Question

I am looking at JavaScript’s number type system.

I’m using Chrome, When I evaluate 15-- for a number literal I get a ReferenceError since it makes no sense to decrement a constant.

When I evaluate var x=10;x--; as expected everything works.

Expectantly var a=Infinity;a-- evaluates to Infinity, this all makes sense and is in accordance to the javascript language spec.

However to my surprise Infinity-- and Infinity++ evaluate to Infinity unlike other literals.

This also happens for Number.POSITIVE_INFINITY which is the same.

tl;dr :

Why does Infinity-- yield infinity as a result when 15-- and (new Number(15))-- yield a reference error?

Answer 1

Infinity as used in your example is not actually a value but refers to the Infinity property of the global object:

15.1 The Global Object

[…]

15.1.1 Value Properties of the Global Object

[…]

15.1.1.2 Infinity
The value of Infinity is +∞ (see 8.5). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

So, Infinity-- is the same as window.Infinity-- which is perfectly valid.