What is the value of the variable “upcased” in the below piece of code?

upcased = ["one", "two", "three"].map {|n| puts n.upcase }

Let’s take a look at puts as below:

>> puts "Hi"
Hi
=> nil

Note the nil at the end: that’s the return value from puts. After all, puts is a method, so it has to return something. As it happens, it always returns nil. The printing out of the string is an action the method performs.

Similarly, evaluating the code in question we can understand that while it’s a common learner mistake to expect the result to be ["ONE", "TWO", "THREE"]. In fact, it’s [nil, nil, nil]. Each time through the block, the block evaluates to the return value of puts to be nil.