ruby中的false && nil和nil && false

Question

Why does nil && false return nil while false && nil returns false ?

I read the top answer in nil || false and false || nil in ruby . It seems to me that both nil and false evaluate as falsy value, but why do these two expressions return different values?

Answer 1

The rule is simple: && returns its first argument if it is falsy. Otherwise, it evaluate and returns its second argument.

So in

nil && false

and

false && nil

In both cases, the first argument is falsy. （ nil and false are the only two values that evaluate falsy), so the result of the expression is the first argument.

The fact that the second argument also happens to be false doesn't matter. In fact, the second argument isn't evaluated because of short-circuit.

Answer 2

It's about operations order. Nil doesn't have a boolean value. In && statement it returns false when at least on argument is false.

So in the expression: false && nil it is discovering that the first argument is false and there is pointless to check the second one, so it returned false. In second case - nil && false it encounters nil, so the value which cannot be compare with boolean, in consequence the expression cannot be evaluated, so it returns 'nil', like in every other similar case in ruby.

In '||' statement it gives false where first or second argument is false.

1st case: false || nil false || nil - it checks that first argument is false and then verifies second one which actually is nothing, so that it gives first value.

2nd case nil || false nil || false it checks first is nothing, then second which is boolean so it returns false.