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Level 4: Module in JavaScript（JavaScript Best Practice @ Code School） - My Frontend Note
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<h1>Level 4: Module in JavaScript（JavaScript Best Practice @ Code School）</h1>
<div class="read-more clearfix">
<span class="date">2017/1/4</span>
<span>posted in </span>
<span class="posted-in"><a href='Best%20Practices.html'>Best Practices</a></span>
<span class="comments">
</span>
</div>
</div><!-- article -->
<div class="article-content">
<blockquote>
<p>本文為 <strong><a href="https://www.codeschool.com/courses/javascript-best-practices">JavaScript Best Practices</a></strong>（CodeSchool）線上課程筆記</p>
</blockquote>
<p><strong>本文目錄：</strong></p>
<ul>
<li>
<a href="#toc_0">Simulate name space by JavaScript objects</a>
</li>
<li>
<a href="#toc_1">Anynomous closures</a>
</li>
<li>
<a href="#toc_2">Global import</a>
</li>
<li>
<a href="#toc_3">Augmentation</a>
</li>
</ul>
<span id="more"></span><!-- more -->
<p>課程中教的主要是最原始的模組寫法，但其實透過擴充或 ES2015 已經有更方便的模組處理方式，不過為了拿徽章還是把他上完了。XD</p>
<p>想瞭解 JavaScript 模組化新的實踐方式詳見以下連結：</p>
<ul>
<li><p>JavaScript 模块化七日谈<br/>
<a href="https://huangxuan.me/2015/07/09/js-module-7day/">https://huangxuan.me/2015/07/09/js-module-7day/</a></p></li>
<li><p>Module（ECMAScript 6 入门）<br/>
<a href="http://es6.ruanyifeng.com/#docs/module">http://es6.ruanyifeng.com/#docs/module</a></p></li>
<li><p>Module（Exploring ES6）<br/>
<a href="http://exploringjs.com/es6/ch_modules.html">http://exploringjs.com/es6/ch_modules.html</a></p></li>
</ul>
<p>想瞭解原始寫法也可參考：</p>
<ul>
<li>深入了解 JavaScript 模組化模式
<a href="http://easonlin.logdown.com/posts/74228-javascript-module-pattern-in-depth">http://easonlin.logdown.com/posts/74228-javascript-module-pattern-in-depth</a></li>
</ul>
<h2 id="toc_0">Simulate name space by JavaScript objects</h2>
<blockquote>
<p>An application that has multiple files which contain variables with the same name will experience a loss of data in the event of a(n) <strong>overwrite</strong>, which erases old data in favor of new (and perhaps wrong) data. This occurs specifically when these variables are declared <strong>globally</strong> in multiple files. This error may not be found until <strong>run-time</strong>.</p>
</blockquote>
<p><strong>Solution: simulate name space by JavaScript objects</strong></p>
<p>global variables => object's properties<br/>
global function => object's methods</p>
<pre><code class="language-js">var CAVESOFCLARITY = {
stalactites: 4235,
stalagmites: 3924,
bats: 345,
treasureChests: 3,
openChest: function() {
this.treasureChests--;
alert('DA DADADA DAAAAAAA!');
}
};
</code></pre>
<h2 id="toc_1">Anynomous closures</h2>
<blockquote>
<p>Use <strong>closure</strong> to cause some properties to be private, bound only to a surrounding function's local scope, and some properties to be public, accessible by all holders of the namespace.</p>
<p><strong>Private</strong> properties are created in the local scope of the function expression (IIFE).</p>
<p><strong>Public</strong> properties are built within the object which is then return to become the namespace.</p>
<p>Access to private data is thus possible only because of closure within the larger module.</p>
</blockquote>
<pre><code class="language-js">var CAVESOFCLARITY = (function() {
var treasureChests = 3; //<= private
var bats: 345; // <= cannot retrieve it
return {
stalactites: 4235,
stalagmites: 3924,
openChest: function() {
treasureChests--;
alert('DA DADADA DAAAAAAA!');
}
};
})();
</code></pre>
<p><strong>Caution</strong>: If the data is never referenced through the public method, we have no way to retrieve it.</p>
<pre><code class="language-js">var CAVESOFCLARITY = (function() {
var treasureChests = 3; //<= private
var bats: 345; // <= private
return {
stalactites: 4235,
stalagmites: 3924,
openChest: function() {
treasureChests--;
alert('DA DADADA DAAAAAAA!');
},
getBats: function() {
return bats;
}
};
})();
</code></pre>
<h2 id="toc_2">Global import</h2>
<p>直接引用 global 變數有兩個問題：</p>
<ol>
<li><p>When non-local var are referenced in a module, the entire length of the scope chain is checked.</p></li>
<li><p>Lengthy namespaces mean that global variables have unclear scope, leading to code that is tough to manage.</p></li>
</ol>
<p>可能會很難找到這個變數哪裡來的，或是錯誤地假設這是一個區域變數而做操作影響到全域變數。</p>
<p>Solution: Global Import</p>
<blockquote>
<p>Your <strong>import</strong> ensures clarity of scope within a module.</p>
<p>By using a <strong>parameter</strong>, you protect the global data that might have been overwritten.</p>
<p>All imported data becomes locally scoped to the anonymous function, to be used in closure.</p>
<p>Thus, when compared with searching the entire scope chain, imports are both clearer and faster.</p>
</blockquote>
<p>Example:</p>
<pre><code class="language-js">var LEXICALLAKE = (function(answer, foundShore) {
var depth = 400;
var fish = 1200;
var unknownSleepers = 3;
return {
getFish: function() { return fish; },
getDepth: function() { return depth; },
getSleepers: function() { return unknownSleepers; },
awakenSleeper: function() {
alert('Far below, in the deep, something awakens. Row faster.');
},
summonBoat: function() {
if (answer === 'agnostic') {
alert('The boat fades into view.');
}
else {
alert('...the lake is silent.');
}
},
stopSleeper: function() {
if (foundShore === true){
alert('That which has awoken shall now return to slumber.');
} else {
alert('A Sleeper cometh. Abandon hope.');
}
}
};
})(explorerAnswer, madeItAcross);
</code></pre>
<h2 id="toc_3">Augmentation</h2>
<p>Augment: Adding or changing properties in a module after the module has already been built</p>
<blockquote>
<p>In simple augmentation, the module file and the augmentation file <strong>do not share their private state</strong>.</p>
<p>Augmented module properties may only access the private data from their file's closure.</p>
<p>Private data from the original closure will not be lost, and will be accessible to all original properties that referenced it.</p>
</blockquote>
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