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

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.

Answer 2

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.

Answer 3

in Javascript already exists constants. You define a constant like this:

const name1 = value;

This cannot change through reassignment.

Answer 4

"use strict";

var constants = Object.freeze({
    "π": 3.141592653589793 ,
    "e": 2.718281828459045 ,
    "i": Math.sqrt(-1)
});

constants.π;        // -> 3.141592653589793
constants.π = 3;    // -> TypeError: Cannot assign to read only property 'π' …
constants.π;        // -> 3.141592653589793

delete constants.π; // -> TypeError: Unable to delete property.
constants.π;        // -> 3.141592653589793

See Object.freeze. You can use const if you want to make the constants reference read-only as well.

Answer 5

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}}

Answer 6

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.