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Level 1: JavaScript 的好用條件語法（JavaScript Best Practice @ Code School） - My Frontend Note
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<h1>Level 1: JavaScript 的好用條件語法（JavaScript Best Practice @ Code School）</h1>
<div class="read-more clearfix">
<span class="date">2016/12/16</span>
<span>posted in </span>
<span class="posted-in"><a href='Best%20Practices.html'>Best Practices</a></span>
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<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">one line condition</a>
<ul>
<li>
<a href="#toc_1">Ternary Conditionals</a>
</li>
<li>
<a href="#toc_2">OR operator <code>||</code></a>
</li>
<li>
<a href="#toc_3">AND operator <code>&&</code></a>
</li>
</ul>
</li>
<li>
<a href="#toc_4">The <code>switch</code> block</a>
</li>
</ul>
<span id="more"></span><!-- more -->
<h2 id="toc_0">one line condition</h2>
<h3 id="toc_1">Ternary Conditionals</h3>
<pre><code class="language-js">判斷式 ? ifTrue : ifFalse
</code></pre>
<h3 id="toc_2">OR operator <code>||</code></h3>
<p>Use to set default value:</p>
<pre><code class="language-js">var something = object.property || defaultValue
</code></pre>
<p><strong>Caution: What is <code>false</code>?</strong></p>
<pre><code class="language-js">// Creating Boolean objects with an initial value of false
var bNull = new Boolean(null);
var bEmptyString = new Boolean('');
var bfalse = new Boolean(false);
// Creating Boolean objects with an initial value of true
var btrue = new Boolean(true);
var btrueString = new Boolean('true');
var bfalseString = new Boolean('false'); // <= !
var bSuLin = new Boolean('Su Lin');
var bArrayProto = new Boolean([]); // <= !
var bObjProto = new Boolean({}); // <= !
</code></pre>
<h3 id="toc_3">AND operator <code>&&</code></h3>
<pre><code class="language-js">var something = condition1 && condition2 && ifAllConditionsisTrue
// same as
var something
if (!condition1) {something = condition1}
else if (!condition2) {something = condition2}
else {something = ifAllConditionsisTrue}
</code></pre>
<p><strong>Caution:</strong> 當 condition1 或 condition2 轉 boolean 是 false 時，運算結果是 condition1 或 condition2 的值，並非一定是 false 或 undefined）</p>
<h2 id="toc_4">The <code>switch</code> block</h2>
<p>和串一堆 <code>if-else</code> 相比，用 <code>switch</code> 的可讀性和效能較好</p>
<p><strong>Caution</strong>: Be aware of <strong>execution fall-through</strong> (Don't forget to <code>break</code> if you don't want that)</p>
<pre><code class="language-js">switch (expression) {
case value1:
//Statements executed when the result of expression matches value1
[break;]
case value2:
//Statements executed when the result of expression matches value2
[break;]
...
case valueN:
//Statements executed when the result of expression matches valueN
[break;]
default:
//Statements executed when none of the values match the value of the expression
[break;]
}
</code></pre>
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