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Javascript Infinity - (infinity minus minus) gives infinity

I'm looking for a JavaScript number type system.

I am using Chrome. When I evaluate 15--

for a numeric literal, I get ReferenceError

as there is no point in decrementing the constant.

When I evaluate var x=10;x--;

as expected everything works.

Expected var a=Infinity;a--

evaluates to Infinity

, this all makes sense and conforms to the javascript spec .

However, to my surprise, Infinity--

and Infinity++

are evaluated Infinity

unlike other literals.

This also happens for Number.POSITIVE_INFINITY

that is the same.

tl; dr:

Why Infinity--

gives infinity as a result when 15--

and (new Number(15))--

gives a reference error?

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2 answers


Infinity

as used in your example is not actually a value but refers to a Infinity

property of the global object :

15.1 Global Object

[...]

15.1.1 Global Object Value Properties

[...]

15.1.1.2 Infinity
The value Infinity

is +∞

(see 8.5 ). This property has attributes {[[Writable]]:, false

[[Enumerable]]:, false

[[Configurable]]: false

}.



So, it Infinity--

coincides with window.Infinity--

, which is perfectly true.

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Since there is no such thing as infinity of a number, this is a concept, and hence when coding it is not built like other constants, but as an object like null or undefined, but with some properties, it behaves well with mathematical methods.



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