事件监听器

\Event listener

为事件 type 注册的事件监听器可以是 JavaScript 函数,也可以是具有值为函数的 handleEvent 属性的对象。

\Event listeners registered for an event type may either be JavaScript functions or objects with a handleEvent property whose value is a function.

在任何一种情况下,句柄函数都是通过传给 eventTarget.dispatchEvent() 函数的 event 参数调用的。

\In either case, the handler function is invoked with the event argument passed to the eventTarget.dispatchEvent() function.

异步函数可用作事件监听器。如果异步处理函数拒绝,则将按照 EventTarget 错误处理 中的描述捕获和处理拒绝。

\Async functions may be used as event listeners. If an async handler function rejects, the rejection is captured and handled as described in EventTarget error handling.

句柄函数抛出的错误不会阻止其他句柄被调用。

\An error thrown by one handler function does not prevent the other handlers from being invoked.

句柄函数的返回值会被忽略。

\The return value of a handler function is ignored.

句柄始终按照其添加的顺序被调用。

\Handlers are always invoked in the order they were added.

句柄函数可能会改变 event 对象。

\Handler functions may mutate the event object.

function handler1(event) {
 console.log(event.type); // Prints 'foo'
 event.a = 1;
}
async function handler2(event) {
 console.log(event.type); // Prints 'foo'
 console.log(event.a); // Prints 1
}
const handler3 = {
 handleEvent(event) {
 console.log(event.type); // Prints 'foo'
 },
};
const handler4 = {
 async handleEvent(event) {
 console.log(event.type); // Prints 'foo'
 },
};
const target = new EventTarget();
target.addEventListener('foo', handler1);
target.addEventListener('foo', handler2);
target.addEventListener('foo', handler3);
target.addEventListener('foo', handler4, { once: true }); 拷贝