what is global properties means in javascript

Question

Today I'm reading "javascript the definitive guide." in chapter 3.5 talk about the global object.There said:

global properties like undefined, infinity, and NaN

Because of my lack knowledge, I feel I could not understand it. In my mind, I feel undefined Infinity and NaN are values I don't know how could these are properties. Thanks for any thoughts.

wow javascript is so confusing I just check this:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined

undefined is a property and this property value is also undefined.
if this understanding is wrong please correct me.

They are both values and properties of the `window` object (for example). Additionally, `undefined` is a type. — Sebastian Simon, Sep 25 '16 at 19:45
@Xufox ok I see thank you so much. by the way the window object is belongs to BOM? — user3658992, Sep 25 '16 at 19:53

score 0 · Answer 1 · answered Sep 25 '16 at 19:48

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It simply means that they could be used in anywhere of your code at any time, i.e, they are always available to you in any scope of code.

Reference: http://www.w3schools.com/jsref/jsref_obj_global.asp

answered Sep 25 '16 at 19:48

Reuel Ribeiro

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@user3658992 Do not post “thanks” comments. Instead upvote helpful answers and accept the most helpful one. – Sebastian Simon Sep 25 '16 at 20:14

score 0 · Answer 2 · answered Sep 25 '16 at 19:50

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Properties of the global object are properties you can use anywhere without specifying the object

Instead of window.undefined you can write undefined
Instead of window.console.log you can write console.log
etc...

answered Sep 25 '16 at 19:50

user3

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score -2 · Accepted Answer · edited May 23 '17 at 10:27

The Global properties refers the variables that can be used anywhere throughout the document and need not be declared.

I would provide you this standard ECMA Language Specification for complete reference.

Update: For Down voters.

The reason to provide ECMA as a reference is here

Update: undefined is a property and this property value is also undefined.

Your understanding this perfectly correct.

Explanation is as:

undefined value

     primitive value used when a variable has not been assigned a value

Undefined type

     type whose sole value is the undefined value

as mentioned in the above standard link in sections 4.3.9 and 4.3.10. I was trying to convey this message but I posted just the link.

This is not an answer, it’s just a link. – Sebastian Simon Sep 25 '16 at 20:13 — Sebastian Simon, Sep 25 '16 at 20:13

what is global properties means in javascript

3 Answers3