As a library developer, it’s possible you’ll create a preferred utility that a whole lot of
1000’s of builders depend on every day, reminiscent of lodash or React. Over time,
utilization patterns may emerge that transcend your preliminary design. When this
occurs, it’s possible you’ll want to increase an API by including parameters or modifying
operate signatures to repair edge instances. The problem lies in rolling out
these breaking adjustments with out disrupting your customers’ workflows.
That is the place codemods are available in—a robust software for automating
large-scale code transformations, permitting builders to introduce breaking
API adjustments, refactor legacy codebases, and keep code hygiene with
minimal handbook effort.
On this article, we’ll discover what codemods are and the instruments you’ll be able to
use to create them, reminiscent of jscodeshift, hypermod.io, and codemod.com. We’ll stroll via real-world examples,
from cleansing up characteristic toggles to refactoring element hierarchies.
You’ll additionally discover ways to break down complicated transformations into smaller,
testable items—a follow referred to as codemod composition—to make sure
flexibility and maintainability.
By the top, you’ll see how codemods can turn out to be a significant a part of your
toolkit for managing large-scale codebases, serving to you retain your code clear
and maintainable whereas dealing with even essentially the most difficult refactoring
duties.
Breaking Adjustments in APIs
Returning to the state of affairs of the library developer, after the preliminary
launch, new utilization patterns emerge, prompting the necessity to lengthen 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 may work. In
extra complicated instances, you may resort to utilizing instruments like sed
or awk
. Nonetheless, when your library is broadly adopted, the
scope of such adjustments turns into tougher to handle. You’ll be able to’t ensure how
extensively the modification will influence your customers, and the very last thing
you need is to interrupt present performance that doesn’t want
updating.
A standard 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.
Contemplate React’s transition from class parts to operate parts
with hooks—a paradigm shift that took years for big codebases to completely
undertake. By the point groups managed emigrate, extra breaking adjustments have been
usually already on the horizon.
For library builders, this case creates a burden. Sustaining
a number of older variations to help customers who haven’t migrated is each
expensive and time-consuming. For customers, frequent adjustments danger eroding belief.
They might hesitate to improve or begin exploring extra steady options,
which perpetuating the cycle.
However what when you might assist customers handle these adjustments routinely?
What when you might launch a software 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 in. 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 previous 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 initiatives 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 1000’s of information throughout totally different repositories was
inefficient and error-prone, so the idea of codemods—automated scripts
that rework code—was launched to deal with this drawback.
The method sometimes includes three principal 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 metamorphosis, reminiscent of renaming a
operate or altering parameters. - Rewriting the modified tree again into the supply code.
Through the use of this method, codemods make sure that adjustments are utilized
constantly throughout each file in a codebase, lowering the possibility of human
error. Codemods can even deal with complicated refactoring eventualities, reminiscent of
adjustments to deeply nested buildings or eradicating deprecated API utilization.
If we visualize the method, it will 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 if you
run refactorings like
Primarily, your IDE parses the supply code into ASTs and applies
predefined transformations to the tree, saving the outcome again into your
information.
For contemporary IDEs, many issues occur underneath the hood to make sure adjustments
are utilized accurately and effectively, reminiscent of figuring out the scope of
the change and resolving conflicts like variable title collisions. Some
refactorings even immediate you to enter parameters, reminiscent of when utilizing
order of parameters or default values earlier than finalizing the change.
Use jscodeshift in JavaScript Codebases
Let’s have a look at a concrete instance to know how we might run a
codemod in a JavaScript mission. The JavaScript neighborhood 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 rework the tree into
different codecs (that is how TypeScript works). Moreover, there are
instruments that assist apply codemods to complete repositories routinely.
Probably the most well-liked 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 use jscodeshift
to determine and change deprecated API calls
with up to date variations throughout a whole mission.
Let’s break down a typical workflow for composing a codemod
manually.
Clear a Stale Characteristic Toggle
Let’s begin with a easy but sensible instance to show the
energy of codemods. Think about you’re utilizing a characteristic
toggle in your
codebase to manage the discharge of unfinished or experimental options.
As soon as the characteristic is reside in manufacturing and dealing as anticipated, the following
logical step is to scrub up the toggle and any associated logic.
For example, contemplate the next code:
const knowledge = featureToggle('feature-new-product-list') ? { title: 'Product' } : undefined;
As soon as the characteristic is totally launched and now not wants a toggle, this
will be simplified to:
const knowledge = { 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 use instruments like AST
Explorer to visualise how supply code and AST
are mapped. It’s useful to know the node varieties you are interacting
with earlier than making use of any adjustments.
The picture under reveals the syntax tree by way of ECMAScript syntax. It
comprises 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 examine
On this AST illustration, the variable knowledge
is assigned utilizing a
ConditionalExpression
. The check a part of the expression calls
featureToggle('feature-new-product-list')
. If the check returns true
,
the consequent department assigns { title: 'Product' }
to knowledge
. If
false
, the alternate department assigns undefined
.
For a activity with clear enter and output, I desire writing checks first,
then implementing the codemod. I begin by defining a unfavourable case to
guarantee we don’t unintentionally change issues we need to depart untouched,
adopted by an actual case that performs the precise conversion. I start with
a easy state of affairs, implement it, then add a variation (like checking if
featureToggle is named inside an if assertion), implement that case, and
guarantee all checks go.
This method aligns nicely with Take a look at-Pushed Growth (TDD), even
when you don’t follow TDD frequently. 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’ll be able to write checks to confirm how the codemod
behaves:
const rework = require("../remove-feature-new-product-list"); defineInlineTest( rework, {}, ` const knowledge = featureToggle('feature-new-product-list') ? { title: 'Product' } : undefined; `, ` const knowledge = { 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 check’s intent.
Now, working the check with a traditional jest
command will fail as a result of the
codemod isn’t written but.
The corresponding unfavourable case would make sure the code stays unchanged
for different characteristic toggles:
defineInlineTest( rework, {}, ` const knowledge = featureToggle('feature-search-result-refinement') ? { title: 'Product' } : undefined; `, ` const knowledge = featureToggle('feature-search-result-refinement') ? { title: 'Product' } : undefined; `, "don't change different characteristic toggles" );
Writing the Codemod
Let’s begin by defining a easy rework operate. Create a file
referred to as rework.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 will begin implementing the rework steps:
- Discover all situations of
featureToggle
. - Confirm that the argument handed is
'feature-new-product-list'
. - Substitute 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 check is featureToggle('feature-new-product-list') root .discover(j.ConditionalExpression, { check: { callee: { title: "featureToggle" }, arguments: [{ value: "feature-new-product-list" }], }, }) .forEach((path) => { // Substitute the ConditionalExpression with the 'consequent' j(path).replaceWith(path.node.consequent); }); return root.toSource(); };
The codemod above:
- Finds
ConditionalExpression
nodes the place the check calls
featureToggle('feature-new-product-list')
. - Replaces all the conditional expression with the ensuing (i.e.,
{
), eradicating the toggle logic and leaving simplified code
title: 'Product' }
behind.
This instance demonstrates how straightforward it’s to create a helpful
transformation and apply it to a big codebase, considerably lowering
handbook effort.
You’ll want to jot down extra check instances to deal with variations like
if-else
statements, logical expressions (e.g.,
!featureToggle('feature-new-product-list')
), and so forth to make the
codemod strong in real-world eventualities.
As soon as the codemod is prepared, you’ll be able to check it out on a goal codebase,
such because the one you are engaged on. jscodeshift offers a command-line
software that you need to use to use the codemod and report the outcomes.
$ jscodeshift -t transform-name src/
After validating the outcomes, examine that each one purposeful checks nonetheless
go and that nothing breaks—even when you’re introducing a breaking change.
As soon as glad, you’ll be able to 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 will be time-consuming and error-prone.
By automating refactoring duties, codemods assist maintain your codebase clear
and freed from legacy patterns. Repeatedly 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 have a look at a extra complicated instance. Suppose you’re working with
a design system that features an Avatar
element tightly coupled with a
Tooltip
. At any time when a person passes a title
prop into the Avatar
, it
routinely 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 ( <Tooltip content material={title}> <CircleImage picture={picture} /> </Tooltip> ); } return <CircleImage picture={picture} />; };
The purpose is to decouple the Tooltip
from the Avatar
element,
giving builders extra flexibility. Builders ought to be capable of 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 <CircleImage picture={picture} />; };
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 ( <Tooltip content material="Juntao Qiu"> <Avatar picture="/juntao.qiu.avatar.png" /> </Tooltip> ); };
The problem arises when there are a whole lot of Avatar usages unfold
throughout the codebase. Manually refactoring every occasion could be extremely
inefficient, so we will use a codemod to automate this course of.
Utilizing instruments like AST Explorer, we will
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 under:
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. - Examine 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 baby of theTooltip
. - Substitute the unique
Avatar
node with the brand newTooltip
.
To start, we’ll discover all situations of Avatar (I’ll omit among the
checks, however you must write comparability checks first).
defineInlineTest(
{ default: rework, parser: "tsx" },
{},
`
<Avatar title="Juntao Qiu" picture="/juntao.qiu.avatar.png" />
`,
`
<Tooltip content material="Juntao Qiu">
<Avatar picture="/juntao.qiu.avatar.png" />
</Tooltip>
`,
"wrap avatar with tooltip when title is offered"
);
Just like the featureToggle
instance, we will use root.discover
with
search standards to find all Avatar nodes:
root .discover(j.JSXElement, { openingElement: { title: { title: "Avatar" } }, }) .forEach((path) => { // now we will deal with every Avatar occasion });
Subsequent, we examine 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 baby. 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 correct is the unique code, and the underside
half is the reworked outcome:
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 could be an enormous burden. Nonetheless, that is not the entire
image. Within the subsequent part, I’ll make clear among the challenges
and the way we will handle these less-than-ideal facets.
Fixing Frequent Pitfalls of Codemods
As a seasoned developer, you realize the “blissful path” is simply a small half
of the total image. There are quite a few eventualities to contemplate when writing
a metamorphosis script to deal with code routinely.
Builders write code in a wide range of kinds. For instance, somebody
may import the Avatar
element however give it a special title as a result of
they could have one other Avatar
element from a special package deal:
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 right
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 all the time the one you’re in search of.
Within the characteristic toggle instance, somebody may 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 may even use the toggle with different situations or apply logical
negation, making the logic extra complicated:
const shouldEnableNewFeature = featureToggle('feature-new-product-list'); if (!shouldEnableNewFeature && someOtherLogic) { //... }
These variations make it tough to foresee each edge case,
rising the danger of unintentionally breaking one thing. Relying solely
on the instances you’ll be able to anticipate just isn’t sufficient. You want thorough testing
to keep away from breaking unintended components of the code.
Leveraging Supply Graphs and Take a look at-Pushed Codemods
To deal with these complexities, codemods ought to be used alongside different
strategies. For example, just a few years in the past, I participated in a design
system parts rewrite mission at Atlassian. We addressed this concern by
first looking out the supply graph, which contained nearly all of inside
element utilization. This allowed us to know 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 ceaselessly used. After this search section, we wrote our
check instances upfront, guaranteeing we coated nearly all of use instances, 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 working the script to deal with particular instances manually. Normally,
there have been solely a handful of such situations, so this method nonetheless proved
useful for upgrading variations.
Using Current Code Standardization Instruments
As you’ll be able to see, there are many edge instances to deal with, particularly in
codebases past your management—reminiscent of exterior dependencies. This
complexity signifies that utilizing codemods requires cautious supervision and a
assessment of the outcomes.
Nonetheless, in case your codebase has standardization instruments in place, reminiscent of a
linter that enforces a specific coding fashion, you’ll be able to leverage these
instruments to scale back edge instances. By imposing a constant construction, instruments
like linters assist slim down the variations in code, making the
transformation simpler and minimizing sudden points.
For example, you possibly can use linting guidelines to limit sure patterns,
reminiscent of 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 complicated transformations into smaller, extra
manageable ones means that you can deal with particular person points extra exactly. As
we’ll quickly see, composing smaller codemods could make dealing with complicated
adjustments extra possible.
Codemod Composition
Let’s revisit the characteristic toggle elimination instance mentioned earlier. Within the code snippet
we’ve got a toggle referred to as feature-convert-new
have to be eliminated:
import { featureToggle } from "./utils/featureToggle"; const convertOld = (enter: string) => { return enter.toLowerCase(); }; const convertNew = (enter: string) => { return enter.toUpperCase(); }; const outcome = featureToggle("feature-convert-new") ? convertNew("Hi there, world") : convertOld("Hi there, world"); console.log(outcome);
The codemod for take away a given toggle works positive, and after working the codemod,
we wish the supply to appear to be this:
const convertNew = (enter: string) => { return enter.toUpperCase(); }; const outcome = convertNew("Hi there, world"); console.log(outcome);
Nonetheless, past eradicating the characteristic toggle logic, there are extra duties to
deal with:
- Take away the unused
convertOld
operate. - Clear up the unused
featureToggle
import.
In fact, you possibly can write one massive codemod to deal with all the things in a
single go and check it collectively. Nonetheless, a extra maintainable method is
to deal with codemod logic like product code: break the duty into smaller,
impartial items—similar to how you’ll usually refactor manufacturing
code.
Breaking It Down
We are able to break the massive transformation down into smaller codemods and
compose them. The benefit of this method is that every transformation
will be examined individually, overlaying totally different instances with out interference.
Furthermore, it means that you can reuse and compose them for various
functions.
For example, you may break it down like this:
- A change to take away a selected characteristic toggle.
- One other transformation to scrub up unused imports.
- A change to take away unused operate declarations.
By composing these, you’ll be able to 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 rework = createTransformer([ removeFeatureConvertNew, removeUnusedImport, removeUnusedFunction, ]); export default rework;
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 now not used.
Determine 6: Compose transforms into a brand new rework
It’s also possible to extract extra codemods as wanted, combining them in
varied orders relying on the specified final result.
Determine 7: Put totally different transforms right into a pipepline to kind one other rework
The createTransformer
Operate
The implementation of the createTransformer
operate is comparatively
simple. It acts as a higher-order operate that takes an inventory of
smaller rework features, iterates via 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((rework) => rework(j, root)); return root.toSource(choices.printOptions || { quote: "single" }); }; export { createTransformer };
For instance, you possibly can have a rework operate that inlines
expressions assigning the characteristic toggle name to a variable, so in later
transforms you don’t have to fret about these instances anymore:
const shouldEnableNewFeature = featureToggle('feature-convert-new'); if (!shouldEnableNewFeature && someOtherLogic) { //... }
Turns into this:
if (!featureToggle('feature-convert-new') && someOtherLogic) { //... }
Over time, you may construct up a group of reusable, smaller
transforms, which may enormously ease the method of dealing with difficult edge
instances. This method proved extremely efficient in our work refining design
system parts. As soon as we transformed one package deal—such because the button
element—we had just a few reusable transforms outlined, like including feedback
in the beginning of features, eradicating deprecated props, or creating aliases
when a package deal is already imported above.
Every of those smaller transforms will be examined and used independently
or mixed for extra complicated transformations, which hurries up subsequent
conversions considerably. Consequently, our refinement work grew to become extra
environment friendly, and these generic codemods at the moment are relevant to different inside
and even exterior React codebases.
Since every rework is comparatively standalone, you’ll be able to fine-tune them
with out affecting different transforms or the extra complicated, composed ones. For
occasion, you may re-implement a rework to enhance efficiency—like
lowering the variety of node-finding rounds—and with complete check
protection, you are able to do this confidently and safely.