if

An if evaluates the given branch if its condition is truthy. Otherwise, it
evaluates the else branch if present.

a = 1
if a > 0
  a = 10
end
a #=> 10
b = 1
if b > 2
  b = 10
else
  b = 20
end
b #=> 20

To write a chain of if-else-if you use elsif:

if some_condition
  do_something
elsif some_other_condition
  do_something_else
else
  do_that
end

After an if, a variable’s type depends on the type of the expressions used in both branches.

a = 1
if some_condition
  a = "hello"
else
  a = true
end
# a : String | Bool
b = 1
if some_condition
  b = "hello"
end
# b : Int32 | String
if some_condition
  c = 1
else
  c = "hello"
end
# c : Int32 | String
if some_condition
  d = 1
end
# d : Int32 | Nil

Note that if a variable is declared inside one of the branches but not in the other one, at the end of the if it will also contain the Nil type.

Inside an if‘s branch the type of a variable is the one it got assigned in that branch, or the one that it had before the branch if it was not reassigned:

a = 1
if some_condition
  a = "hello"
  # a : String
  a.size
end
# a : String | Int32

That is, a variable’s type is the type of the last expression(s) assigned to it.

If one of the branches never reaches past the end of an if, like in the case of a return, next, break or raise, that type is not considered at the end of the if:

if some_condition
  e = 1
else
  e = "hello"
  # e : String
  return
end
# e : Int32