JavaScript基础对象创建模式之对象的常量(028)

Math.PI; // 3.141592653589793
Math.SQRT2; // 1.4142135623730951
Number.MAX_VALUE; // 1.7976931348623157e+308

// constructor
var Widget = function () {
    // implementation...
};
// constants
Widget.MAX_HEIGHT = 320;
Widget.MAX_WIDTH = 480;

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;
        }
    };
}());

// 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