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JavaScript

Posted by Ryan Scharfer
Ryan Scharfer
Courses Plus Student 12,537 Points

Why does calling window.myVariable get me undefined when the variable hasn't been declared yet?

If I console.log(myVariable) before declaring it, I will get an Uncaught ReferenceError because myVariable is not yet defined. But when I console.log(window.myVariable), I get the value undefined, as if myVariable already has a memory space. What gives? Why don't I get a Uncaught ReferenceError?

1 Answer

Jason Anello
Jason Anello
Courses Plus Student 94,610 Points

Hi Ryan,

I wasn't sure about this myself so I did some research on it.

I came across this blog post which I think explains the results you're getting: https://javascriptweblog.wordpress.com/2010/08/09/variables-vs-properties-in-javascript/

I would have a look at that. In particular, the section "What is a variable?" and the sub-section "hoisting".

From "What is a variable?" 3rd paragraph:

"Already we can see the essential difference emerging. Properties belong to objects; Variables belong to contexts (and context happens to have an object representation – the VariableObject)."

From "hoisting" note 2):

"2) We could have avoided the ReferenceError by simply accessing b by property syntax window.b. When confronted with b without an object qualifier JavaScript assumes we are referencing a variable and so checks its VariableObject (which has no property named b) When an identifier is not found in the VariableObject we get a ReferenceError. Conversely simple property accessors will return the result of a hash lookup on the parent object (in this case the value undefined). More on ReferenceErrors in my next post."

Ryan Scharfer
Ryan Scharfer
Courses Plus Student 12,537 Points

Thanks, Jason! Sounds like good stuff. I'll have a look at it.