Javascript未定义对象

Question

I know the best way to check if a variable is undefined is

if ( typeof( something ) == "undefined") 

What I don't understand is when is a variable undefined and when is an object undefined. For instance when I console.log() the a variable I get

Object {detailedView: undefined}

My question is why I am getting this and not a plain undefined.

Answer 1

a isn't undefined. It's defined. It's an object. That object just so happens to contain a property that is undefined , but the variable itself isn't undefined.

something isn't defined, so it's undefined .

Answer 2

There are two things here: the "value" undefined , which is a value that a variable can point to when it has been declared but not assigned anything yet (ie var a; ), or when it's been assigned something that doesn't actually have a value or exists, and there is the string "undefined" , which is a string similar to "monkey" or "collywobble".

In your conditional, you're testing to see what the result of the typeof operator is, which is a string "object" or "function", or in this case "undefined". Those are just words:

if(typeof a == "undefined")

is the same as testing

if(a == undefined)

So, when you console.log the actual object, you'll see it has a value undefined , rather than being a string "undefined" .

Answer 3

I believe you're asking about the difference between "undefined variables" (variables never declared) and "undefined values" (the value undefined inside a variable or property).

You define/declare a variable with the var keyword:

var myVariable;

If you just do that, the value of that variable is undefined :

console.log(myVariable); // undefined

If you don't declare a variable, you cannot use it:

console.log(myOtherVariable); // throws a ReferenceError

... except in typeof :

typeof myOtherVariable == "undefined"; // true