Dojo参考指南： dojo/has

dojo/has提供了标准化的特征检测。 它是基于 has.js 项目（约定了方法签名，比如 has方法，有哪些参数，参数顺序等，相当于提供了一个标准，大家都按这个标准去做的话，那么特征检测在所有的库中就可以通用了）

介绍

源文档：http://dojotoolkit.org/reference-guide/1.9/dojo/has.html

在客户端代码里使用浏览器的user-agent检测 或者特征检测都有各自的缺陷。dojo/has API先添加新的特征测试代码，在需要检测某个特征时，在执行特征测试特码， 并缓存结果， 这句有点不好理解，可以使用代码来解释

has.add("dom",function(){return true}); //特征测试
if(has(dom){  // has(dom) 就是需要先检测某个特征。如果有缓存结果，直接调用缓存，如果没有，执行上面的测试代码
 // do someting
}

dojo/has 模块主要是为dojo的模块提供标准的特征测试和特征检测。 虽然 dojo/has API 实现了 has.js 规范中的方法和 规定的特征名，但dojo模块用的是自己的特征检测。 

dojo/has 特征检测是懒实例化， 什么意思呢？ 就是特征测试代码只会在请求某个特征时才会运行，并且为之后相同的特征检测 缓存结果。

Dojo Core 和 Dijit 模块使用 dojo/has 做特征检测。 但还是有很多 DojoX  项目是继续使用dojo.isXXX  的浏览器代理 user agent侦测技术。 这是一个慢慢演进的过程。

dojo/has 可以作为一个插件，通过三元表达式(?:)，实现有条件加载模块。

基本的测试是定义在dojo/has模块，在附加在相应模块上的特征和测试，可以增强相应模块的特征检测。 最常使用的 dojo/sniff 就是扩展 dojo/has, 并添加自己额外的测试。

使用staticHasFeatures 在编译配置里， 并结合 Google 的 closure 可以删除无效代码的路径。 查看 builder 文档获得更多的信息.

使用

为了使用这个模块， 它必须被添加到你模块的依赖列表里， 例如：

define(["dojo/has"], function(has){
  if(has("dom")){
    // Do something based on feature
  }
});

特征检测一般会返回一个 "truthy"(Something which evaluates to TRUE, 看了下goole的解释，代表的这个值是需要评估的，这里主要是突出评估，对像，函数，数组都是truthy. 但比如特征检测用到的测试函数，函数就是一个truthy, 但这个测试函数可能会回返一个false的值。这样理解，不知道是对还是错。) 的值。意思是如果功能存在， 返回一个true, 如果不顾在，返回false.

这个模块可以被用于loader的插件， 这么做， 插件的参数是一个三元逻辑表达式， 第一个参数就是feature, 第二个是当 feature为true时要加载的模块，第三个是为false要加载的模块。

require(["dojo/has!feature?package/module:package/other"], function(featureModule){
  // If feature is true, package/module loaded
  // If feature is false, package/other loaded
});

注册一个特征检测需要提供特征检测的名称和测试函数：

has.add("some-test-name", function(global, document, anElement){
  // Feature dection code, returning a truthy value if true
  return true;
});

测试函数应该接受三个参数：

Argument Description global 引用全局作域 document 引用document对像 anElement 是否需要使用到某个HTML元素 
注意， 任何测试函数都应该自行清理， 这样不会导致内存的溢出。

当你在测试函数后添加一个 true时，会立即运行测试函数。

has.add("some-other-test", function(){
  return false; // Boolean
}, true);

虽然你可以如下，传递一个非封装的函数，但这是不被建议的：

// this is not wrapped in a function, and should be:
has.add("some-other-test", ("foo" in bar)); // or whatever

没有被函数封装， 它会立即执行，而不是之前的那样，在实际需要进行代码检测的路径里执行。

特征名称

 以下的特征测试在Dojo中是有效的。 这个表显示了 哪些模块中有哪些特征测试和测试的名字。 Module Feature Description dojo/_base/browser config-selectorEngine Pre-configured selector engine to use in dojo/query, defaults to dojo/selector/acme. dojo/_base/connect events-keypress-typed Keypresses should only occur a printable character is hit dojo/_base/kernel extend-dojo Defined Dojo modules should push their definitions into the dojo object. In 2.0, it will likely be unusual to augment another object as a result of defining a module. This has feature gives a way to force 2.0 behavior as the code is migrated. dojo/_base/kernel dojo-guarantee-console Ensure that console.log, console.warn, etc. methods are defined dojo/_base/kernel dojo-debug-messages Log internal debug messages generated by Dojo, these include deprecated/experimental warnings along with parser auto-required module names. dojo/_base/kernel dojo-modulePaths Consume module paths defined in config.modulePaths. dojo/_base/kernel dojo-moduleUrl Expose dojo.moduleUrl method, returns a URL relative to a module. Deprecated in 2.0, should use require.toUrl() dojo/_base/lang bug-for-in-skips-shadowed Test for bug where the for-in iterator skips object properties that exist in Object’s prototype (IE6 - ?). dojo/_base/loader dojo-fast-sync-require All dojoRequireCallbacks can be released when all non-dojo/require!, dojo/loadInit! modules are either executed, not requested, or arrived. Potential weakness of this algorithm is that dojo/require will not execute callbacks untilall dependency trees are ready. dojo/_base/loader config-publishRequireResult Publish resolved module values, resulting from a require call, as JavaScript objects referenced by module identifiers in the global namespace. dojo/_base/window quirks Browser is running in Quirks-Mode dojo/dojo host-node 设置Dojo的运行环境在Nodejs平台 dojo/dojo host-rhino Dojo 的运行环境 on the Rhino 平台 dojo/dojo dojo-xhr-factory   dojo/dojo dojo-force-activex-xhr Force XHR provider to use ActiveX API (MSXMLHTTP). dojo/dojo native-xhr Browser has native XHR API, XMLHttpRequest. dojo/dojo dojo-gettext-api Dojo provides API for retrieving text resource contents from a URL. dojo/dojo dojo-loader-eval-hint-url Module location should be used as source hint during eval rather than module identifier. dojo/dojo ie-event-behavior Browser supports legacy IE event behaviour API (attachEvent versus attachEventListener). dojo/has host-browser  Dojo 运行在浏览器平台 dojo/has dom 当前平台支持 DOM  dojo/has dojo-dom-ready-api 当前平台上支持 DOMReady API, 当DOM加载完时，会通知一个监听器,  代码在dojo.js dojo/has dojo-sniff  允许扫描dojo.js 角本中的 data-dojo-config 和 djConfig 标签 dojo/has dom-addeventlistener Standard DOM event API (addEventListener) supported on the current platform. dojo/has touch Touch events are supported on the current platform dojo/has device-width Amount of horizontal space in pixels available on the window dojo/has dom-attributes-explicit DOM node attributes array only lists explicitly user specified attributes, (W3C standard) dojo/has dom-attributes-specified-flag DOM node attribute values provide specified flag to skip attributes user didn’t specify, (IE8) dojo/hccss highcontrast Browser is in ‘high-contrast’ mode dojo/i18n dojo-preload-i18n-Api Define the preload localizations machinery, allow loading of special rollup modules which contain one or more flattened, localized bundles. dojo/i18n dojo-v1x-i18n-Api Define legacy (v1.x) i18n functions dojo/json json-parse Platform supports parsing JSON text to JavaScript objects through native API dojo/json json-stringify Platform supports ‘stringify’ method on native JSON API, allowing serialisation of JavaScript objects to JSON text. dojo/mouse dom-quirks Browser is running in Quirks-Mode dojo/mouse events-mouseenter Browser supports the onmouseenter DOM event dojo/mouse events-mousewheel Browser supports the onmousewheel DOM event dojo/on jscript JavaScript environment provided by the JScript platform, dialect of ECMAScript standard that is used in Microsoft’s Internet Explorer. dojo/on event-orientationchange Browser supports the orientationchange DOM event, used to detect orientation changes in the target device. dojo/on event-stopimmediatepropagation Browser supports the stopImmediatePropagation method on DOM events, used to prevent other event listeners being called. dojo/query array-extensible Native array implementation supports manual extension (not supported in older versions of IE). dojo/ready dojo-config-addOnLoad Consume addOnLoad configuration property. dojo/request/handlers activex Browser platform has ActiveX API methods, provided by Internet Explorer dojo/request/script script-readystatechange DOM supports onreadystatechange event, fired when document.readyState changes dojo/request/xhr native-xhr Browser has native XHR API, XMLHttpRequest dojo/request/xhr dojo-force-activex-xhr Force XHR provider to use ActiveX API (MSXMLHTTP). dojo/request/xhr native-xhr2 Browser’s native XHR implementation supports XHR Level 2 API dojo/request/xhr native-formdata Browser has a native FormData implementation, letting user compile set of key/value pairs to send using XMLHttpRequest dojo/selector/_loader dom-qsa2.1 Browser supports the DOM QuerySelectorAll method available, with Level 2.1 CSS selectors dojo/selector/_loader dom-qsa3 Browser supports DOM QuerySelectorAll method, with Level 3 CSS selectors dojo/selector/lite dom-matches-selector Browser supports the matchesSelector method for testing selector queries directly against DOM nodes.dojo/selector/lite, dom-qsa, Browsers supports the DOM QuerySelectorAll method.” dojo/sniff air Environment is running on the Adobe Air platform dojo/sniff khtml Environment is running on the Konqueror-based platform dojo/sniff webkit Environment is running on the WebKit rendering engine platform dojo/sniff chrome Environment is running on the Chrome browser platform dojo/sniff safari Environment is running on the Safari browser platform dojo/sniff mac Environment is running on the Mac OS X platform dojo/sniff quirks Browser is running in Quirks-Mode dojo/sniff ios Environment is running on the iOS mobile operating system dojo/sniff android Environment is running on the Android mobile operating system dojo/sniff opera Environment is running on the Opera browser platform dojo/sniff mozilla Environment is running on the Mozilla browser platform dojo/sniff ff Environment is running on the Firefox browser platform dojo/sniff ie Environment is running on the Microsoft Internet Explorer browser platform dojo/sniff wii Environment is running on the Nintendo Wii browser platform dijt/BackgroundIframe config-bgIframe Flag to create background iframe behind popups like Menus and Dialog. A background iframe prevents problems with popups appearing behind applets/pdf viewers, and also prevents the bleed through select problem on IE6 and IE7. dijit/_WidgetBase dijit-legacy-requires Make dijit load modules the application didn’t explicitly require, e.g. dijit/_base/manager, backwards compatibility in non-async mode. dijit/form/_ExpandingTextAreaMixin textarea-needs-help-shrinking Browser platform’s <textarea> element needs manual help to shrink as content changes. dojox/form/uploader/Base FormData Browser has a native FormData implementation, letting user compile set of key/value pairs to send using XMLHttpRequest dojox/form/uploader/Base xhr-sendAsBinary Browser’s native XHR implementation supports the sendAsBinary method, for sending binary data over XHR. dojox/form/uploader/Base file-multiple Browser supports file input DOM element with multiple file selection attribute, allowing user to select more than one file. dojox/mobile/Audio mobile-embed-audio-video-support Platform supports creating embed tags with audio and video elements. dojox/mobile/common mblAndroidWorkaround Test for Android 2.X transition animation flicker issue dojox/mobile/common mblAndroid3Workaround Test for Android 3.X transition animation flicker issue dojox/mobile/scrollable translate3d Browser supports the WebKit-specific CSS transform property, translate3d. dojox/mobile/sniff bb Environment is running on the RIM Blackberry mobile browser platform dojox/mobile/sniff android Environment is running on the Android mobile browser platform dojox/mobile/sniff iphone Environment is running on the iPhone mobile browser platform dojox/mobile/sniff touch Touch events are supported on the current platform dojox/mvc/_InlineTemplateMixin dom-qsa Browser supports the DOM QuerySelectorAll method dojox/mvc/parserExtension dom-qsa Browser supports the DOM QuerySelectorAll method dojox/mvc/parserExtension dojo-parser Browser has loaded the dojo/parser module dojox/mvc/parserExtension dojo-mobile-parser Browser has loaded the dojox/mobile/parser module dojox/mvc/sync mvc-bindings-log-api Enable debugging messages for MVC module.