exp1 || exp2
evaluates exp1. If exp1 is true then exp2 is not evaluated (known as short circuit evaluation). If exp1 returns false then exp 2 is evaluated. If exp1 OR exp2 is true then (exp1||exp2) evaluates as true.
But in Javascript, you can set values using the operator.
a = something
if (prop)
a = prop
can be rewritten as
a = prop || something
