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JavaScript Patterns 5.7 Object Constants

2024-07-10 12:31:41   221人阅读

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;

 

  1. 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)


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