Why 'err.status = 404' and 'err.status(404)' are both used?

A definitive answer would require seeing a link to the course page and/or more of the code for context, but I might guess that these things are being done to different objects (of different types) that happen to have the same name.

For example, "err.status = 404;" might be a case where a new status property is being added to an object that did not have one before (perhaps one newly created); and "err.status(404);" could be a middleware method call on an argument passed to a use method.

Hmm, for whatever reason the video on which I asked the question did not get linked properly to the question. It's from the Build a REST API with Express course (https://teamtreehouse.com/library/next-steps-2).

I downloaded and unzipped the project files, navigated to qa-rest-api-completed and opened the code there in my editor to see:

// catch 404 and forward to error handler
app.use(function(req, res, next){
    var err = new Error("Not Found");
    err.status = 404;
    next(err);
});

// Error Handler

app.use(function(err, req, res, next){
    res.status(err.status || 500);
    res.json({
        error: {
            message: err.message
        }
    });
});

In seeing this now rather than as it was quickly going by in the video, I am realizing that the call in the first is err.status = 404 whereas in the second middleware function it is using the response object like this res.status(500)

Maybe that answers my question? In one case the Error object is having it's property "status" set to a value and then the error is passed onto the error handler using next. In the other case, the response object is having it's "status" method called to set the property to 500.

Would it work to write it the other way. EG:

res.status = err.status || 500;

I'm not seeing in the MDN on JS' Error has a method called status(). So it likely would not work to write:

err.status(500);

Let me know if I am misunderstanding something after this further review. Thanks all!