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?

Answer 1

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.

1. Copy all the code in the above script and paste it into the top textarea on the javascriptcompressor.com page.
2. Check the Base62 encode checkbox, check the Shrink Variables checkbox.
3. Press the Compress button.
4. Paste and save it all in a .js file and add it to your page in the head of your page.

Answer 2

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
});

> constant
=> 7
> constant = 5
=> 7

note: assignment will give you back the assigned value in the console, but the variable's value will not change

Answer 3

If it is worth mentioning, you can define constants in angular using $provide.constant()

angularApp.constant('YOUR_CONSTANT', 'value');

Answer 4

Forget IE and use the const keyword.

Answer 5

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.}

Answer 6

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";