7 Patterns to Refactor JavaScript Applications: Service Objects

June 9, 2014 | By Michael Phillips, Engineer

On October 17, 2012, Bryan Helmkamp, founder of Code Climate, wrote a blog post outlining 7 patterns to refactor fat ActiveRecord models in Ruby on Rails. Here at Crush & Lovely, this post is a core reference for all Rails developers on how to separate concerns, write modular, concise and expressive code, and make testing exceedingly simple.

This series of posts demonstrates these concepts in the JavaScript environment; they are no less applicable to data models in JavaScript, and are equally as valuable. Each week, one of the seven patterns will be explained. This week, we’ll be talking about Service Objects.

Patterns

  1. Service Objects
  2. Value Objects
  3. Form Objects
  4. Query Objects
  5. View Objects
  6. Policy Objects
  7. Decorators

Service Objects

Service Objects are objects that perform a discrete operation or action. When a process becomes complex, hard to test, or touches more than one type of model, a Service Object can come in handy for cleaning up your code base.

The goal of a Service Object is to isolate an operation, and should aim to follow these principles:

  • Strict with input and output. Service Objects should be designed to handle a very specific process so we can forego the Robustness Principle in favor of creating a tool for a very discrete purpose.
  • Documented thoroughly. This module will be extracted out of the place it’s being executed, therefore it’s even more imperative that the object’s intent and use be well-explained.
  • Terminates after operation is complete. This pattern should not be conflated with a worker process, which could set an interval, listen for web socket messages continuously, or some other operation to which there is no immediate end. Service Objects should be invoked, perform their immediate operations (whether synchronous or asynchronous), and terminate.

Example

A program written for a teacher to grade their students has an end-of-year process which determines whether or not the student passed. The process takes all assignments to be evaluated, finds the average of the percentage grades, and then assigns the grade to the student.

By extracting this logic into a single module, we provide a centralized location for any future hooks in this action. For example, if an email needs to be sent to the parent if the student is not passing, we can easily add this to the object’s workflow by either adding another method or, even better, use another Service Object.

Testing

Even if the action grows in complexity, the test suite can still stay focused on this single operation, preventing large test files and cumbersome environment preparation.

Service Objects can be an extremely valuable tool in cleaning up and refactoring your code. Isolating actions keeps logic clean, orderly, easy to test, and ultimately leads to a more maintainable code base.


In the next post, we’ll take a look at Form Objects, which can make form validation and persistence easy and context-specific.

Special thanks to Justin Reidy and Anisha Vasandani for editorial and code review.