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<!doctype html>
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Level 2: 寫出效率較好的 JavaScript（JavaScript Best Practice @ Code School） - My Frontend Note
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<h1>Level 2: 寫出效率較好的 JavaScript（JavaScript Best Practice @ Code School）</h1>
<div class="read-more clearfix">
<span class="date">2017/1/2</span>
<span>posted in </span>
<span class="posted-in"><a href='Best%20Practices.html'>Best Practices</a></span>
<span class="comments">
</span>
</div>
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<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">Loop optimization</a>
<ul>
<li>
<a href="#toc_1">Cache the property to avoid repetitive access at depth:</a>
</li>
<li>
<a href="#toc_2">Stick with <code>for-loops</code> instead of <code>for-in loops</code> when your goal is only to reach every index.</a>
</li>
</ul>
</li>
<li>
<a href="#toc_3">Script execution in HTML</a>
</li>
<li>
<a href="#toc_4">Use a <code>prototype</code> for shared stuff for object constructor</a>
</li>
<li>
<a href="#toc_5">Use document <code>fragment</code> to insert additions all at once</a>
<ul>
<li>
<a href="#toc_6">Declare variables as few times as possible</a>
</li>
<li>
<a href="#toc_7">Use <code>join()</code> method instead of for-loop to merge an array into one jumbo string</a>
</li>
</ul>
</li>
<li>
<a href="#toc_8">Test the speed of your code</a>
<ul>
<li>
<a href="#toc_9">console.test()</a>
</li>
<li>
<a href="#toc_10">class test</a>
</li>
</ul>
</li>
</ul>
<span id="more"></span><!-- more -->
<h2 id="toc_0">Loop optimization</h2>
<h3 id="toc_1">Cache the property to avoid repetitive access at depth:</h3>
<pre><code class="language-js">treasureChest = {
necklaces: ['ruby', 'pear', 'sapphire', 'diamond']
}
function returnLoop() {
var list = treasureChest.necklaces;
for(var i=0, x = treasureChest.necklaces.length; i<x : i++) {
console.log(list[i])
}
// same as:
// var x = treasureChest.necklaces.length;
// for(var i=0; i<x : i++) {
// console.log(list[i])
// }
}
</code></pre>
<h3 id="toc_2">Stick with <code>for-loops</code> instead of <code>for-in loops</code> when your goal is only to reach every index.</h3>
<p>Methods we add to the prototype become “enumerable” just like indices</p>
<h2 id="toc_3">Script execution in HTML</h2>
<p><strong>Problems:</strong></p>
<ul>
<li>Loading huge script will block the page loading.</li>
<li>Cannot manipulate DOM elements before they have been construct.</li>
</ul>
<p><strong>Solutions:</strong></p>
<ul>
<li>Move <code><script></code> as low as possible.</li>
<li>Add <code>async</code> attribute in <code><script></code> to prevent blocking.</li>
</ul>
<h2 id="toc_4">Use a <code>prototype</code> for shared stuff for object constructor</h2>
<p>We don’t need to build all of these methods within every single object, which would use extra memory AND take longer to create.</p>
<h2 id="toc_5">Use document <code>fragment</code> to insert additions all at once</h2>
<p>Each new addition to the DOM causes document "reflow"</p>
<p><strong>fragments</strong> are invisible containers that hold multiple DOM elements without being a node itself</p>
<p>原始版，每次for-loop都reflow：</p>
<pre><code class="language-js">var list = document.getElementById("kotwList")
var kotw = ["a", "b", "c"]
for (var i = 0, x = kotw.length; i<x ; i++) {
var element = document.createElement("li")
element.appendChild(document.creatementTextNode(kotw[i])
list.appendChild(element)
}
</code></pre>
<p>優化，只reflow一次：</p>
<pre><code class="language-js">var list = document.getElementById("kotwList")
var kotw = ["a", "b", "c"]
var fragment = document.createDocumentFragment() // <=
for (var i = 0, x = kotw.length; i<x ; i++) {
var element = document.createElement("li")
element.appendChild(document.creatementTextNode(kotw[i])
fragment.appendChild(element) // <=
}
list.appendChild(fragment) // <=
</code></pre>
<h3 id="toc_6">Declare variables as few times as possible</h3>
<p>Each <code>var</code> keywords adds a look-up for the javascript parser that can be avoided with comma extensions.</p>
<pre><code class="language-js">var list = document.getElementById("kotwList"),
kotw = ["a", "b", "c"],
fragment = document.createDocumentFragment(),
element // <=
for (var i = 0, x = kotw.length; i<x ; i++) {
element = document.createElement("li") //<=
element.appendChild(document.creatementTextNode(kotw[i])
fragment.appendChild(element)
}
list.appendChild(fragment)
</code></pre>
<h3 id="toc_7">Use <code>join()</code> method instead of for-loop to merge an array into one jumbo string</h3>
<p>is faster </p>
<h2 id="toc_8">Test the speed of your code</h2>
<h3 id="toc_9">console.test()</h3>
<pre><code class="language-js">var newbs = ["Tim", "Tom", "Sam"]
var knights = [ /*...tons of Knight objects...*/ ]
console.time("Time for adding" + newbs.length + "Knights")
for (var i = 0, x = newbs.length; i<x; i++) {
var newGuy = new Knight(newbs[i])
knights.push(newGuy)
}
console.timeEnd("Time for adding" + newbs.length + "Knights")
</code></pre>
<h3 id="toc_10">class test</h3>
<pre><code class="language-js">var SpeedTest = function(testImplement,testParams,repetitions) {
this.testImplement = testImplement;
this.testParams = testParams;
this.repetitions = repetitions || 10000;
this.average = 0;
};
SpeedTest.prototype = {
startTest: function() {
if (this.testImplement(this.testParams) === false) {
alert('Test failed with those parameters.');
return;
}
var beginTime, endTime, sumTimes = 0;
for (var i = 0, x = this.repetitions; i < x; i++) {
beginTime = +new Date();
this.testImplement(this.testParams);
endTime = +new Date();
sumTimes += endTime - beginTime;
}
this.average = sumTimes / this.repetitions;
return console.log('Average execution across ' + this.repetitions + ': ' + this.average);
}
};
var IP = ['Nipping Global Variable', 'Sneaky Forin', 'Bulging Blocking Script'],
GH = ['Switch Blocks', 'Pesky Gnat', 'Aiedra'],
inhabitants = [IP,GH];
function populationGetter(popn) {
var list = '';
for (var i = 0, x = popn.length; i < x; i++) {
for (var j = 0; j < popn[i].length; j++) {
list += (popn[i][j] + ', ');
}
}
return list;
}
populationGetter(inhabitants);
var concatTest = new SpeedTest(populationGetter, inhabitants, 100000);
concatTest.startTest();
</code></pre>
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