JavaScript TypeError, prints object as '#<Object>' string

Question

Probably a naive question - if this snippet is executed (I'm running with Node.js v12.16.3, same result in Chrome apart from the error type):

const obj = {}

Object.defineProperty(obj, 'a', {
  writable: false,
  value: 13
})

obj.a = 14

It will, obviously, throw an error, but the message string of that error has strange representation of an object:

TypeError: Cannot assign to read only property 'a' of object '#<Object>'

The question is - why it's stringified as #<Object>?

Defining a toString method on the object will render it as if it were called with Object.prototype.toString.call(obj):

const obj = {
  toString () {
    return 'stringified obj'
  }
}

Object.defineProperty(obj, 'a', {
  writable: false,
  value: 13
})

console.log(`${obj}`)

obj.a = 14

TypeError: Cannot assign to read only property 'a' of object '[object Object]'

While console.log outputs correctly. How to make that TypeError output the correct string representation? What does it use internally?

interestingly this happens only with "use strict";. Youre using babel in your snippet. Babel uses strict-mode. Without strict mode no error will be shown. Maybe it has something to do with the strict mode and the error - handling of it. — Matthias Gwiozda
– Matthias Gwiozda, Commented Mar 13, 2021 at 21:29
Correct, this error is explicitly thrown only in strict mode, but I have no evidence that the strict mode represents objects differently. Probably it's just an internal switch to throw or silently do nothing. — Damaged Organic
– Damaged Organic, Commented Mar 15, 2021 at 13:19

Benjamin Gruenbaum · Accepted Answer · 2021-03-31 20:01:47Z

1

+50

Node.js and Chrome share the same JavaScript engine called V8. That engine hard-codes the string "#" in the error message.

It uses the template:

T(StrictReadOnlyProperty,                                                    \
    "Cannot assign to read only property '%' of % '%'")

Which in turn gets interpolated as the object's constructor name. In this case Object. You can see this:

const obj = {};
obj.constructor.name; // Object

const obj2 = new class Bar {};
obj2.constructor.name; // Bar

Or, in your example:

const obj = new class SomeValue {}

Object.defineProperty(obj, 'a', {
  writable: false,
  value: 13
})

// TypeError: Cannot assign to read only property 'a' of object '#<SomeValue>'
obj.a = 14

Note the spec just says: "throw a TypeError exception." - this behaviour is entirely defined by the V8 engine and not JavaScript.

answered Mar 31, 2021 at 20:01

Benjamin Gruenbaum

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5 Comments

Damaged Organic Over a year ago

Thank you for the explanation. It is interesting though that defining toString on the object changes the object representation in the error message to [object Object].

Benjamin Gruenbaum Over a year ago

@DamagedOrganic it is, and enough so that a special symbol Symbol.toStringTag is defined to control this behaviour and the behaviour of what happens when you call Object.prototype.toString.call(myObject). I am not sure why V8 chose to use the constructor name here - Thanks for the interesting question.

Damaged Organic Over a year ago

Apparently, at the moment there's no way to control this behavior from Node.js runtime directly, so the answer above is exhaustive. Thank you.

Benjamin Gruenbaum Over a year ago

@DamagedOrganic I am a maintainer of said Node.js runtime - if there is a legitimate need to customize this please open an issue! (or email me (my email is in the Node.js repo home page at GitHub) and I will if you'd rather not).

Benjamin Gruenbaum Over a year ago

Also - happy to help with the actual question - always fun to dig in V8 and figure why they do stuff ^^

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