Where is toString of Number stored?

Question

Some code for example.

let a = 1 a.__proto__.toString = function(){ return 'test'} a.toString() //"test" a + '2' //"12" 

I realy can't understand where is toString method stored. If I do this with mutable objects I got

let o = {} o.__proto__.toString = function(){ return 'test'} o.toString() //"test" o + '2' //"test2" 

That works as I expected. So the question is where toString of Number or other immutable types stored and called when type conversion occurs.

Answer 1

So the question is where toString of Number or other immutable types stored and called when type conversion occurs

It's stored on Number.prototype. The more important question is:

How is a number converted to a string when doing 1 + 'a'?

Not via the toString method!

The toString method is only used when converting an object to a primitive value. However, a number is already a primitive value. Instead there is an internal ToString routine that is called to convert the number to a string. The details can be found in the ES2017 specification at 12.8.3, 7.7.12 and 7.1.12.1 . The details are a bit long, but it starts like this:

1. If m is NaN, return the String "NaN".
2. If m is +0 or -0, return the String "0".
3. ...

as you can see, these are very specific instructions for how to convert a number value to a string, which have nothing to do with the actual toString method defined on Number.prototype.toString.