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JavaScript Build a REST API With Express Building API Routes in Express Error Handling in Express

Matthew Olivares
Matthew Olivares
4,181 Points
Posted by Matthew Olivares
Matthew Olivares
4,181 Points

Why does the 1st anon func. "next(err)" skip the 2nd anon. function in the "vote-:dir" handler to the error handler?

In the "/vote-:dir" handler, the "if" block creates an error with a 404 status for any request other than "up" or "down".

It is then passed through the next() function as such: next(err). However there is 2nd anonymous function after this that accepts (req, res). This anonymous function never calls next();

My understanding was that next() calls the next available middleware function sequentially.

But the variable "err" seems to bypass the 2nd anonymous function AND the main 404 catcher in "app.js" and moves directly to the error handler in "app.js" file.

What is going on? Does the 2nd anonymous function see that the variable passed into it's "req" parameter is an "Error" object and skip to the next available middleware function with 4 parameters (if it exists)?

1 Answer

Seth Kroger
Seth Kroger
56,414 Points
Seth Kroger
Seth Kroger
56,414 Points

It's not entirely true that "next() calls the next available middleware function sequentially". If next is called without an argument that is true but not if called with an argument. If next is called with an error as an argument, next(err) it will skip all the remaining regular middleware and execute the first 4-argument (i.e. error handling) middleware.