JavaScript Patterns 5.7 Object Constants

javaScript Patterns 5.7 Object Constants

PRinciple

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

// constructorvar Widget = function () {    // implementation...};// constantsWidget.MAX_HEIGHT = 320;Widget.MAX_WIDTH = 480;

3. General-purpose constant object

set(name, value)　// To define a new constantisDefined(name)　// To check whether a constant existsget(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 definedconstant.isDefined("maxwidth"); // false// defineconstant.set("maxwidth", 480); // true// check againconstant.isDefined("maxwidth"); // true// attempt to redefineconstant.set("maxwidth", 320); // false// is the value still intact?constant.get("maxwidth"); // 480

References:

Javascript Patterns -by Stoyan Stefanov(O`Reilly)