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