Why does JSON.stringify on TypeError return an empty object [duplicate]

Question

I'm using node 4.2, and I'm catching an error and using JSON.stringify on it. For most objects, this works fine. But when a [TypeError: callback is not a function] is thrown, it returns an empty object. if I console.log it directly, it works fine.

Mozilla's page says:

Boolean, Number, and String objects are converted to the corresponding primitive values during stringification, in accord with the traditional conversion semantics.

try {
    ...
} catch (err) {
    console.log('error: ' + JSON.stringify(err)) // outputs {}
}

Answer 1

When you use stringify on a TypeError, you're stringifying an object with no enumerable properties.

So if you do

var typeError = new TypeError("hey")
for(var prop in typeError) {
  console.log(prop) // this does not run
}

When you log using console.log, you're using valueOf, so

var typeError = new TypeError("hey")
typeError.valueOf()  // TypeError: hey(…)

Also, an error knows how to turn itself into a string, so this works too:

var typeError = new TypeError("hey")
typeError.toString() // "TypeError: hey"

---

If you want to log the properties of an object you can't see using a normal log, you can console.dir the object.

When you do it on the typeError, you'll see that it has a message property: