# ES6 Promise 對象 ## 概述 是異步編程的一種解決方案。 從語法上說，Promise 是一個對象，從它可以獲取異步操作的消息。 ## Promise 狀態 ### 狀態的特點 Promise 異步操作有三種狀態：pending（進行中）、fulfilled（已成功）和 rejected（已失敗）。除了異步操作的結果，任何其他操作都無法改變這個狀態。 Promise 對象只有：從 pending 變為 fulfilled 和從 pending 變為 rejected 的狀態改變。只要處于 fulfilled 和 rejected ，狀態就不會再變了即 resolved（已定型）。 ``` <pre class="calibre13">``` <span class="token2">const</span> p1 <span class="token">=</span> <span class="token2">new</span> <span class="token3">Promise</span><span class="token1">(</span><span class="token2">function</span><span class="token1">(</span>resolve<span class="token1">,</span>reject<span class="token1">)</span><span class="token1">{</span> <span class="token3">resolve</span><span class="token1">(</span><span class="token4">'success1'</span><span class="token1">)</span><span class="token1">;</span> <span class="token3">resolve</span><span class="token1">(</span><span class="token4">'success2'</span><span class="token1">)</span><span class="token1">;</span> <span class="token1">}</span><span class="token1">)</span><span class="token1">;</span> <span class="token2">const</span> p2 <span class="token">=</span> <span class="token2">new</span> <span class="token3">Promise</span><span class="token1">(</span><span class="token2">function</span><span class="token1">(</span>resolve<span class="token1">,</span>reject<span class="token1">)</span><span class="token1">{</span> <span class="token3">resolve</span><span class="token1">(</span><span class="token4">'success3'</span><span class="token1">)</span><span class="token1">;</span> <span class="token3">reject</span><span class="token1">(</span><span class="token4">'reject'</span><span class="token1">)</span><span class="token1">;</span> <span class="token1">}</span><span class="token1">)</span><span class="token1">;</span> p1<span class="token1">.</span><span class="token3">then</span><span class="token1">(</span><span class="token2">function</span><span class="token1">(</span>value<span class="token1">)</span><span class="token1">{</span> console<span class="token1">.</span><span class="token3">log</span><span class="token1">(</span>value<span class="token1">)</span><span class="token1">;</span> <span class="token6">// success1 </span> <span class="token1">}</span><span class="token1">)</span><span class="token1">;</span> p2<span class="token1">.</span><span class="token3">then</span><span class="token1">(</span><span class="token2">function</span><span class="token1">(</span>value<span class="token1">)</span><span class="token1">{</span> console<span class="token1">.</span><span class="token3">log</span><span class="token1">(</span>value<span class="token1">)</span><span class="token1">;</span> <span class="token6">// success3 </span> <span class="token1">}</span><span class="token1">)</span><span class="token1">;</span> ``` ``` ### 狀態的缺點 無法取消 Promise ，一旦新建它就會立即執行，無法中途取消。 如果不設置回調函數，Promise 內部拋出的錯誤，不會反應到外部。 當處于 pending 狀態時，無法得知目前進展到哪一個階段（剛剛開始還是即將完成）。 ## then 方法 then 方法接收兩個函數作為參數，第一個參數是 Promise 執行成功時的回調，第二個參數是 Promise 執行失敗時的回調，兩個函數只會有一個被調用。