Master the Art of the AST

//TODO Write linting rules to enforce your team’s code conventions Write your own javascript transpiling code Write powerful “code-mods” to automatically refactor thousands of legacy scripts from ES5 to ES6

//TODO Write linting rules to enforce your team’s code conventions ESLint Write your own javascript transpiling code Write powerful “code-mods” to automatically refactor thousands of legacy scripts from ES5 to ES6 jscodeshift

//TODO Write linting rules to enforce your team’s code conventions ESLint ESTreebased AST Write your own javascript transpiling code Write powerful “code-mods” to automatically refactor thousands of legacy scripts from ES5 to ES6 + Visitor Pattern jscodeshift

Yonatan Mevorach • Javascript infrastructure at • code, blog, speak, repeat blog.cowchimp.com github.com/cowchimp @cowchimp

Webpack Babel Uglifyjs Rollup ESLint Istanbul Ternjs jscodeshift

Abstract Syntax Tree cb http://flic.kr/p/gC4EkS

var foo = 'bar'; Program Body Variable Declaration Kind Declarations Variable Declarator Id Init Identifier Literal

var foo = 'bar'; { "type": "Program", "body": [ { "type": "VariableDeclaration", "declarations": [ { "type": "VariableDeclarator", "id": { "type": "Identifier", "name": "foo" }, "init": { "type": "Literal", "value": "bar" } } ], "kind": "var" } ] }

var foo = 'bar'; { "type": "Program", "body": [ { "type": "VariableDeclaration", "declarations": [ { "type": "VariableDeclarator", "id": { "type": "Identifier", "name": "foo" }, "init": { "type": "Literal", "value": "bar" } } ], "kind": "var" } ] }

I/ { O{ Parse Traverse Manipulate Generate code cbd http://flic.kr/p/3xNFue

I/ { O{ Parse cbd http://flic.kr/p/3xNFue

astexplorer.net

I/ { Traverse O{ cbd http://flic.kr/p/3xNFue

ESLint

DEMO 1\3 ESLint and then… Questions?

astexplorer.net/#/bF7PLcS6GQ

astexplorer.net/#/rKC56tJvJJ

astexplorer.net/#/x9ULEp6XWw

cb http://flic.kr/p/s8G9qy

I/ { Manipulate O{ cbd http://flic.kr/p/3xNFue

DEMO 2\3 and then… Questions?

astexplorer.net/#/aV9EoqGS2Q

astexplorer.net/#/D8mMdOO1gz

I/ { O { Generate code cbd http://flic.kr/p/3xNFue

jscodeshift

Babel jscodeshift Input ➡ Output Javascript ➡ Javascript Javascript ➡ Javascript Plugin-based Yes Yes Building Block AST-to-AST Transform function AST-to-AST Transform function Endorsed by Facebook Facebook cb http://flic.kr/p/91r3Wz

Babel jscodeshift Input ➡ Output Javascript ➡ Javascript Javascript ➡ Javascript Plugin-based Yes Yes Building Block AST-to-AST Transform function AST-to-AST Transform function Endorsed by Facebook Facebook Goal Compiler "Codemod" (refactoring) Example Use ES6 ➡ ES5 ES5 ➡ ES6 Output level Lower level than source Same level as source Output readability Best-effort (boilerplate code) High Usage frequency All the time One-time cb http://flic.kr/p/91r3Wz

DEMO 3\3 jscodeshift and then… Questions?

astexplorer.net/#/1WJItUlZFN

astexplorer.net/#/sAVP0oNPMd

github.com/cowchimp/awesome-ast blog.cowchimp.com @cowchimp yonatan@sears.co.il