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Principle

1. Make variables shouldn't be changed stand out using all caps.
2. Add constants as static properties to the constructor function.

// constructor

var Widget = function () {

    // implementation...

};

// constants

Widget.MAX_HEIGHT = 320;

Widget.MAX_WIDTH = 480;

1. General-purpose constant object

set(name, value)　// To define a new constant

isDefined(name)　// To check whether a constant exists

get(name)　// To get the value of a constant

var constant = (function () {

    var constants = {},

        ownProp = Object.prototype.hasOwnProperty,

        allowed = {

            string: 1,

            number: 1,

            boolean: 1

        },

        prefix = (Math.random() + "_").slice(2);

    return {

        set: function (name, value) {

            if (this.isDefined(name)) {

                return false;

            }

            if (!ownProp.call(allowed, typeof value)) {

                return false;

            }

            constants[prefix + name] = value;

            return true;

        },

        isDefined: function (name) {

            return ownProp.call(constants, prefix + name);

        },

        get: function (name) {

            if (this.isDefined(name)) {

                return constants[prefix + name];

            }

            return null;

        }

    };

}());

Testing the implementation：

// check if defined

constant.isDefined("maxwidth"); // false

// define

constant.set("maxwidth", 480); // true

// check again

constant.isDefined("maxwidth"); // true

// attempt to redefine

constant.set("maxwidth", 320); // false

// is the value still intact?

constant.get("maxwidth"); // 480

References: 

JavaScript Patterns - by Stoyan Stefanov (O`Reilly)