What does the tap method do in Ruby?

Object#tap yields the object it is called upon to a block but the resulting value is the value of the object (not the value of the block). For example:

> 1.tap { |obj| puts obj * 2 }
2
=> 1

The value 1 is passed to the block where we output the result of obj * 2 (2). However, the value of the expression is the value of the object: 1.

Say we have a User class:

class User
  attr_accessor :name
end

To create a new user, we might do something like this:

def user
  new_user = User.new
  new_user.name = @name
  new_user
end

The return value of the above method is a new user object with its name assigned to the value of @name.

You can see we have to create the temporary variable new_user to hold the value of the new user in order for us to assign its name. We then return the value of the temporary variable.

#tap removes the need for the temporary variable.

def user
  User.new.tap { |obj| obj.name = @name }
end

In the above example, we call #tap on the newly instantiated user object. Inside the block, we assign a name to the new user. The resulting value is the newly instantiated user object with its name assigned the value of @name.