In terms of convention over configuration, Ruby's metaprogramming is crucial for the name-based class conventions that make Rails so convenient. For instance, if you have a class User with the attribute name, at runtime Rails dynamically adds methods to the User class, allowing you to do things like User.find_by_name("Alice"). Rails uses Ruby's metaprogramming capabilities to accomplish this.

The same is true for defining associations between models. Consider the following model:

   class User < ApplicationRecord
     belongs_to :organization
     scope :active, -> { where(active: true) }
   end

You mentioned that DSLs aren't important, but the DSL for defining associations is extremely convenient. Adding the belongs_to line defines numerous methods on the User class, allowing you to do things like User.last.organization. The scope line dynamically defines the active method, allowing you to call User.active, which executes the code in the provided block. There are also DSLs for defining routes and migrations, which are extremely important when using Rails.

Ruby's metaprogramming is also utilized in Rails controllers to load the correct code. For example, the index method in the UsersController knows to look for the views/users/index.html file because it can reflect on the method and the class to determine where to load the correct code using Rails' autoloader, Zeitwerk.

Ruby's open classes and monkey patching (something that should be done with extreme caution) allow ActiveSupport to add extremely useful methods to Ruby's base classes. This enables conveniences such as Date.today - 10.days or DateTime.now.beginning_of_day.

These are just a few examples that I can think of off the top of my head, but nearly every major convenience in Rails relies on the flexibility provided by Ruby. While it might be possible to reproduce many aspects of Rails in JavaScript, I don't believe you could replicate all of it with the same readability and intuitiveness that Rails offers.