Why is a Javascript Error object not treated as an object by expressjs

Question

I have a node/express backend. In my backend I create an error object if there is an error, but when I try to send it in the response it is not there. I was able to make my code work by building a new object in the response, but I would like to know why it didn't work.

The relevant code is:

var error = new Error('some error message')

app.send({error}) // returns {}

app.send({error: error.message}) // returns {error: 'some error message}

According to MDN docs, an error object is an object, and we should therefore be able to pass it directly into app.send(). This didn't work in practice, and I would like to be able to explain why. Thanks for your help!

Answer 1

I assume that app.send converts the value to JSON. JSON.stringify only considers (own) enumerable properties and message is not enumerable:

> Object.getOwnPropertyDescriptor(new Error('foo'), 'message');
Object {value: "foo", writable: true, enumerable: false, configurable: true}