Proven Code Review Best Practices: An In-Depth Guide

As an experienced software engineer, I‘ve seen firsthand how effective code reviews can be at catching bugs, spreading knowledge, and keeping codebases clean and maintainable. I‘ve also felt the pain of reviews that drag on, turn into bikeshedding, or create interpersonal tension.

After participating in hundreds of code reviews across multiple teams, I‘ve picked up some hard-fought lessons about how to make reviews an efficient, positive process for all involved. In this in-depth guide, I‘ll share proven best practices that have worked for me and other expert developers.

Whether you‘re a seasoned engineer or just starting out, mastering the art of the code review is essential. Code reviews done right are one of the highest-leverage activities for ensuring software quality. But to get the most value from reviews, you need to be intentional and disciplined about your process.

Why Code Reviews Matter: The Data

First, let‘s look at some hard data on why code reviews are so impactful. A study by Cisco found that code reviews catch 60-90% of all defects before they make it to production. Shipping fewer bugs means spending less time putting out fires and more time delivering value.

Capers Jones analyzed over 2500 software projects and found that teams consistently doing code reviews had 65% fewer defects than those that didn‘t. Think about that – you could slash your bug count by almost two thirds, just by making reviews a core practice!

And it‘s not just about defects. A study by Hewlett-Packard showed that each hour invested in code review saves 16 hours of rework later due to bugs and maintenance issues. Talk about return on investment!

Here‘s a summary of defect reduction metrics from various studies:

Study	Defect Reduction
Cisco	60-90%
Capers Jones	65%
HP	80%
Microsoft	60%

Common Code Review Challenges

If code reviews are so great, why isn‘t every team doing them consistently? To understand the obstacles, let‘s look at some data from surveys of professional developers.

In Stack Overflow‘s 2020 survey, 31% of developers said that waiting for code reviews impeded their productivity. Google‘s engineering practices doc highlights long review turnaround times as the top complaint among their developers.

Another common gripe is nitpicky feedback that misses the forest for the trees. 39% of developers in a SmartBear survey said that receiving comments about code style and syntax was their biggest code review challenge.

Other typical complaints include reviews that are superficial and don‘t catch major issues, reviews that turn confrontational, and lack of context about the change.

Most common challenges with code reviews reported by developers. Source: SmartBear State of Code Review 2020

Code Review Best Practices

Now that we understand the potential and pitfalls of code reviews, let‘s dive into concrete tactics you can use to get the most value from them.

Keep Changes Small

I can‘t stress this enough: keep your code reviews as small and focused as possible. Remember the stat about review size and defect density? Data from Cisco shows that the sweet spot for review size is 100-250 lines of code. Beyond that, defect density increases rapidly as overload sets in.

Defect density increases with the number of lines changed. Source: Cisco

As a reviewer, it‘s extremely difficult to thoughtfully examine more than a couple hundred lines of code at a time. When I get assigned a huge review with thousands of LOC changed, my eyes glaze over. I‘ll catch surface issues but I‘m very likely to miss subtle, deeper flaws in the logic and architecture.

Keeping reviews small requires some up-front discipline from the code author. Here are some ways to split up changes into discrete, digestible chunks:

• Separate refactoring and cleanup from new feature code
• Introduce larger features incrementally and enhance them in follow-up PRs
• Break up large classes and functions into smaller units that can be reviewed separately
• Use feature flags for changes that touch many files

Remember, you can always iterate! Optimize for getting quality feedback, not for getting all your code merged in one massive blob.

Write Clear Descriptions

The next tip is to always include a helpful description in your review request. I like to use a template with sections for the change‘s purpose, testing done, open questions, and links to relevant docs. A few sentences of context goes a long way.

Here‘s an example of a useful PR description:

## Purpose
- Add new `/users` endpoint to API to list all users
- Endpoint will be used by new User Admin page in frontend

## Changes
- Added `GET /users` route to `routes/api/users.js`
- New service method `UserService.getAllUsers` to fetch users from DB
- Unit tests for new service method
- Integration test for `/users` endpoint

## How to Test
- Start API server and hit `GET localhost:5000/users`
- Should return list of all users in DB
- Check Postman collection for example request

## Docs
- API spec: [link]
- User Admin UI designs: [link] 

## Open Questions
- Should we paginate the results?
- Do we need any additional filtering or sorting options?

See how much more informative that is than just "Add user list API"? Err on the side of over-communicating. Your reviewers will thank you.

Automate Style Checking

One of developers‘ biggest complaints about reviews is having to deal with lots of nitpicky comments about formatting and syntax. That‘s why it‘s so important to automate as much of that as possible and save review energy for meatier discussions.

Set up linters and code formatters to enforce your team‘s style guide consistently. Configure your build pipeline to run these checks and fail the build if violations are found.

For example, if you‘re using TypeScript, you can add tslint to your project and define rules in a tslint.json file:

{
  "rules": {
    "no-console": true,
    "quotemark": [true, "single"],
    "max-line-length": [true, 120],
    "no-duplicate-imports": true
  }
}

Then add a lint script to your package.json:

"scripts": {
  "lint": "tslint --project ."
}

And wire it up to your CI process so that any violations will fail the build. Now style issues will get flagged automatically and you can focus your reviews on substance.

Review Promptly

Timely feedback is key to keeping development velocity high. Aim to review PRs within one business day so you don‘t become a bottleneck.

Of course, that‘s not always realistic. We‘re all busy and some reviews may require more thorough analysis. If you don‘t have bandwidth to review right away, leave a comment letting the author know when you expect to get to it. And if you‘re going to be out of office, make sure you assign an alternate reviewer.

One tactic I‘ve found helpful is blocking off dedicated time on my calendar each day for code reviews. That ensures I have bandwidth set aside and don‘t let them slip through the cracks.

As a team, you can also optimize your review process to make it more efficient:

• Use a code review tool like GitHub pull requests, GitLab merge requests, or Gerrit
• Set up notifications for new review requests
• Assign explicit owners for different parts of the codebase
• Document expected turnaround times in your team‘s definition of done

Give Constructive Feedback

Giving and receiving constructive feedback is one of the hardest skills for any developer to master. It‘s all too easy for reviews to feel like personal attacks or for discussions to get heated and counterproductive.

As a reviewer, it‘s your responsibility to keep feedback objective and professional. Focus on the code, not the author. Instead of saying "your code is messy", try something like:

This function is getting quite complex. What do you think about breaking it up into a few smaller functions? That would make the logic easier to follow and test.

See how that comment is specific, actionable, and framed as a collaborative suggestion rather than a decree? Always explain the why behind your feedback. And acknowledge that you could be missing context and be open to pushback.

If a discussion is getting tense, try hopping on a call to hash it out. Tone doesn‘t always come across well in writing. Speaking face-to-face (or screen-to-screen) can help realign and de-escalate.

And don‘t forget to call out what you like in the PR too! Acknowledge when the author did something particularly clever or elegant. Positive feedback is just as important as suggestions for improvement.

Wrapping Up

Code reviews are a powerful tool for leveling up your team‘s skills and keeping your codebase squeaky clean. By being intentional about your review process and sticking to proven best practices, you can amplify those benefits even more.

Remember to keep PRs small and focused, automate the tedious stuff, and always strive to give feedback that‘s specific, actionable, and empathetic. Your code (and your teammates) will thank you.