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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

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."

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