How to check if a variable exists with a value without "undefined local variable or method"?

Change the present? to != '' and use the && operator which only tries to evaluate the seond expression if the first one is true:

if defined?(mmm) && (mmm != '') then puts "yes" end

But actually as of 2019 this is no longer needed as both the below work

irb(main):001:0> if (defined? mm) then
irb(main):002:1* if mm.present? then
irb(main):003:2* p true
irb(main):004:2> end
irb(main):005:1> end
=> nil
irb(main):006:0> if (defined? mm) then
irb(main):007:1* p mm
irb(main):008:1> end
=> nil

On Ruby on Rails

if defined?(mm) && mm.present?
  puts "acceptable variable"
end

On IRB

if defined?(mm) && !mm.blank? && !mm.nil?
  puts "acceptable variable"
end

It can make sure you won't get undefined variable or nil or empty value.

Understand how defined? works

a = 1
defined?(a) # => "local-variable"

b = nil
defined?(b) # => "local-variable"

c = ""
defined?(c) # => "local-variable"

d = []
defined?(d) # => "local-variable"

$e = 'text'
defined?($e) # => "global-variable"

defined?(f) # => nil
defined?($g) # => nil

Note that defined? checks variable in the scope it is.

Why you need defined?

When there is possible of undefined variable presence, you cannot just check it with only .nil? for eaxample, you will have a chance to get NameError.

a = nil
a.nil? # => true
b.nil? # => NameError: undefined local variable or method `b'