javascript中的全局属性是什么意思

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.

Answer 1

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

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

Answer 2

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...

Answer 3

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.

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1. Update: For Down voters.

The reason to provide ECMA as a reference is here

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

Your understanding this perfectly correct.

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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.