ESLint Fixers vs. Codemods, When to Use Each and Their Trade-offs
When it comes to automating code transformations, developers often have two primary tools at their disposal: ESLint fixers and codemods. While both can modify code programmatically, they serve different purposes—one is designed for continuous enforcement, while the other is meant for large-scale one-time migrations.
In this post, we’ll compare ESLint fixers and codemods, covering when to use each, their limitations, and why codemods are single-use while ESLint is better suited for ongoing maintenance.
What Are ESLint Fixers?
ESLint fixers are built-in or custom rules within ESLint that automatically modify code to match predefined best practices. These fixers help enforce consistency and prevent common mistakes across a codebase.
Common Use Cases
- Linting & formatting: Fixing indentation, spacing, semicolons, and other stylistic concerns.
- Simple rule enforcement: Converting
vartolet, enforcing===over==, or ensuring hooks follow best practices. - Preventing common pitfalls: Catching unused variables, missing dependencies, or incorrect function calls.
ESLint Ensures Code Shape Ongoing
ESLint is designed for continuous enforcement. Unlike codemods, which you run once, ESLint fixers operate every time a developer writes or commits code, ensuring a project remains in a consistent shape over time.
- ✅ Best when you need rules to persist across the lifecycle of a project.
- ✅ Integrates into CI/CD pipelines, pre-commit hooks, and IDEs to catch issues early.
However, this comes with trade-offs:
Downsides of ESLint Fixers
- ❌ Limited to small, localized fixes – They are great for enforcing code style but not for complex, project-wide refactoring.
- ❌ Not ideal for one-time upgrades – If a rule isn’t needed permanently, adding a temporary ESLint rule just to refactor code can be inefficient.
- ❌ Performance concerns in large codebases – Enforcing too many lint rules can slow down linting, increasing build times and slowing development workflows. For large projects, you may not want certain rules running indefinitely, as they can introduce unnecessary overhead.
What Are Codemods?
Codemods are scripts that perform large-scale, one-time code transformations. They typically use AST (Abstract Syntax Tree) manipulation via tools like jscodeshift, recast, or babel-codemod to make complex changes at scale.
Common Use Cases
- Migrating deprecated APIs – e.g., updating React class components to hooks.
- Performing large-scale refactoring – e.g., renaming a function across an entire codebase.
- Upgrading framework conventions – e.g., transitioning from
styled-componentsto Tailwind CSS. - Project-wide transformations – e.g., converting thousands of instances of a function call to a new format.
Codemods Are Single-Use for One-Time Migrations
Unlike ESLint, which is meant to enforce rules continuously, codemods are designed for one-time transformations. You run a codemod once, review the changes, and commit them—after that, it’s no longer needed.
- ✅ Best when making large, sweeping changes across an entire project.
- ✅ Ideal for framework updates and breaking changes, where the goal is to modify all occurrences of an outdated pattern.
- ✅ No long-term performance overhead, since codemods don’t run indefinitely.
Downsides of Codemods
- ❌ Not meant for ongoing enforcement – Once a codemod is run, it doesn’t persist as a rule like ESLint does. Developers must manually ensure new code follows the updated pattern.
- ❌ Higher effort to implement – Writing codemods requires AST knowledge and testing to avoid unintended consequences.
- ❌ Potential for breaking changes – If not carefully tested, a codemod can introduce regressions across a large codebase.
Which One Should You Use?
| Scenario | Use ESLint Fixer | Use Codemod |
|---|---|---|
| Enforcing consistent code style | ✅ Yes | ❌ No |
| Fixing minor syntax issues | ✅ Yes | ❌ No |
| Large-scale API migration | ❌ No | ✅ Yes |
| Updating framework conventions | ❌ No | ✅ Yes |
| Applying a one-time upgrade | ❌ No | ✅ Yes |
| Automatically fixing common mistakes | ✅ Yes | ❌ No |
| Modifying multiple files at once | ❌ No | ✅ Yes |
Final Thoughts
- Use ESLint fixers when you want ongoing rule enforcement to maintain code quality over time. However, be mindful that excessive rules in large codebases may slow down performance.
- Use codemods for one-time, large-scale code transformations that aren’t needed on an ongoing basis. They are powerful but require careful testing.
For teams maintaining large codebases, a combination of ESLint for continuous enforcement and codemods for major migrations can help balance performance, maintainability, and developer efficiency.