Why is if (a,b = [nil, nil]) truthy in Ruby?

user_1983337 0
Viewed 68

I'm studying Ruby now, and I don't understand why the test condition

if (a,b = [nil, nil]) # <--
  true
else
  false
end

evaluates as truthy, while

if (a,b = nil) # <--
  true
else
  false
end

is falsy as advertised.

Tested on ruby 2.5 and 2.7 via irb. Added a complete if-else statement for your copy-paste testing.

if-statement ruby
2 Answers

Assignments evaluate to the same value as the right operand of the assignment. This is still true for destructuring assignments.

So a,b = [nil, nil] evaluates to [nil, nil], which truthy because it's an array and arrays are truthy, whereas a,b = nil evaluates to nil, which is falsy.

user_1493920

In ruby, only 2 values are false, which are nil and false itself.

All other values are truth, including arrays of nils.

Also, assign operation returns right side of assignment, so

(a,b = [nil, nil]) == [nil, nil]
(a,b = nil) == nil
user_7645460
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