throw is a statement only; it may not exist in a position where an expression is required. For similar reasons, you can’t put an if statement there, for example
var something = false || if (cond) { /* something */ }
is invalid syntax as well.
Only expressions (things that evaluate to a value) are permitted to be assigned to variables. If you want to throw, you have to throw as a statement, which means you can’t put it on the right-hand side of an assignment.
I suppose one way would be to use an IIFE on the right-hand side of the ||, allowing you to use a statement on the first line of that function:
var un = undefined
var v2 = un || (() => { throw new Error('nope') })();But that’s pretty weird. I’d prefer the explicit if – throw.