先回答我:為什麼模組很重要?
答案:因為有了模組,我們就可以更方便使用別人的程式碼,想要什麼功能,就載入什麼模組。
但是,這樣做有一個前提,那就是大家必須以同樣的方式寫模組,否則你有你的寫法,我有我的寫法,豈不是亂了套!
於是下面三個模組規範出來了,這篇文章也出來了(拼出來的 {摀臉笑})。
JS中的模組規格(CommonJS,AMD,CMD),如果你聽過js模組化這個東西,那麼你就應該聽過或CommonJS或AMD甚至是CMD這些規範咯,我也聽過,但之前也真的是聽聽看而已。 現在就看看吧,這些規範到底是啥東西,幹嘛的。本文包括這三個規範的來源及對應的產物的原理。
一、CommonJS
1.一開始大家都認為JS是辣雞,沒什麼用,官方定義的API只能構建基於瀏覽器的應用程序,逗我呢,這太狹隘了吧(用了個高端詞,嘎嘎),CommonJS就按耐不住了,CommonJS API定義很多普通應用程式(主要指非瀏覽器的應用程式)所使用的API,從而填補了這個空白。它的終極目標是提供一個類似Python,Ruby和Java標準函式庫。這樣的話,開發者可以使用CommonJS API編寫應用程序,然後這些應用可以運行在不同的JavaScript解釋器和不同的主機環境中。
在相容於CommonJS的系統中,你可以使用JavaScript開發以下程式:
(1).伺服器端JavaScript應用程式
(2).命令列工具
(3).圖形介面應用程式
(4).混合應用程式(如, Titanium或Adobe AIR)
2009年,美國程式設計師Ryan Dahl創造了node.js項目,將javascript語言用於伺服器端程式設計。這標誌"Javascript模組化程式設計"正式誕生。因為老實說,在瀏覽器環境下,沒有模組也不是特別大的問題,畢竟網頁程式的複雜性有限;但是在伺服器端,一定要有模組,與作業系統和其他應用程式互動,否則根本沒法程式設計. NodeJS是CommonJS規格的實現,webpack 也是以CommonJS的形式來書寫。
node.js的模組系統,就是參考CommonJS規格實現的。在CommonJS中,有一個全域性方法require(),用於載入模組。假定有一個數學模組math.js,就可以像下面這樣載入。
var math = require('math');
#然後,就可以呼叫模組提供的方法:
var math = require('math');
math.add(2,3); // 5
CommonJS定義的模組分為:{模組引用(require)} {模組定義(exports)} {模組識別(module)}
require()用來引入外部模組;exports物件用於導出當前模組的方法或變量,唯一的導出口;module物件就代表模組本身。
雖然說Node遵循CommonJS的規範,但相比也是做了一些取捨,填了一些新東西的。
不過,我說了CommonJS也說了Node,那我覺得也得先了解下NPM了。 NPM作為Node的套件管理器,不是為了幫助Node解決依賴套件的安裝問題嘛,那它肯定也要遵循CommonJS規範啦,它遵循套件規範(還是理論)的。 CommonJS WIKI講了它的歷史,也介紹了modules和packages等。
下面講commonJS的原理以及簡易實作:
##瀏覽器不相容CommonJS的根本原因,在於缺少四個Node.js環境的變數。
module
#exports
require
global
只要能提供這四個變量,瀏覽器就能載入CommonJS 模組。
以下是一個簡單的範例。
<code class="language-javascript"><span style="font-family:'Microsoft YaHei';font-size:16px;"><code class="language-javascript"><br><span class="token keyword">var module <span class="token operator">= <span class="token punctuation">{<br> exports<span class="token punctuation">: <span class="token punctuation">{<span class="token punctuation">}<br><span class="token punctuation">}<span class="token punctuation">;<br><br><span class="token punctuation">(<span class="token keyword">function<span class="token punctuation">(module<span class="token punctuation">, exports<span class="token punctuation">) <span class="token punctuation">{<br> exports<span class="token punctuation">.multiply <span class="token operator">= <span class="token keyword">function <span class="token punctuation">(n<span class="token punctuation">) <span class="token punctuation">{ <span class="token keyword">return n <span class="token operator">* <span class="token number">1000 <span class="token punctuation">}<span class="token punctuation">;<br><span class="token punctuation">}<span class="token punctuation">(module<span class="token punctuation">, module<span class="token punctuation">.exports<span class="token punctuation">)<span class="token punctuation">)<br><br><span class="token keyword">var f <span class="token operator">= module<span class="token punctuation">.exports<span class="token punctuation">.multiply<span class="token punctuation">;<br><span class="token function">f<span class="token punctuation">(<span class="token number">5<span class="token punctuation">)<span class="token comment"> // 5000 <br></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></code></span></code>
上面程式碼向一個立即執行函數提供module 和exports 兩個外部變量,模組就放在這個立即執行函數裡面。模組的輸出值放在 module.exports 之中,這樣就實作了模組的載入。
2、Browserify 的實作
知道了原理,就能做出工具了。 Browserify 是目前最常使用的 CommonJS 格式轉換的工具。
請看一個例子,main.js 模組載入 foo.js 模組。
<code class="language-javascript"><span style="font-family:'Microsoft YaHei';font-size:16px;"><code class="language-javascript"><span class="token comment"><br>// foo.js<br>module<span class="token punctuation">.exports <span class="token operator">= <span class="token keyword">function<span class="token punctuation">(x<span class="token punctuation">) <span class="token punctuation">{<br> console<span class="token punctuation">.<span class="token function">log<span class="token punctuation">(x<span class="token punctuation">)<span class="token punctuation">;<br><span class="token punctuation">}<span class="token punctuation">;<br><span class="token comment"><br>// main.js<br><span class="token keyword">var foo <span class="token operator">= <span class="token function">require<span class="token punctuation">(<span class="token string">"./foo"<span class="token punctuation">)<span class="token punctuation">;<br><span class="token function">foo<span class="token punctuation">(<span class="token string">"Hi"<span class="token punctuation">)<span class="token punctuation">;<br></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></code></span></code>
使用下面的指令,就能將main.js轉換為瀏覽器可用的格式。
<span style="font-family:'Microsoft YaHei';font-size:16px;"><code class="language-bash"><br/>$ browserify main<span class="token punctuation">.js <span class="token operator">> compiled<span class="token punctuation">.js<br/></span></span></span></code></span>
Browserify到底做了什麼?安裝一下browser-unpack,就能看清楚了。
<span style="font-family:'Microsoft YaHei';font-size:16px;"><code class="language-bash"><br/>$ npm install browser<span class="token operator">-unpack <span class="token operator">-g<br/></span></span></code></span>
然後,前面產生的compile.js解封包。
<span style="font-family:'Microsoft YaHei';font-size:16px;"><code class="language-bash"><br/>$ browser<span class="token operator">-unpack <span class="token operator">< compiled<span class="token punctuation">.js<br/><br/><span class="token punctuation">[<br/><span class="token punctuation">{<br/><span class="token string">"id"<span class="token punctuation">:<span class="token number">1<span class="token punctuation">,<br/><span class="token string">"source"<span class="token punctuation">:<span class="token string">"module.exports = function(x) {\n console.log(x);\n};"<span class="token punctuation">,<br/><span class="token string">"deps"<span class="token punctuation">:<span class="token punctuation">{<span class="token punctuation">}<br/><span class="token punctuation">}<span class="token punctuation">,<br/><span class="token punctuation">{<br/><span class="token string">"id"<span class="token punctuation">:<span class="token number">2<span class="token punctuation">,<br/><span class="token string">"source"<span class="token punctuation">:<span class="token string">"var foo = require(\"./foo\");\nfoo(\"Hi\");"<span class="token punctuation">,<br/><span class="token string">"deps"<span class="token punctuation">:<span class="token punctuation">{<span class="token string">"./foo"<span class="token punctuation">:<span class="token number">1<span class="token punctuation">}<span class="token punctuation">,<br/><span class="token string">"entry"<span class="token punctuation">:<span class="token boolean">true<br/><span class="token punctuation">}<br/><span class="token punctuation">]<br/></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></code></span>
可以看到,browerify 將所有模組放入一個數組,id 屬性是模組的編號,source 屬性是模組的原始碼,deps 屬性是模組的依賴。
因為 main.js 裡面載入了 foo.js,所以 deps 屬性就指定 ./foo 對應1號模組。執行的時候,瀏覽器遇到 require('./foo') 語句,就自動執行1號模組的 source 屬性,並將執行後的 module.exports 屬性值輸出。
#雖然 Browserify 很強大,但無法在瀏覽器操作,有時就很不方便。
我根據 mocha 的內部實現,做了一個純瀏覽器的 CommonJS 模組載入器 tiny-browser-require 。完全不需要命令列,直接放進瀏覽器即可,所有程式碼只有30多行。
它的邏輯非常簡單,就是把模組讀入數組,載入路徑就是模組的id。
<code class="language-javascript"><span style="font-family:'Microsoft YaHei';font-size:16px;"><code class="language-javascript"><br/><span class="token keyword">function <span class="token function">require<span class="token punctuation">(p<span class="token punctuation">)<span class="token punctuation">{<br/><span class="token keyword">var path <span class="token operator">= require<span class="token punctuation">.<span class="token function">resolve<span class="token punctuation">(p<span class="token punctuation">)<span class="token punctuation">;<br/><span class="token keyword">var mod <span class="token operator">= require<span class="token punctuation">.modules<span class="token punctuation">[path<span class="token punctuation">]<span class="token punctuation">;<br/><span class="token keyword">if <span class="token punctuation">(<span class="token operator">!mod<span class="token punctuation">) <span class="token keyword">throw <span class="token keyword">new <span class="token class-name">Error<span class="token punctuation">(<span class="token string">'failed to require "' <span class="token operator">+ p <span class="token operator">+ <span class="token string">'"'<span class="token punctuation">)<span class="token punctuation">;<br/><span class="token keyword">if <span class="token punctuation">(<span class="token operator">!mod<span class="token punctuation">.exports<span class="token punctuation">) <span class="token punctuation">{<br/> mod<span class="token punctuation">.exports <span class="token operator">= <span class="token punctuation">{<span class="token punctuation">}<span class="token punctuation">;<br/> mod<span class="token punctuation">.<span class="token function">call<span class="token punctuation">(mod<span class="token punctuation">.exports<span class="token punctuation">, mod<span class="token punctuation">, mod<span class="token punctuation">.exports<span class="token punctuation">, require<span class="token punctuation">.<span class="token function">relative<span class="token punctuation">(path<span class="token punctuation">)<span class="token punctuation">)<span class="token punctuation">;<br/><span class="token punctuation">}<br/><span class="token keyword">return mod<span class="token punctuation">.exports<span class="token punctuation">;<br/><span class="token punctuation">}<br/><br/>require<span class="token punctuation">.modules <span class="token operator">= <span class="token punctuation">{<span class="token punctuation">}<span class="token punctuation">;<br/><br/>require<span class="token punctuation">.resolve <span class="token operator">= <span class="token keyword">function <span class="token punctuation">(path<span class="token punctuation">)<span class="token punctuation">{<br/><span class="token keyword">var orig <span class="token operator">= path<span class="token punctuation">;<br/><span class="token keyword">var reg <span class="token operator">= path <span class="token operator">+ <span class="token string">'.js'<span class="token punctuation">;<br/><span class="token keyword">var index <span class="token operator">= path <span class="token operator">+ <span class="token string">'/index.js'<span class="token punctuation">;<br/><span class="token keyword">return require<span class="token punctuation">.modules<span class="token punctuation">[reg<span class="token punctuation">] <span class="token operator">&& reg<br/><span class="token operator">|| require<span class="token punctuation">.modules<span class="token punctuation">[index<span class="token punctuation">] <span class="token operator">&& index<br/><span class="token operator">|| orig<span class="token punctuation">;<br/><span class="token punctuation">}<span class="token punctuation">;<br/><br/>require<span class="token punctuation">.register <span class="token operator">= <span class="token keyword">function <span class="token punctuation">(path<span class="token punctuation">, fn<span class="token punctuation">)<span class="token punctuation">{<br/> require<span class="token punctuation">.modules<span class="token punctuation">[path<span class="token punctuation">] <span class="token operator">= fn<span class="token punctuation">;<br/><span class="token punctuation">}<span class="token punctuation">;<br/><br/>require<span class="token punctuation">.relative <span class="token operator">= <span class="token keyword">function <span class="token punctuation">(parent<span class="token punctuation">) <span class="token punctuation">{<br/><span class="token keyword">return <span class="token keyword">function<span class="token punctuation">(p<span class="token punctuation">)<span class="token punctuation">{<br/><span class="token keyword">if <span class="token punctuation">(<span class="token string">'.' <span class="token operator">!<span class="token operator">= p<span class="token punctuation">.<span class="token function">charAt<span class="token punctuation">(<span class="token number">0<span class="token punctuation">)<span class="token punctuation">) <span class="token keyword">return <span class="token function">require<span class="token punctuation">(p<span class="token punctuation">)<span class="token punctuation">;<br/><span class="token keyword">var path <span class="token operator">= parent<span class="token punctuation">.<span class="token function">split<span class="token punctuation">(<span class="token string">'/'<span class="token punctuation">)<span class="token punctuation">;<br/><span class="token keyword">var segs <span class="token operator">= p<span class="token punctuation">.<span class="token function">split<span class="token punctuation">(<span class="token string">'/'<span class="token punctuation">)<span class="token punctuation">;<br/> path<span class="token punctuation">.<span class="token function">pop<span class="token punctuation">(<span class="token punctuation">)<span class="token punctuation">;<br/><br/><span class="token keyword">for <span class="token punctuation">(<span class="token keyword">var i <span class="token operator">= <span class="token number">0<span class="token punctuation">; i <span class="token operator">< segs<span class="token punctuation">.length<span class="token punctuation">; i<span class="token operator">++<span class="token punctuation">) <span class="token punctuation">{<br/><span class="token keyword">var seg <span class="token operator">= segs<span class="token punctuation">[i<span class="token punctuation">]<span class="token punctuation">;<br/><span class="token keyword">if <span class="token punctuation">(<span class="token string">'..' <span class="token operator">== seg<span class="token punctuation">) path<span class="token punctuation">.<span class="token function">pop<span class="token punctuation">(<span class="token punctuation">)<span class="token punctuation">;<br/><span class="token keyword">else <span class="token keyword">if <span class="token punctuation">(<span class="token string">'.' <span class="token operator">!<span class="token operator">= seg<span class="token punctuation">) path<span class="token punctuation">.<span class="token function">push<span class="token punctuation">(seg<span class="token punctuation">)<span class="token punctuation">;<br/><span class="token punctuation">}<br/><br/><span class="token keyword">return <span class="token function">require<span class="token punctuation">(path<span class="token punctuation">.<span class="token function">join<span class="token punctuation">(<span class="token string">'/'<span class="token punctuation">)<span class="token punctuation">)<span class="token punctuation">;<br/><span class="token punctuation">}<span class="token punctuation">;<br/><span class="token punctuation">}<span class="token punctuation">;<br/></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></code></span></code>登入後複製
使用的时候,先将上面的代码放入页面。然后,将模块放在如下的立即执行函数里面,就可以调用了。
<span style="font-family:'Microsoft YaHei';font-size:16px;"><code class="language-html"><br/><script src="require.js" /><br/><br/><script><br/>require.register("moduleId", function(module, exports, require){<br/> // Module code goes here<br/>});<br/>var result = require("moduleId");<br/></script><br/></code></span>登入後複製
还是以前面的 main.js 加载 foo.js 为例。
<code class="language-javascript"><span style="font-family:'Microsoft YaHei';font-size:16px;"><code class="language-javascript"><br/>require<span class="token punctuation">.<span class="token function">register<span class="token punctuation">(<span class="token string">"./foo.js"<span class="token punctuation">, <span class="token keyword">function<span class="token punctuation">(module<span class="token punctuation">, exports<span class="token punctuation">, require<span class="token punctuation">)<span class="token punctuation">{<br/> module<span class="token punctuation">.exports <span class="token operator">= <span class="token keyword">function<span class="token punctuation">(x<span class="token punctuation">) <span class="token punctuation">{<br/> console<span class="token punctuation">.<span class="token function">log<span class="token punctuation">(x<span class="token punctuation">)<span class="token punctuation">;<br/><span class="token punctuation">}<span class="token punctuation">;<br/><span class="token punctuation">}<span class="token punctuation">)<span class="token punctuation">;<br/><br/><span class="token keyword">var foo <span class="token operator">= <span class="token function">require<span class="token punctuation">(<span class="token string">"./foo.js"<span class="token punctuation">)<span class="token punctuation">;<br/><span class="token function">foo<span class="token punctuation">(<span class="token string">"Hi"<span class="token punctuation">)<span class="token punctuation">;<br/></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></code></span></code>登入後複製
注意,这个库只模拟了 require 、module 、exports 三个变量,如果模块还用到了 global 或者其他 Node 专有变量(比如 process),就通过立即执行函数提供即可。
二、AMD
基于commonJS规范的nodeJS出来以后,服务端的模块概念已经形成,很自然地,大家就想要客户端模块。而且最好两者能够兼容,一个模块不用修改,在服务器和浏览器都可以运行。但是,由于一个重大的局限,使得CommonJS规范不适用于浏览器环境。还是上面的代码,如果在浏览器中运行,会有一个很大的问题,你能看出来吗?
var math = require('math');
math.add(2, 3);
第二行math.add(2, 3),在第一行require('math')之后运行,因此必须等math.js加载完成。也就是说,如果加载时间很长,整个应用就会停在那里等。您会注意到 require
是同步的。
这对服务器端不是一个问题,因为所有的模块都存放在本地硬盘,可以同步加载完成,等待时间就是硬盘的读取时间。但是,对于浏览器,这却是一个大问题,因为模块都放在服务器端,等待时间取决于网速的快慢,可能要等很长时间,浏览器处于"假死"状态。
因此,浏览器端的模块,不能采用"同步加载"(synchronous),只能采用"异步加载"(asynchronous)。这就是AMD规范诞生的背景。
CommonJS是主要为了JS在后端的表现制定的,他是不适合前端的,AMD(异步模块定义)出现了,它就主要为前端JS的表现制定规范。
AMD是"Asynchronous Module Definition"的缩写,意思就是"异步模块定义"。它采用异步方式加载模块,模块的加载不影响它后面语句的运行。所有依赖这个模块的语句,都定义在一个回调函数中,等到加载完成之后,这个回调函数才会运行。
AMD也采用require()语句加载模块,但是不同于CommonJS,它要求两个参数:
require([module], callback);
第一个参数[module],是一个数组,里面的成员就是要加载的模块;第二个参数callback,则是加载成功之后的回调函数。如果将前面的代码改写成AMD形式,就是下面这样:
require(['math'], function (math) {
math.add(2, 3);
});
math.add()與math模組載入不是同步的,瀏覽器不會發生假死。所以很顯然,AMD比較適合瀏覽器環境。 目前,主要有兩個Javascript函式庫實作了AMD規格:require.js和curl.js。
RequireJS就是實作了AMD規範的呢。
詳細概括:以下以RequireJS為例說明AMD規格
一、為什麼要用require .js?
最早的時候,所有Javascript程式碼都寫在一個檔案裡面,只要載入這一個檔案就夠了。後來,程式碼越來越多,一個文件不夠了,必須分成多個文件,依序載入。下面的網頁程式碼,相信很多人都看過
#
#這段程式碼依序載入多個js檔案。
這樣的寫法有很大的缺點。首先,載入的時候,瀏覽器會停止網頁渲染,載入檔案越多,網頁失去回應的時間就會越長;其次,由於js檔案之間存在依賴關係,因此必須嚴格保證載入順序(例如上例的1.js要在2.js的前面),依賴性最大的模組一定要放到最後加載,當依賴關係很複雜的時候,程式碼的編寫和維護都會變得困難。
require.js的誕生,就是為了解決這兩個問題:
# (1)實作js檔案的非同步加載,避免網頁失去回應;
(2)管理模組之間的依賴性,便於程式碼的編寫和維護。
二、require.js的載入
使用require.js的第一步,是先到官方網站下載最新版本。
下載後,假定把它放在js子目錄下面,就可以載入了。
有人可能會想到,載入這個文件,也可能造成網頁失去回應。解決方法有兩個,一個是把它放在網頁底部加載,另一個是寫成下面這樣:
async屬性顯示這個檔案需要非同步加載,避免網頁失去回應。 IE不支援這個屬性,只支援defer,所以把defer也寫上。
載入require.js以後,下一步就要載入我們自己的程式碼了。假定我們自己的程式碼檔案是main.js,也放在js目錄下面。那麼,只要寫成下面這樣就行了:
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