Javascript Infinity-- (infinity minus minus) yields Infinity - Stack Overflow

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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?

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?

• javascript
• infinity

Share Improve this question Follow edited Feb 8, 2013 at 17:14 Salman Arshad 273k8484 gold badges444444 silver badges534534 bronze badges asked Feb 8, 2013 at 17:09 Benjamin GruenbaumBenjamin Gruenbaum 277k8989 gold badges520520 silver badges517517 bronze badges

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2 Answers 2

Sorted by: Reset to default 11

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.

Because there is no such thing as a the number infinity, it is a concept, and thus in coding it isn't built as other constants but as an object like null or undefined but with some properties thrown in to make it behave nice with Math methods.

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