In my journey to learn and master Ruby, I keep coming across new syntactic sugar every day that makes me happy. This morning, I stumbled upon the double-splat ** operator while learning about the FileUtils standard library.

Now, I’d already seen the splat *, which converts one or more arguments to an array, so I was curious what the double-splat did.

The results were impressive.

def run(a, *b, **c)
  pp [a, b, c]
end

# [10, [], {}]
run 10 

# [10, [20], {}]
run 10, 20 

# [10, [20, 30, 40], {}]
run 10, 20, 30, 40

# [10, [20, 30, 40, [50, 60]], {}]
run 10, 20, 30, 40, [50, 60]

# [10, [20, 30, 40], {:name=>"akshay", :age=>29}]
run 10, 20, 30, 40, name: "akshay", age: 29

# [10, [], {:name=>"akshay", :age=>29}]
run 10, name: "akshay", age: 29

The double-splat operator was introduced in Ruby 2.0. It does the same thing for keyword arguments what the splat operator does for plain arguments. It converts all the key-value pairs to a single hash.