Logo of magicast


Programmatically modify JavaScript and TypeScript source codes.

Programmatically modify JavaScript and TypeScript source codes with a simplified, elegant and familiar syntax. Built on top of the AST parsed by recast and babel.

❯ 🧙🏼 Magical modify a JS/TS file and write back magically just like JSON!
❯ 🔀 Exports/Import manipulate module's imports and exports at ease
❯ 💼 Function Arguments easily manipulate arguments passed to a function call, like defineConfig()
❯ 🎨 Smart Formatting preseves the formatting style (quotes, tabs, etc.) from the original code
❯ 🧑‍💻 Readable get rid of the complexity of AST manipulation and make your code super readable


Install npm package:

# using yarn
yarn add --dev magicast

# using npm
npm install -D magicast

# using pnpm
pnpm add -D magicast

Import utilities:

// ESM / Bundler
import { parseModule, generateCode, builders, createNode } from "magicast";

// CommonJS
const { parseModule, generateCode, builders, createNode } = require("magicast");


Example: Modify a file:


export default {
  foo: ["a"],

Code to modify and append b to foo prop of defaultExport:

import { loadFile, writeFile } from "magicast";

const mod = await loadFile("config.js");


await writeFile(mod, "config.js");

Updated config.js:

export default {
  foo: ["a", "b"],

Example: Directly use AST utils:

import { parseModule, generateCode } from "magicast";

// Parse to AST
const mod = parseModule(`export default { }`);

// Ensure foo is an array
mod.exports.default.foo ||= [];
// Add a new array member

// Generate code
const { code, map } = generateCode(mod);

Generated code:

export default {
  foo: ["a", "b"],

Example: Get the AST directly:

import { parseModule, generateCode } from "magicast";

const mod = parseModule(`export default { }`);

const ast = mod.exports.default.$ast
// do something with ast

Example: Function arguments:

import { parseModule, generateCode } from "magicast";

const mod = parseModule(`export default defineConfig({ foo: 'bar' })`);

// Support for both bare object export and `defineConfig` wrapper
const options = mod.exports.default.$type === 'function-call'
  ? mod.exports.default.$args[0]
  : mod.exports.default;

console.log(options.foo) // bar

Example: Create a function call:

import { parseModule, generateCode, builders } from "magicast";

const mod = parseModule(`export default {}`);

const options = mod.exports.default.list = builders.functionCall('create', [1, 2, 3])

console.log(mod.generateCode()) // export default { list: create([1, 2, 3]) }


As JavaScript is a very dynamic language, you should be aware that Magicast's convention CAN NOT cover all possible cases. Magicast serves as a simple and maintainable interface to update static-ish JavaScript code. When interacting with Magicast node, be aware that every option might have chance to throw an error depending on the input code. We recommend to always wrap the code in a try/catch block (even better to do some defensive coding), for example:

import { loadFile, writeFile } from "magicast";

function updateConfig() {
  try {
    const mod = await loadFile("config.js");


    await writeFile(mod);
  } catch (e) {
    console.error('Unable to update config.js')
    console.error('Please update it manually with the following instructions: ...')
    // handle error

High Level Helpers

We also experiment to provide a few high level helpers to make common tasks easier. You could import them from magicast/helpers. They might be moved to a separate package in the future.

import {
  // ...
} from "magicast/helpers";

We recommend to check out the source code and test cases for more details.


  • Clone this repository
  • Install latest LTS version of Node.js
  • Enable Corepack using corepack enable
  • Install dependencies using pnpm install
  • Run interactive tests using pnpm dev


Made with 💛

Published under MIT License.