Expand type aliases from a typescript file

reveal.ts

import { readFileSync } from 'node:fs'
import { resolve } from 'node:path'
import JSON5 from 'json5'
import ts from 'typescript'
import * as prettier from 'prettier'

/**
 * reveal all top level `type` declarations from a file
 */
export const reveal = async (sourcePath: string) => {
  const program = ts.createProgram([sourcePath], getConfig())
  const checker = program.getTypeChecker()

  const revealed = program
    .getSourceFile(sourcePath)!
    .getChildren()
    .flatMap(c => c.getChildren())
    .filter(ts.isTypeAliasDeclaration)
    .map(c => getFullDeclarationText(checker, c))
    .join('\n\n')

  return await prettier.format(revealed, {
    parser: 'typescript',
    semi: false,
  })
}

const getFullDeclarationText = (
  checker: ts.TypeChecker,
  c: ts.TypeAliasDeclaration,
) => getTypeDeclarationText(c) + typeToString(checker, c)

const typeToString = (checker: ts.TypeChecker, c: ts.TypeAliasDeclaration) =>
  checker.typeToString(
    checker.getTypeAtLocation(c.type),
    undefined,
    ts.TypeFormatFlags.InTypeAlias | ts.TypeFormatFlags.NoTruncation,
  )

const getTypeDeclarationText = (c: ts.TypeAliasDeclaration) =>
  c.getText().slice(0, c.getText().lastIndexOf(c.type.getText()))

const getConfig = () => {
  const configFile = ts.findConfigFile(
    '.',
    ts.sys.fileExists,
    'tsconfig.json',
  ) as string
  // return JSON5.parse(readFileSync(configFile, { encoding: 'utf-8' }))
  return JSON5.parse(readFileSync(configFile) as any)
}

/**
 * I usually put this file in a scratch folder in the project and run as
 *
 * ```bash
 *  npx ts-node-dev --respawn -T --compiler-options '{"module": "commonjs"}' ./scratch/reveal.ts ./test.ts
 * ```
 *
 * In order for this to rerun when the sourcePath file changes,
 * a good trick is to `import` it here, so ts-node-dev will know
 * to rerun this, also is better to run `reveal` only after
 * the file is fully imported, I had problems with reveal getting
 * previous source if not
 */
const sourcePath = resolve(process.argv.at(-1)!)

import(sourcePath).then(() => reveal(sourcePath)).then(console.log)