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4-5 Map - My Frontend Note
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<h1>4-5 Map</h1>
<div class="read-more clearfix">
<span class="date">2016/11/12</span>
<span>posted in </span>
<span class="posted-in"><a href='ES2015.html'>ES2015</a></span>
<span class="comments">
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<blockquote>
<p>本文為 <strong><a href="https://www.codeschool.com/courses/es2015-the-shape-of-javascript-to-come">ES2015: The Shape of JavaScript to Come</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">key 會自動轉為字串（只能用字串當 key）</a>
</li>
<li>
<a href="#toc_1">使用map的時機：</a>
</li>
<li>
<a href="#toc_2">maps are iterable （可以用 fro...of）</a>
</li>
<li>
<a href="#toc_3">Weakmap</a>
<ul>
<li>
<a href="#toc_4">All methods on a <code>WeakMap</code> required a key (object)</a>
</li>
</ul>
</li>
</ul>
<span id="more"></span><!-- more -->
<p>使用 object 作為 map 的壞處：</p>
<h3 id="toc_0">key 會自動轉為字串（只能用字串當 key）</h3>
<pre><code class="language-js">let user1 = { name: Sam }
let user2 = { name: Tyler }
let totalReplies ={}
totalReplies[user1] = 5
totalReplies[user2] = 42
</code></pre>
<pre><code class="language-js">let user1 = { name: Sam }
let user2 = { name: Tyler }
let totalReplies = new Map();
totalReplies.set( user1, 5 )
totalReplies.set( user2, 42 )
</code></pre>
<p>設值： <code>object.set(key, value)</code> =><br/>
取值： <code>object.get(key)</code> => return value</p>
<h3 id="toc_1">使用map的時機：</h3>
<ol>
<li><p>Use maps when key are unknown until runtime</p>
<p>Since keys like perPage and canSort are previously defined, we don't run the risk of accidentally overwriting values.</p>
<p><img src="media/14789626942710/when_to_use_maps.png" alt="when_to_use_maps"/></p></li>
<li><p>when all keys are the same type and all values are the same type</p>
<p><img src="media/14789626942710/when_to_use_maps_2.png" alt="when_to_use_maps_2"/></p></li>
</ol>
<h3 id="toc_2">maps are iterable （可以用 fro...of）</h3>
<p>each run of the loops returns a [key, value] pair for an entry in the map</p>
<p><img src="media/14789626942710/maps_iterable.png" alt="maps_iterable"/></p>
<h3 id="toc_3">Weakmap</h3>
<p>A type of <code>Map</code> where only objects can be passed as keys. Primitive data types (such as strings, numbers, booleans, etc) are not allowed.</p>
<h4 id="toc_4">All methods on a <code>WeakMap</code> required a key (object)</h4>
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