CODEWITHSUNDEEP
javascriptnode.jsobjecttostring
Damaged Organic
Damaged Organic

Reputation: 8467

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

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?

Upvotes: 2

Views: 308

Answers (1)

Benjamin Gruenbaum
Benjamin Gruenbaum

Reputation: 276266

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.

Upvotes: 1

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