# ES6 Map 與 Set ## Map 對象 Map 對象保存鍵值對。任何值(對象或者原始值) 都可以作為一個鍵或一個值。 ``` <pre class="calibre13">``` ### Map 中的 key <span class="token">**</span>key 是字符串<span class="token">**</span> <span class="token2">var</span> myMap <span class="token">=</span> <span class="token2">new</span> <span class="token3">Map</span><span class="token1">(</span><span class="token1">)</span><span class="token1">;</span> <span class="token2">var</span> keyString <span class="token">=</span> <span class="token4">"a string"</span><span class="token1">;</span> myMap<span class="token1">.</span><span class="token2">set</span><span class="token1">(</span>keyString<span class="token1">,</span> <span class="token4">"和鍵'a string'關聯的值"</span><span class="token1">)</span><span class="token1">;</span> myMap<span class="token1">.</span><span class="token2">get</span><span class="token1">(</span>keyString<span class="token1">)</span><span class="token1">;</span> <span class="token6">// "和鍵'a string'關聯的值" </span> myMap<span class="token1">.</span><span class="token2">get</span><span class="token1">(</span><span class="token4">"a string"</span><span class="token1">)</span><span class="token1">;</span> <span class="token6">// "和鍵'a string'關聯的值" </span> <span class="token6">// 因為 keyString === 'a string'</span> ``` ``` ``` <pre class="calibre13">``` ### Map 對象的操作 <span class="token">**</span>Map 與 Array的轉換<span class="token">**</span> <span class="token2">var</span> kvArray <span class="token">=</span> \<span class="token1">[</span>\<span class="token1">[</span><span class="token4">"key1"</span><span class="token1">,</span> <span class="token4">"value1"</span>\<span class="token1">]</span><span class="token1">,</span> \<span class="token1">[</span><span class="token4">"key2"</span><span class="token1">,</span> <span class="token4">"value2"</span>\<span class="token1">]</span>\<span class="token1">]</span><span class="token1">;</span> <span class="token6">// Map 構造函數可以將一個 二維 鍵值對數組轉換成一個 Map 對象 </span> <span class="token2">var</span> myMap <span class="token">=</span> <span class="token2">new</span> <span class="token3">Map</span><span class="token1">(</span>kvArray<span class="token1">)</span><span class="token1">;</span> <span class="token6">// 使用 Array.from 函數可以將一個 Map 對象轉換成一個二維鍵值對數組 </span> <span class="token2">var</span> outArray <span class="token">=</span> Array<span class="token1">.</span><span class="token2">from</span><span class="token1">(</span>myMap<span class="token1">)</span><span class="token1">;</span> ``` ``` ## Set 對象 Set 對象允許你存儲任何類型的唯一值，無論是原始值或者是對象引用。 ### Set 中的特殊值 Set 對象存儲的值總是唯一的，所以需要判斷兩個值是否恒等。有幾個特殊值需要特殊對待： - +0 與 -0 在存儲判斷唯一性的時候是恒等的，所以不重復； - undefined 與 undefined 是恒等的，所以不重復； - NaN 與 NaN 是不恒等的，但是在 Set 中只能存一個，不重復 ``` <pre class="calibre14">``` ### Set 對象作用 <span class="token">**</span>數組去重<span class="token">**</span> <span class="token2">var</span> mySet<span class="token">=</span> <span class="token2">new</span> <span class="token3">Set</span><span class="token1">(</span>\<span class="token1">[</span><span class="token5">1</span><span class="token1">,</span> <span class="token5">2</span><span class="token1">,</span> <span class="token5">3</span><span class="token1">,</span> <span class="token5">4</span><span class="token1">,</span> <span class="token5">4</span>\<span class="token1">]</span><span class="token1">)</span><span class="token1">;</span> \<span class="token1">[</span><span class="token">...</span>mySet\<span class="token1">]</span><span class="token1">;</span> <span class="token6">// \[1, 2, 3, 4\]</span> ``` ```