Expand type aliases from a typescript file

reveal.ts

import { readFileSync } from 'fs'
import { resolve } from 'path'
import ts from 'typescript'
import * as prettier from 'prettier'

/**
 * reveal all top level `type` declarations from a file
 */
export const reveal = (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 =>
        c.getText().slice(0, c.getText().lastIndexOf(c.type.getText())) +
        checker.typeToString(
          checker.getTypeAtLocation(c.type),
          undefined,
          ts.TypeFormatFlags.InTypeAlias | ts.TypeFormatFlags.NoTruncation
        )
    )
    .join('\n\n')

  let result = revealed

  // optional formatting
  result = prettier.format(result, {
    parser: 'typescript',
    semi: false,
  })

  return result
}

const getConfig = () =>
  JSON.parse(
    readFileSync(
      ts.findConfigFile('.', ts.sys.fileExists, 'tsconfig.json') as string
    ).toString()
  )

/**
 * 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)