The Art of Crafting Codemods
About me
twitter/rajasegar_c
Front-end Developer @ Freshworks
github/rajasegar
hangaroundtheweb.com
We are hiring!
pradeep.naik@freshworks.com
The Problem(s)
1. Making changes in code
Let's start with a small change
Replace variable "hello" with "world"
let hello = "world";
function hello() {
console.log("hello world")
}
$ sed 's/hello/world' input.js
$ sed 's/hello/world' project1/app/**/*.js project2/app/**/*.js
let world = "world";
function world() {
console.log("world world")
}
let hello = "world";
function hello() {
console.log("hello world")
}
Change with(in) a context
Replace only the variable name "hello" with "world"
Replace variable "hello" with "world"
The Context
-
Which one is a variable name?
-
Which one is a function name?
-
Which one is a string literal?
2. Upgrading Code-base
Types of Upgrade
- Upgrade to new language syntax / features
- Upgrade a lib / framework to a newer version
Upgrade to new language syntax / features
mounted() {
var self = this
window.addEventListener('scroll', function() {
self.scrolled = true
})
}
mounted() {
window.addEventListener('scroll', () => {
this.scrolled = true
})
}
Arrow functions
Upgrade a lib / framework to a newer version
attached: function () {
doSomething()
}
mounted: function () {
this.$nextTick(function () {
doSomething()
})
}
Vue 1.x to 2.0
Making Changes to code
- Not just find/replace
- Remove code
- Add code
- And much more...
What is a Codemod
Code to rewrite code
- Partial Automation
- Human oversight
- Occasional intervention
What codemods can do?
-
Update source code to fit a team’s coding conventions
-
Make widespread changes when an API is modified
-
Automate large-scale refactoring tasks
-
Easily update your code to take advantage of newer language features
Code Transformations
What is an AST?
A Tree representation of the abstract syntactic structure of source code written in a programming language.
An AST is basically a DOM for your code.
How Compilers create Abstract Syntax Trees?
Scanner
( Lexical Analyzer )
function helloWorld() {
console.log('hello world');
}
Parser
( Syntax Analyzer )
function helloWorld() {
console.log('hello world');
}
{
"type": "Program",
"start": 0,
"end": 44,
"body": [
{
"type": "FunctionDeclaration",
"start": 0,
"end": 44,
"id": {
"type": "Identifier",
"start": 9,
"end": 14,
"name": "hello"
},
"expression": false,
"generator": false,
"params": [],
"body": {
"type": "BlockStatement",
"start": 17,
"end": 44,
"body": [
{
"type": "ExpressionStatement",
"start": 21,
"end": 42,
"expression": {
"type": "CallExpression",
"start": 21,
"end": 41,
"callee": {
"type": "MemberExpression",
"start": 21,
"end": 32,
"object": {
"type": "Identifier",
"start": 21,
"end": 28,
"name": "console"
},
"property": {
"type": "Identifier",
"start": 29,
"end": 32,
"name": "log"
},
"computed": false
},
"arguments": [
{
"type": "Literal",
"start": 33,
"end": 40,
"value": "hello",
"raw": "'hello'"
}
]
}
}
]
}
}
],
"sourceType": "module"
}
AST
Where is it used?
- Syntax Highlighting
- Code Completion
- Static Analysis (aka ESLint, etc.,)
- Code Coverage
- Minification
- JIT Compilation
- Source Maps
- Compiling to JS languages
- Code Refactoring & Migrations
- And much more ...
Codemods
jscodeshift
A JavaScript codemod toolkit.
RUNNER
WRAPPER
jscodeshift
RUNNER
WRAPPER
jscodeshift
( CLI )
( recast )
RUNNER
jscodeshift
( CLI )
recast
esprima
ast-types
AST => DOM
jscodeshift => jQuery
recast
const recast = require('recast');
const code = fs.readFileSync('code.js','utf-8');
const ast = recast.parse(code);
const faster = transform(ast);
const output = recast.print(faster).code;
/**
* This replaces every occurrence of variable "foo".
*/
module.exports = function(fileInfo, api) {
return api.jscodeshift(fileInfo.source)
.findVariableDeclarators('foo')
.renameTo('bar')
.toSource();
}
jscodeshift
Let's create a Codemod!
Reference Videos
- https://www.youtube.com/watch?v=d0pOgY8__JM
- https://www.youtube.com/watch?v=1X9p-RUUkak
- https://www.youtube.com/watch?v=8r_sXUDoPYo
- https://www.youtube.com/watch?v=mkg3NWcloOw&list=LLVxvrINFNKL9kCbakNFjstg&index=9&t=0s
- https://frontendmasters.com/courses/linting-asts/introducing-codemods-and-ast/
- https://www.youtube.com/watch?v=C06MohLG_3s
Blog Posts
- https://medium.com/@cpojer/effective-javascript-codemods-5a6686bb46fb
- https://benmccormick.org/2018/06/18/codemod-survival/
- https://www.sitepoint.com/getting-started-with-codemods/
- https://www.toptal.com/javascript/write-code-to-rewrite-your-code
- http://hangaroundtheweb.com/2019/03/codemods-the-new-age-saviors-for-js-developers/
Codemod tooling
- https://github.com/facebook/jscodeshift
- https://github.com/benjamn/recast
- https://github.com/benjamn/ast-types
- https://rajasegar.github.io/ast-builder/
- https://astexplorer.net
- https://github.com/rajasegar/awesome-codemods
Vue codemods
Take aways
#1
The idea is to not break things and to build transformation tools that we can be confident in.
#2
Tool-assisted code modification is set to profoundly transform the way people evolve and maintain very large scale codebases.
Questions?
Thank you
The Art of Crafting Codemods
By Rajasegar Chandiran
The Art of Crafting Codemods
A presentation about Codemods in Vue Hyderabad meetup #2
- 2,152