What is a Block?

You've already been writing blocks. Every time you wrote each do |x| ... end, the part between do and end was a block: a chunk of code passed to a method.

[1, 2, 3].each do |n|
  puts n * 2
end

Here, each is a method on the array, and do |n| ... end is the block. each runs the block once for every element, passing it in as n.

Blocks are not separate objects you store in variables, they're tied directly to the method call. A method either gets a block or doesn't, and most iteration methods (each, map, select, times) are designed to take one.

3.times do |i|
  puts "hi #{i}"
end

Calling a method without its block changes what it does. [1,2,3].map with no block returns an enumerator instead of a transformed array, so the block is part of what the method means.