Why does typeof only sometimes throw ReferenceError?

Question

In Chrome and Firefox,

typeof foo

evalulates to \'undefined\'.

But

typeof (function() { return foo; })         


        

Answer 1

Digging through the spec, I think this all comes down to when the operator in question attempts to run GetValue() on its operand.

typeof attempts to determine its operand's Type first. If that type is a Reference and is IsUnresolvableReference(), then it bails out and returns undefined. In essence, it does not fully evaluate the operand; if it did, anything that was undefined would throw an exception, so instead it short circuits and returns a nice, useful string.

In the examples, self-executing functions and the addition operator call GetValue without first checking for IsUnresolvableReference() like typeof does: they call GetValue and throw an exception if the reference is unresolved (foo is undefined in our case). (I think! This is my best guess from reading through the spec.)