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---
title: PromiseRejectionEvent.promise
slug: Web/API/PromiseRejectionEvent/promise
translation_of: Web/API/PromiseRejectionEvent/promise
---
<div>{{APIRef("HTML DOM") }}</div>
<p> {{domxref("PromiseRejectionEvent")}}事件对象的<code><strong>promise</strong></code>属性是只读的，表明Promise被reject的原因。您可以通过检查{{domxref("PromiseRejectionEvent.reason")}}来了解Promise为什么被reject。</p>
<h2 id="Syntax">Syntax</h2>
<pre class="syntaxbox"><em>promise</em> = <em>PromiseRejectionEvent</em>.promise</pre>
<h3 id="Value">Value</h3>
<p>一个被reject的，并且错误未被处理的{{jsxref("Promise")}} </p>
<h2 id="Examples">Examples</h2>
<p>下面的例子监听了未被处理的promise，如果{{domxref("PromiseRejectionEvent.reason", "reason")}} 是一个对象，并且其<code>code</code>属性包含了一段文本“Module not read.”，一个空闲的回调函数被声明，当任务执行错误时会进行重试</p>
<p>{{domxref("event.preventDefault()")}} 用来表明该promise已被处理</p>
<pre class="brush: js">window.onunhandledrejection = function(event) {
if (event.reason.code && event.reason.code == "Module not ready") {
window.requestIdleCallback(function(deadline) {
loadModule(event.reason.moduleName)
.then(performStartup);
});
event.preventDefault();
}
}</pre>
<h2 id="Specifications">Specifications</h2>
<table class="standard-table">
<thead>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
</thead>
<tbody>
<tr>
<td>{{SpecName('HTML WHATWG', 'webappapis.html#dom-promiserejectionevent-promise', 'PromiseRejectionEvent.promise')}}</td>
<td>{{ Spec2('HTML WHATWG') }}</td>
<td>Initial definition.</td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容性">浏览器兼容性</h2>
<p>{{Compat("api.PromiseRejectionEvent.promise")}}</p>
<h2 id="See_also">See also</h2>
<ul>
<li>{{jsxref("Promise")}}</li>
<li>{{domxref("PromiseRejectionEvent")}}</li>
<li>{{event("rejectionhandled")}}</li>
<li>{{event("unhandledrejection")}}</li>
</ul>
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