Using Explicit Resource Management with TypeScript and Postgres

Overview

Explicit Resource Management is a newish JavaScript feature that lets you add tear-down functionality to function scopes. This means if your code sets something up, you can have it automatically clean things when that code completes.

It's available in Node.js 20.4.0 +, Deno 1.38+ and TypeScript 5.2+, so you should be able to use it on newer projects.

It looks something like this:

using file = openFile('notes.txt')

Then openFile implements this API to close the file when it is not used anymore.

My Code

So I had some code, I wanted to see how the footprint changed. Roughly my code:

import postgres from 'postgres'
import { getPostgresMigrator } from 'gruber'

export async function runMigrations(direction: string) {
  const sql = postgres('postgres://...')

  const migrator = getPostgresMigrator({ sql, directory })

  try {
    if (direction === 'up') await migrator.up()
    else if (direction === 'down') await migrator.down()
    else throw new Error('Unknown direction <up|down>')
  } finally {
    await sql.end()
  }
}

And it turned into this:

export async function runMigrations(direction: string) {
  await using sql = getPostgresClient('postgres://...')

  const migrator = getPostgresMigrator({ sql, directory })

  if (direction === 'up') await migrator.up()
  else if (direction === 'down') await migrator.down()
  else throw new Error('Unknown direction <up|down>')
}

Which needed this magic:

function getPostgresClient(url) {
  const sql = postgres(url)

  Object.assign(sql, {
    [Symbol.asyncDispose]: async () => {
      await sql.end()
    },
  })

  return sql
}

Notes

• It takes an extra level of nesting out of the code
• I wish TypeScript let you assign symbols to things without complaining
• I could remove the use of return to simplify the code
• Synchronous and async are different, one uses Symbol.dispose and the other Symbol.asyncDispose and requires the await using keywords.
• It feels like this could be very powerful, especially when all your favourite libraries start using it.