code check   The argument      v  is the name of one of the first four functions in this script that you want to check  I make a different     variable to make it harder for a user to figure out whats going on  You call this function in your code to check     if any of the functions has been changed by the user         self  doCodeCheck   function v          l   v         doFunctionCheck l               It also seems that security is really a problem and there is not way to  hide  you programming from the client side  A good idea for me is to compress your code so that it is really hard for anyone  including you  the programmer  to read and understand it  There is a site you can go to  http   javascriptcompressor com    This is not my site  don t worry I m not advertising   This is a site that will let you compress and obfuscate Javascript code for free    Copy all the code in the above script and paste it into the top textarea on the javascriptcompressor com page  Check the Base62 encode checkbox  check the Shrink Variables checkbox  Press the Compress button  Paste and save it all in a  js file and add it to your page in the head of your page

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If you don t mind using functions   var constant   function val       return function             return val            This approach gives you functions instead of regular variables  but it guarantees  that no one can alter the value once it s set   a   constant 10    a       10  b   constant 20    b       20   I personally find this rather pleasant  specially after having gotten used to this pattern from knockout observables    Unless someone redefined the function constant before you called it

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The const keyword is in the ECMAScript 6 draft but it thus far only enjoys a smattering of browser support  http   kangax github io compat-table es6   The syntax is   const CONSTANT NAME   0

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Mozillas MDN Web Docs contain good examples and explanations about const  Excerpt      define MY FAV as a constant and give it the value 7 const MY FAV   7      this will throw an error - Uncaught TypeError  Assignment to constant variable  MY FAV   20    But it is sad that IE9 10 still does not support const  And the reason it s absurd      So  what is IE9 doing with const  So   far  our decision has been to not   support it  It isn   t yet a consensus   feature as it has never been available   on all browsers                In the end  it seems like the best   long term solution for the web is to   leave it out and to wait for   standardization processes to run their   course    They don t implement it because other browsers didn t implement it correctly   Too afraid of making it better  Standards definitions or not  a constant is a constant  set once  never changed   And to all the ideas  Every function can be overwritten  XSS etc    So there is no difference in var or function   return   const is the only real constant   Update  IE11 supports const      IE11 includes support for the well-defined and commonly used features of the emerging ECMAScript 6 standard including let  const  Map  Set  and WeakMap  as well as   proto   for improved interoperability

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In JavaScript my practice has been to avoid constants as much as I can and use strings instead  Problems with constants appear when you want to expose your constants to the outside world   For example one could implement the following Date API   date add 5  MyModule Date DAY  add 12  MyModule Date HOUR    But it s much shorter and more natural to simply write   date add 5   days   add 12   hours     This way  days  and  hours  really act like constants  because you can t change from the outside how many seconds  hours  represents  But it s easy to overwrite MyModule Date HOUR   This kind of approach will also aid in debugging  If Firebug tells you action     18 it s pretty hard to figure out what it means  but when you see action      save  then it s immediately clear

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with the  new  Object api you can do something like this    var obj       Object defineProperty obj   CONSTANT       configurable  false   enumerable  true    writable  false    value   your constant value        take a look at this on the Mozilla MDN for more specifics  It s not a first level variable  as it is attached to an object  but if you have a scope  anything  you can attach it to that  this should work as well   So for example doing this in the global scope will declare a pseudo constant value on the window  which is a really bad idea  you shouldn t declare global vars carelessly   Object defineProperty this   constant       enumerable  true     writable  false     value  7     configurable  false       gt  constant   gt  7  gt  constant   5   gt  7   note  assignment will give you back the assigned value in the console  but the variable s value will not change

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Clearly this shows the need for a standardized cross-browser const keyword   But for now   var myconst   value    or  Object  myconst     value    Both seem sufficient and anything else is like shooting a fly with a bazooka

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For a while  I specified  constants   which still weren t actually constants  in object literals passed through to with   statements  I thought it was so clever  Here s an example   with        MY CONST    some really important value           alert MY CONST       In the past  I also have created a CONST namespace where I would put all of my constants  Again  with the overhead  Sheesh   Now  I just do var MY CONST    whatever   to KISS

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I use const instead of var in my Greasemonkey scripts  but it is because they will run only on Firefox    Name convention can be indeed the way to go  too  I do both

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No  not in general  Firefox implements const but I know IE doesn t      John points to a common naming practice for consts that has been used for years in other languages  I see no reason why you couldn t use that  Of course that doesn t mean someone will not write over the variable s value anyway

[javascript] Are there constants in JavaScript?

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Examples related to constants