How to declare string constants in JavaScript? [duplicate]

Answer 1

Standard freeze function of built-in Object can be used to freeze an object containing constants.

var obj = {
    constant_1 : 'value_1'
};
Object.freeze(obj);
obj.constant_1 = 'value_2';   //Silently does nothing
obj.constant_2 = 'value_3';   //Silently does nothing

In strict mode, setting values on immutable object throws TypeError. For more details, see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/freeze

Answer 2

You can use freeze method of Object to create a constant. For example:

var configObj ={timeOut :36000};
Object.freeze(configObj);

In this way you can not alter the configObj.

Answer 3

Are you using JQuery? Do you want to use the constants in multiple javascript files? Then read on. (This is my answer for a related JQuery question)

There is a handy jQuery method called 'getScript'. Make sure you use the same relative path that you would if accessing the file from your html/jsp/etc files (i.e. the path is NOT relative to where you place the getScript method, but instead relative to your domain path). For example, for an app at localhost:8080/myDomain:

$(document).ready(function() {
  $.getScript('/myDomain/myScriptsDir/constants.js');
  ...

then, if you have this in a file called constants.js:

var jsEnum = { //not really an enum, just an object that serves a similar purpose
  FOO : "foofoo",
  BAR : "barbar",
}

You can now print out 'foofoo' with

jsEnum.FOO

Answer 4

Well, you can do it like so:

(function() {
    var localByaka;
    Object.defineProperty(window, 'Byaka', {
        get: function() {
            return localByaka;
        },
        set: function(val) {
            localByaka = window.Byaka || val;
        }
    });
}());
window.Byaka = "foo"; //set constant
window.Byaka = "bar"; // try resetting it for shits and giggles
window.Byaka; // will allways return foo!

If you do this as above in global scope this will be a true constant, because you cannot overwrite the window object.

I've created a library to create constants and immutable objects in javascript. Its still version 0.2 but it does the trick nicely. http://beckafly.github.io/insulatejs

Answer 5

Starting ECMAScript 2015 (a.k.a ES6), you can use const

const constantString = 'Hello';

But not all browsers/servers support this yet. In order to support this, use a polyfill library like Babel.