CODEWITHSUNDEEP
javascript
Александр Чиркин
Александр Чиркин

Reputation: 241

Where is toString of Number stored?

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.

Upvotes: 21

Views: 1089

Answers (4)

Felix Kling
Felix Kling

Reputation: 816364

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.

Upvotes: 28

Ry-
Ry-

Reputation: 224886

The issue here isn’t with Number.prototype; it’s just that Number.prototype.toString isn’t called when converting numbers to strings. JavaScript uses its internal ToString operation for string conversions, and it has special behaviour for primitives.

Number: Return NumberToString(argument).

Upvotes: 4

mehulmpt
mehulmpt

Reputation: 16557

JavaScript doesn't use toString to convert primitive data types to string. It uses its own implementation. As a matter of fact, all these examples below follow your example:

let a = 1, b = 'string', c = false, d = 100.5, e = {}
a.__proto__.toString = _ => 'test'
b.__proto__.toString = _ => 'test'
c.__proto__.toString = _ => 'test'
d.__proto__.toString = _ => 'test'
e.__proto__.toString = _ => 'test'

console.log(a + 'X', b + 'X', c + 'X', d + 'X', e + 'X')

Upvotes: 1

kumesana
kumesana

Reputation: 2490

It's stored as a property of the prototype...

Remember that a number is a number. When you use it to be evaluated in an expression, such as with operator +, its toString method is not invoked. Instead, its value is resolved, and used to evaluate the rest of the expression.

Upvotes: -1

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