As a library developer, you could create a well-liked utility that a whole bunch of
hundreds of builders depend on each day, similar to lodash or React. Over time,
utilization patterns would possibly emerge that transcend your preliminary design. When this
occurs, you could want to increase an API by including parameters or modifying
operate signatures to repair edge circumstances. The problem lies in rolling out
these breaking adjustments with out disrupting your customers’ workflows.
That is the place codemods are available—a robust device for automating
large-scale code transformations, permitting builders to introduce breaking
API adjustments, refactor legacy codebases, and preserve code hygiene with
minimal handbook effort.
On this article, we’ll discover what codemods are and the instruments you may
use to create them, similar to jscodeshift, hypermod.io, and codemod.com. We’ll stroll by means of real-world examples,
from cleansing up characteristic toggles to refactoring element hierarchies.
You’ll additionally discover ways to break down advanced transformations into smaller,
testable items—a apply often called codemod composition—to make sure
flexibility and maintainability.
By the tip, you’ll see how codemods can turn into an important a part of your
toolkit for managing large-scale codebases, serving to you retain your code clear
and maintainable whereas dealing with even probably the most difficult refactoring
duties.
Breaking Adjustments in APIs
Returning to the situation of the library developer, after the preliminary
launch, new utilization patterns emerge, prompting the necessity to prolong an
API—maybe by including a parameter or modifying a operate signature to
make it simpler to make use of.
For easy adjustments, a primary find-and-replace within the IDE would possibly work. In
extra advanced circumstances, you would possibly resort to utilizing instruments like sed
or awk
. Nevertheless, when your library is extensively adopted, the
scope of such adjustments turns into tougher to handle. You’ll be able to’t make certain how
extensively the modification will influence your customers, and the very last thing
you need is to interrupt current performance that doesn’t want
updating.
A typical method is to announce the breaking change, launch a brand new
model, and ask customers emigrate at their very own tempo. However this workflow,
whereas acquainted, usually does not scale nicely, particularly for main shifts.
Take into account React’s transition from class parts to operate parts
with hooks—a paradigm shift that took years for big codebases to totally
undertake. By the point groups managed emigrate, extra breaking adjustments have been
usually already on the horizon.
For library builders, this example creates a burden. Sustaining
a number of older variations to help customers who haven’t migrated is each
pricey and time-consuming. For customers, frequent adjustments threat eroding belief.
They might hesitate to improve or begin exploring extra secure options,
which perpetuating the cycle.
However what in the event you may assist customers handle these adjustments mechanically?
What in the event you may launch a device alongside your replace that refactors
their code for them—renaming features, updating parameter order, and
eradicating unused code with out requiring handbook intervention?
That’s the place codemods are available. A number of libraries, together with React
and Subsequent.js, have already embraced codemods to clean the trail for model
bumps. For instance, React offers codemods to deal with the migration from
older API patterns, just like the outdated Context API, to newer ones.
So, what precisely is the codemod we’re speaking about right here?
What’s a Codemod?
A codemod (code modification) is an automatic script used to rework
code to observe new APIs, syntax, or coding requirements. Codemods use
Summary Syntax Tree (AST) manipulation to use constant, large-scale
adjustments throughout codebases. Initially developed at Fb, codemods helped
engineers handle refactoring duties for big tasks like React. As
Fb scaled, sustaining the codebase and updating APIs grew to become
more and more tough, prompting the event of codemods.
Manually updating hundreds of information throughout totally different repositories was
inefficient and error-prone, so the idea of codemods—automated scripts
that remodel code—was launched to sort out this downside.
The method usually includes three important steps:
- Parsing the code into an AST, the place every a part of the code is
represented as a tree construction. - Modifying the tree by making use of a change, similar to renaming a
operate or altering parameters. - Rewriting the modified tree again into the supply code.
By utilizing this method, codemods be sure that adjustments are utilized
persistently throughout each file in a codebase, decreasing the prospect of human
error. Codemods may deal with advanced refactoring situations, similar to
adjustments to deeply nested buildings or eradicating deprecated API utilization.
If we visualize the method, it might look one thing like this:

Determine 1: The three steps of a typical codemod course of
The concept of a program that may “perceive” your code after which carry out
computerized transformations isn’t new. That’s how your IDE works once you
run refactorings like
Primarily, your IDE parses the supply code into ASTs and applies
predefined transformations to the tree, saving the consequence again into your
information.
For contemporary IDEs, many issues occur underneath the hood to make sure adjustments
are utilized accurately and effectively, similar to figuring out the scope of
the change and resolving conflicts like variable title collisions. Some
refactorings even immediate you to enter parameters, similar to when utilizing
order of parameters or default values earlier than finalizing the change.
Use jscodeshift in JavaScript Codebases
Let’s take a look at a concrete instance to grasp how we may run a
codemod in a JavaScript challenge. The JavaScript group has a number of
instruments that make this work possible, together with parsers that convert supply
code into an AST, in addition to transpilers that may remodel the tree into
different codecs (that is how TypeScript works). Moreover, there are
instruments that assist apply codemods to whole repositories mechanically.
Probably the most common instruments for writing codemods is jscodeshift, a toolkit maintained by Fb.
It simplifies the creation of codemods by offering a robust API to
manipulate ASTs. With jscodeshift, builders can seek for particular
patterns within the code and apply transformations at scale.
You should utilize jscodeshift
to establish and change deprecated API calls
with up to date variations throughout a whole challenge.
Let’s break down a typical workflow for composing a codemod
manually.
Clear a Stale Function Toggle
Let’s begin with a easy but sensible instance to display the
energy of codemods. Think about you’re utilizing a characteristic
toggle in your
codebase to regulate the discharge of unfinished or experimental options.
As soon as the characteristic is dwell in manufacturing and dealing as anticipated, the subsequent
logical step is to scrub up the toggle and any associated logic.
For example, think about the next code:
const information = featureToggle('feature-new-product-list') ? { title: 'Product' } : undefined;
As soon as the characteristic is totally launched and not wants a toggle, this
might be simplified to:
const information = { title: 'Product' };
The duty includes discovering all situations of featureToggle
within the
codebase, checking whether or not the toggle refers to
feature-new-product-list
, and eradicating the conditional logic surrounding
it. On the similar time, different characteristic toggles (like
feature-search-result-refinement
, which can nonetheless be in growth)
ought to stay untouched. The codemod must perceive the construction
of the code to use adjustments selectively.
Understanding the AST
Earlier than we dive into writing the codemod, let’s break down how this
particular code snippet appears to be like in an AST. You should utilize instruments like AST
Explorer to visualise how supply code and AST
are mapped. It’s useful to grasp the node sorts you are interacting
with earlier than making use of any adjustments.
The picture beneath exhibits the syntax tree by way of ECMAScript syntax. It
accommodates nodes like Identifier
(for variables), StringLiteral
(for the
toggle title), and extra summary nodes like CallExpression
and
ConditionalExpression
.

Determine 2: The Summary Syntax Tree illustration of the characteristic toggle verify
On this AST illustration, the variable information
is assigned utilizing a
ConditionalExpression
. The take a look at a part of the expression calls
featureToggle('feature-new-product-list')
. If the take a look at returns true
,
the consequent department assigns { title: 'Product' }
to information
. If
false
, the alternate department assigns undefined
.
For a activity with clear enter and output, I choose writing exams first,
then implementing the codemod. I begin by defining a unfavorable case to
guarantee we don’t by chance change issues we need to go away untouched,
adopted by an actual case that performs the precise conversion. I start with
a easy situation, implement it, then add a variation (like checking if
featureToggle is known as inside an if assertion), implement that case, and
guarantee all exams cross.
This method aligns nicely with Check-Pushed Growth (TDD), even
in the event you don’t apply TDD recurrently. Figuring out precisely what the
transformation’s inputs and outputs are earlier than coding improves security and
effectivity, particularly when tweaking codemods.
With jscodeshift, you may write exams to confirm how the codemod
behaves:
const remodel = require("../remove-feature-new-product-list"); defineInlineTest( remodel, {}, ` const information = featureToggle('feature-new-product-list') ? { title: 'Product' } : undefined; `, ` const information = { title: 'Product' }; `, "delete the toggle feature-new-product-list in conditional operator" );
The defineInlineTest
operate from jscodeshift means that you can outline
the enter, anticipated output, and a string describing the take a look at’s intent.
Now, operating the take a look at with a traditional jest
command will fail as a result of the
codemod isn’t written but.
The corresponding unfavorable case would make sure the code stays unchanged
for different characteristic toggles:
defineInlineTest( remodel, {}, ` const information = featureToggle('feature-search-result-refinement') ? { title: 'Product' } : undefined; `, ` const information = featureToggle('feature-search-result-refinement') ? { title: 'Product' } : undefined; `, "don't change different characteristic toggles" );
Writing the Codemod
Let’s begin by defining a easy remodel operate. Create a file
known as remodel.js
with the next code construction:
module.exports = operate(fileInfo, api, choices) { const j = api.jscodeshift; const root = j(fileInfo.supply); // manipulate the tree nodes right here return root.toSource(); };
This operate reads the file right into a tree and makes use of jscodeshift’s API to
question, modify, and replace the nodes. Lastly, it converts the AST again to
supply code with .toSource()
.
Now we are able to begin implementing the remodel steps:
- Discover all situations of
featureToggle
. - Confirm that the argument handed is
'feature-new-product-list'
. - Change all the conditional expression with the consequent half,
successfully eradicating the toggle.
Right here’s how we obtain this utilizing jscodeshift
:
module.exports = operate (fileInfo, api, choices) { const j = api.jscodeshift; const root = j(fileInfo.supply); // Discover ConditionalExpression the place the take a look at is featureToggle('feature-new-product-list') root .discover(j.ConditionalExpression, { take a look at: { callee: { title: "featureToggle" }, arguments: [{ value: "feature-new-product-list" }], }, }) .forEach((path) => { // Change the ConditionalExpression with the 'consequent' j(path).replaceWith(path.node.consequent); }); return root.toSource(); };
The codemod above:
- Finds
ConditionalExpression
nodes the place the take a look at calls
featureToggle('feature-new-product-list')
. - Replaces all the conditional expression with the resultant (i.e.,
{
), eradicating the toggle logic and leaving simplified code
title: 'Product' }
behind.
This instance demonstrates how simple it’s to create a helpful
transformation and apply it to a big codebase, considerably decreasing
handbook effort.
You’ll want to put in writing extra take a look at circumstances to deal with variations like
if-else
statements, logical expressions (e.g.,
!featureToggle('feature-new-product-list')
), and so forth to make the
codemod sturdy in real-world situations.
As soon as the codemod is prepared, you may try it out on a goal codebase,
such because the one you are engaged on. jscodeshift offers a command-line
device that you should utilize to use the codemod and report the outcomes.
$ jscodeshift -t transform-name src/
After validating the outcomes, verify that each one purposeful exams nonetheless
cross and that nothing breaks—even in the event you’re introducing a breaking change.
As soon as glad, you may commit the adjustments and lift a pull request as
a part of your regular workflow.
Codemods Enhance Code High quality and Maintainability
Codemods aren’t simply helpful for managing breaking API adjustments—they’ll
considerably enhance code high quality and maintainability. As codebases
evolve, they usually accumulate technical debt, together with outdated characteristic
toggles, deprecated strategies, or tightly coupled parts. Manually
refactoring these areas might be time-consuming and error-prone.
By automating refactoring duties, codemods assist hold your codebase clear
and freed from legacy patterns. Usually making use of codemods means that you can
implement new coding requirements, take away unused code, and modernize your
codebase with out having to manually modify each file.
Refactoring an Avatar Element
Now, let’s take a look at a extra advanced instance. Suppose you’re working with
a design system that features an Avatar
element tightly coupled with a
Tooltip
. Every time a consumer passes a title
prop into the Avatar
, it
mechanically wraps the avatar with a tooltip.

Determine 3: A avatar element with a tooltip
Right here’s the present Avatar
implementation:
import { Tooltip } from "@design-system/tooltip"; const Avatar = ({ title, picture }: AvatarProps) => { if (title) { return (); } return ; };
The objective is to decouple the Tooltip
from the Avatar
element,
giving builders extra flexibility. Builders ought to have the ability to determine
whether or not to wrap the Avatar
in a Tooltip
. Within the refactored model,
Avatar
will merely render the picture, and customers can apply a Tooltip
manually if wanted.
Right here’s the refactored model of Avatar
:
const Avatar = ({ picture }: AvatarProps) => { return; };
Now, customers can manually wrap the Avatar
with a Tooltip
as
wanted:
import { Tooltip } from "@design-system/tooltip"; import { Avatar } from "@design-system/avatar"; const UserProfile = () => { return (); };
The problem arises when there are a whole bunch of Avatar usages unfold
throughout the codebase. Manually refactoring every occasion can be extremely
inefficient, so we are able to use a codemod to automate this course of.
Utilizing instruments like AST Explorer, we are able to
examine the element and see which nodes characterize the Avatar
utilization
we’re concentrating on. An Avatar
element with each title
and picture
props
is parsed into an summary syntax tree as proven beneath:

Determine 4: AST of the Avatar element utilization
Writing the Codemod
Let’s break down the transformation into smaller duties:
- Discover
Avatar
utilization within the element tree. - Verify if the
title
prop is current. - If not, do nothing.
- If current:
- Create a
Tooltip
node. - Add the
title
to theTooltip
. - Take away the
title
fromAvatar
. - Add
Avatar
as a toddler of theTooltip
. - Change the unique
Avatar
node with the brand newTooltip
.
To start, we’ll discover all situations of Avatar (I’ll omit a number of the
exams, however it’s best to write comparability exams first).
defineInlineTest( { default: remodel, parser: "tsx" }, {}, ``, ` `, "wrap avatar with tooltip when title is offered" );
Much like the featureToggle
instance, we are able to use root.discover
with
search standards to find all Avatar nodes:
root .discover(j.JSXElement, { openingElement: { title: { title: "Avatar" } }, }) .forEach((path) => { // now we are able to deal with every Avatar occasion });
Subsequent, we verify if the title
prop is current:
root
.discover(j.JSXElement, {
openingElement: { title: { title: "Avatar" } },
})
.forEach((path) => {
const avatarNode = path.node;
const nameAttr = avatarNode.openingElement.attributes.discover(
(attr) => attr.title.title === "title"
);
if (nameAttr) {
const tooltipElement = createTooltipElement(
nameAttr.worth.worth,
avatarNode
);
j(path).replaceWith(tooltipElement);
}
});
For the createTooltipElement
operate, we use the
jscodeshift API to create a brand new JSX node, with the title
prop utilized to the Tooltip
and the Avatar
element as a toddler. Lastly, we name replaceWith
to
change the present path
.
Right here’s a preview of the way it appears to be like in
Hypermod, the place the codemod is written on
the left. The highest half on the fitting is the unique code, and the underside
half is the remodeled consequence:

Determine 5: Run checks inside hypermod earlier than apply it to your codebase
This codemod searches for all situations of Avatar
. If a
title
prop is discovered, it removes the title
prop
from Avatar
, wraps the Avatar
inside a
Tooltip
, and passes the title
prop to the
Tooltip
.
By now, I hope it’s clear that codemods are extremely helpful and
that the workflow is intuitive, particularly for large-scale adjustments the place
handbook updates can be an enormous burden. Nevertheless, that is not the entire
image. Within the subsequent part, I’ll make clear a number of the challenges
and the way we are able to tackle these less-than-ideal facets.
Fixing Widespread Pitfalls of Codemods
As a seasoned developer, you recognize the “joyful path” is just a small half
of the total image. There are quite a few situations to think about when writing
a change script to deal with code mechanically.
Builders write code in quite a lot of kinds. For instance, somebody
would possibly import the Avatar
element however give it a distinct title as a result of
they may have one other Avatar
element from a distinct bundle:
import { Avatar as AKAvatar } from "@design-system/avatar"; const UserInfo = () => ( <AKAvatar title="Juntao Qiu" picture="/juntao.qiu.avatar.png" /> );
A easy textual content seek for Avatar
gained’t work on this case. You’ll want
to detect the alias and apply the transformation utilizing the proper
title.
One other instance arises when coping with Tooltip
imports. If the file
already imports Tooltip
however makes use of an alias, the codemod should detect that
alias and apply the adjustments accordingly. You’ll be able to’t assume that the
element named Tooltip
is at all times the one you’re on the lookout for.
Within the characteristic toggle instance, somebody would possibly use
if(featureToggle('feature-new-product-list'))
, or assign the results of
the toggle operate to a variable earlier than utilizing it:
const shouldEnableNewFeature = featureToggle('feature-new-product-list'); if (shouldEnableNewFeature) { //... }
They could even use the toggle with different situations or apply logical
negation, making the logic extra advanced:
const shouldEnableNewFeature = featureToggle('feature-new-product-list'); if (!shouldEnableNewFeature && someOtherLogic) { //... }
These variations make it tough to foresee each edge case,
growing the chance of unintentionally breaking one thing. Relying solely
on the circumstances you may anticipate is just not sufficient. You want thorough testing
to keep away from breaking unintended elements of the code.
Leveraging Supply Graphs and Check-Pushed Codemods
To deal with these complexities, codemods ought to be used alongside different
methods. For example, a couple of years in the past, I participated in a design
system parts rewrite challenge at Atlassian. We addressed this difficulty by
first looking the supply graph, which contained nearly all of inner
element utilization. This allowed us to grasp how parts have been used,
whether or not they have been imported underneath totally different names, or whether or not sure
public props have been continuously used. After this search section, we wrote our
take a look at circumstances upfront, making certain we coated nearly all of use circumstances, and
then developed the codemod.
In conditions the place we could not confidently automate the improve, we
inserted feedback or “TODOs” on the name websites. This allowed the
builders operating the script to deal with particular circumstances manually. Often,
there have been solely a handful of such situations, so this method nonetheless proved
helpful for upgrading variations.
Using Present Code Standardization Instruments
As you may see, there are many edge circumstances to deal with, particularly in
codebases past your management—similar to exterior dependencies. This
complexity signifies that utilizing codemods requires cautious supervision and a
overview of the outcomes.
Nevertheless, in case your codebase has standardization instruments in place, similar to a
linter that enforces a selected coding fashion, you may leverage these
instruments to cut back edge circumstances. By imposing a constant construction, instruments
like linters assist slim down the variations in code, making the
transformation simpler and minimizing surprising points.
For example, you would use linting guidelines to limit sure patterns,
similar to avoiding nested conditional (ternary) operators or imposing named
exports over default exports. These guidelines assist streamline the codebase,
making codemods extra predictable and efficient.
Moreover, breaking down advanced transformations into smaller, extra
manageable ones means that you can sort out particular person points extra exactly. As
we’ll quickly see, composing smaller codemods could make dealing with advanced
adjustments extra possible.
Codemod Composition
Let’s revisit the characteristic toggle elimination instance mentioned earlier. Within the code snippet
we now have a toggle known as feature-convert-new
should be eliminated:
import { featureToggle } from "./utils/featureToggle"; const convertOld = (enter: string) => { return enter.toLowerCase(); }; const convertNew = (enter: string) => { return enter.toUpperCase(); }; const consequence = featureToggle("feature-convert-new") ? convertNew("Hey, world") : convertOld("Hey, world"); console.log(consequence);
The codemod for take away a given toggle works wonderful, and after operating the codemod,
we would like the supply to appear like this:
const convertNew = (enter: string) => { return enter.toUpperCase(); }; const consequence = convertNew("Hey, world"); console.log(consequence);
Nevertheless, past eradicating the characteristic toggle logic, there are further duties to
deal with:
- Take away the unused
convertOld
operate. - Clear up the unused
featureToggle
import.
After all, you would write one large codemod to deal with all the things in a
single cross and take a look at it collectively. Nevertheless, a extra maintainable method is
to deal with codemod logic like product code: break the duty into smaller,
impartial items—similar to how you’d usually refactor manufacturing
code.
Breaking It Down
We will break the large transformation down into smaller codemods and
compose them. The benefit of this method is that every transformation
might be examined individually, protecting totally different circumstances with out interference.
Furthermore, it means that you can reuse and compose them for various
functions.
For example, you would possibly break it down like this:
- A metamorphosis to take away a particular characteristic toggle.
- One other transformation to scrub up unused imports.
- A metamorphosis to take away unused operate declarations.
By composing these, you may create a pipeline of transformations:
import { removeFeatureToggle } from "./remove-feature-toggle"; import { removeUnusedImport } from "./remove-unused-import"; import { removeUnusedFunction } from "./remove-unused-function"; import { createTransformer } from "./utils"; const removeFeatureConvertNew = removeFeatureToggle("feature-convert-new"); const remodel = createTransformer([ removeFeatureConvertNew, removeUnusedImport, removeUnusedFunction, ]); export default remodel;
On this pipeline, the transformations work as follows:
- Take away the
feature-convert-new
toggle. - Clear up the unused
import
assertion. - Take away the
convertOld
operate because it’s not used.

Determine 6: Compose transforms into a brand new remodel
It’s also possible to extract further codemods as wanted, combining them in
numerous orders relying on the specified end result.

Determine 7: Put totally different transforms right into a pipepline to type one other remodel
The createTransformer
Perform
The implementation of the createTransformer
operate is comparatively
simple. It acts as a higher-order operate that takes a listing of
smaller remodel features, iterates by means of the listing to use them to
the basis AST, and eventually converts the modified AST again into supply
code.
import { API, Assortment, FileInfo, JSCodeshift, Choices } from "jscodeshift"; kind TransformFunction = { (j: JSCodeshift, root: Assortment): void }; const createTransformer = (transforms: TransformFunction[]) => (fileInfo: FileInfo, api: API, choices: Choices) => { const j = api.jscodeshift; const root = j(fileInfo.supply); transforms.forEach((remodel) => remodel(j, root)); return root.toSource(choices.printOptions || { quote: "single" }); }; export { createTransformer };
For instance, you would have a remodel operate that inlines
expressions assigning the characteristic toggle name to a variable, so in later
transforms you don’t have to fret about these circumstances anymore:
const shouldEnableNewFeature = featureToggle('feature-convert-new'); if (!shouldEnableNewFeature && someOtherLogic) { //... }
Turns into this:
if (!featureToggle('feature-convert-new') && someOtherLogic) { //... }
Over time, you would possibly construct up a set of reusable, smaller
transforms, which might enormously ease the method of dealing with difficult edge
circumstances. This method proved extremely efficient in our work refining design
system parts. As soon as we transformed one bundle—such because the button
element—we had a couple of reusable transforms outlined, like including feedback
at the beginning of features, eradicating deprecated props, or creating aliases
when a bundle is already imported above.
Every of those smaller transforms might be examined and used independently
or mixed for extra advanced transformations, which hastens subsequent
conversions considerably. In consequence, our refinement work grew to become extra
environment friendly, and these generic codemods at the moment are relevant to different inner
and even exterior React codebases.
Since every remodel is comparatively standalone, you may fine-tune them
with out affecting different transforms or the extra advanced, composed ones. For
occasion, you would possibly re-implement a remodel to enhance efficiency—like
decreasing the variety of node-finding rounds—and with complete take a look at
protection, you are able to do this confidently and safely.