dispatchEvent 如何同步执行 addEventListener 的处理程序？

Question

 let section = document.querySelector('section'); function handler(e) { console.log(e.detail.color); } section.addEventListener('hello', handler); let event = new CustomEvent("hello", {detail: {color: "red"}}); if (section.dispatchEvent(event)) { console.log("true") } else { console.log("false"); }

 <section></section>

In this example, I'm struggling to understand how handler is executed during the initial execution of the file - leading us to log red before true . I thought that handlers waited for the event somewhere, and when the event happened, they were put in the macro-task queue which means they'd only be put on the call-stack after the initial execution of the file.

Am I safe to assume that dispatchEvent has a unique way of executing .addEventListener 's handler synchronously?

Or do I have a misunderstanding of the way the handler in .addEventListener executes? I thought it executed asynchronously.

Answer 1

I thought that handlers waited for the event somewhere

They do - but you just happened to fire the event immediately, so the handler is run immediately.

and when the event happened, they were put in the macro-task queue which means they'd only be put on the call-stack after the initial execution of the file.

I don't know what "initial execution of the file" means. Handlers are run whenever an event is dispatched.

Am I safe to assume that dispatchEvent has a unique way of executing .addEventListener's handler synchronously

No, nothing strange is happening here, this is all normal stuff.

Or do I have a misunderstanding of the way the handler in .addEventListener executes? I thought it executed asynchronously.

Handlers get executed synchronously. Whenever an event is dispatched, handlers are checked and any matching ones are executed synchronously in the order they have been added.