babel-dead-code-elimination

Composable primitives for dead code elimination in Babel

This package is not a Babel plugin. It is a library for authoring your own Babel transforms and plugins.

Install

npm install babel-dead-code-elimination

Usage

You have a Babel AST and want to transform it:

import type { ParseResult } from "@babel/parser"
import type { File } from "@babel/types"

type AST = ParseResult<File>

export function transform(ast: AST) {
  // your code to mutate the AST goes here...
}

[!TIP] This example assumes that you already have a Babel AST. If you need to parse source code or generate source code, use @babel/parser and @babel/generator.

Dead code elimination

Use deadCodeElimination when you want to remove unreferenced identifiers:

import { deadCodeElimination } from "babel-dead-code-elimination"

export function transform(ast: AST) {
  deadCodeElimination(ast)
}

For example:

// Before
const name = "Ada"
const greeting = "Hello, " + name
const unusedMessage = "Goodbye"

console.log(greeting)

// After
const name = "Ada"
const greeting = "Hello, " + name

console.log(greeting)

Only target certain identifiers for removal

deadCodeElimination has a second argument for specifying which identifiers to consider for removal. When provided, any identifiers not specified will not be removed.

This can be extremely useful if you want to run dead code elimination after a custom transform. In such cases, you may want to preserve identifiers that were already unused in the original source. That way dead code elimination only removes identifiers that are unused because of your transform.

For example, use findReferencedIdentifiers to get a set of referenced identifiers prior to your transform and only target those for elimination:

import traverse from "@babel/traverse"

import {
  deadCodeElimination,
  findReferencedIdentifiers,
} from "babel-dead-code-elimination"

export function transform(ast: AST) {
  const referenced = findReferencedIdentifiers(ast)

  traverse(ast, {
    // your custom transform goes here
  })

  deadCodeElimination(ast, referenced)
}

For example, if the custom transform removes the console.log call:

// Before the custom transform
const startup = initializeApplication()
const greeting = "Hello"

console.log(greeting)

// After the custom transform
const startup = initializeApplication()
const greeting = "Hello"

// After dead code elimination
const startup = initializeApplication()

greeting was referenced before the custom transform and became unused afterward, so it can be removed. startup was already unused, so it is preserved along with its potentially side-effectful initializer.

Prior art

Credit to Jason Miller for the initial implementation. Thanks to these projects for exploring dead code elimination:

• Next.js
• babel-plugin-eliminator
• bling