In express, why app.use function can use route argument?

In express framework 'app.use('/users', users);' defined in app.js and then use 'get('/', function(req, res, next){})' in route, why it can work when I search localhost:3000/users ?

Because I see native code , the path is always undefined, if use 'use' middleware, this is layer.js https://github.com/expressjs/express/blob/master/lib/router/layer.js#L44 and also I debug the path is also undefined

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