How to add a method to an existing object instance at runtime in Ruby?

Question

In Javascript I could do this.

var a = {};
a.f1 = function(){};

How can I do this in Ruby?

---

Update.

In JS code, a is an object instantiated without class. And function(){} is an anonymous function, and a.f1 = adds the function to the object instance. So the function is bound to the instance only, and nothing is related to the class or similar definitions.

Answer 1

Ruby and JS's object models are very different, but a direct translation may look like this:

a = Object.new
def a.f1(x) 
  2*x
end

a.f1(5) #=> 10

You can also use the Ruby's eigenclass:

class Object
  def metaclass
    class << self; self; end
  end
end

a = Object.new    
# Also: (class << a; self; end).send(:define_method, :f1) do |x|
a.metaclass.send(:define_method, :f1) { |x| 2*x }

A warning note: you'll see this kind of code in meta-programming/monkeypatching/... but it's not usual when writing "normal" code, where other techniques (module mix-ins, mainly) apply.

Answer 2

There's difference between JavaScript and other languages because JavaScript does inheritance in another way. So here's couldn't be direct analogue.

Let's pretend that {} is the A class in ruby and you create object from it

class A
end

a = A.new

And f1 function seats in some module B

module B
  def f1
    puts "you've extended your object"
  end
end

Now you can do what you want in similar way

a.extend(B)

a.f1 #=> "you've extended your object"