Question

I don’t get why

(var ||= []) << 1

works as expected but

(var ||= true) = false

doesn’t.

Could anyone explain why it doesnt work and what is going on here?

Answer 1

a ||= b behaves like a || a = b.

An assignment returns the assigned value, i.e., var = true returns true.

var ||= true will evaluate to the assignment var = true, because var is undefined at that point. If var is defined and its value is true, it will return the value of var, that is true; if it’s false, it will return the value of true, which is true.

var ||= [] returns [], and your first expression evaluated to [] << 1, which is legal.

However, your second expression evaluates to true = false, which throws a compile error.

tl;dr

(var ||= []) << 1 ⟺ (var = []) << 1 ⟺ [] << 1 ✔

(var ||= true) = false ⟺ (var = true) = false ⟺ true = false ✘