Express中_router是什么？为何执行parent._router.stack.pop()？

Hey there! Let's break down this code snippet piece by piece since you're new to Express—no confusing jargon, I promise.

First: What is _router? #

Every Express app (whether it's a main app or a sub-app you're mounting) has an internal _router object. Think of it as Express's behind-the-scenes organizer: it keeps track of all the middleware, route handlers, and routing rules you've set up.

The _router.stack part is an array that stores all these items in the order they were added. When a request comes in, Express runs through this stack from top to bottom to process the request.

Next: The mount Event & Why once()? #

When you mount a sub-Express app to a parent app (like doing parent.use('/api', mySubApp)), the sub-app triggers a mount event right after it's attached. The once('mount') means we only listen for this event one time—since a sub-app can only be mounted once, there's no need to keep the listener around after that.

The callback gets a parent parameter, which is a reference to the main Express app you just mounted the sub-app onto.

Finally: Why parent._router.stack.pop()? #

Here's the tricky part. By default, when you mount a sub-app, Express automatically adds a new middleware to the parent app's _router.stack. This middleware's job is to forward any requests matching the mount path (like /api/*) to the sub-app.

But some developers want to override this default behavior. For example:

• They might want to add custom logic before forwarding requests to the sub-app
• They might want to handle the mount path themselves instead of letting Express auto-forward
• They might be building a custom routing setup that conflicts with the auto-added middleware

The pop() call removes that auto-added middleware from the parent's stack (since it's the last item added to the array, pop() targets it perfectly).

A Quick Warning #

Keep in mind that _router is an internal private property (the underscore is a hint from Express maintainers that this isn't part of the public API). Directly modifying it like this is a bit of a hack—Express could change how _router works in future versions, which would break this code. Only use this if you have a specific, unavoidable need!

内容的提问来源于stack exchange，提问作者batman