Modules, Runtime & Tooling

Environment Variables & Config

Configuration can exist at build time, server runtime, client runtime, and test runtime. In browser bundles, exposed environment variables are public values baked into shipped JavaScript.

Build-time config is substituted into output
Server runtime config is read by the process
Client config is visible to users
Config should be validated
Tests need isolated config

Config locations
Location	When resolved	Visible to
Build-time client env	During build	Anyone with the bundle
Server env	At process runtime	Server process
Test env	During test run	Test process
Runtime public config endpoint	When client loads	Anyone who calls endpoint
Secrets manager	At deploy/runtime	Authorized server code

Validate server config once

function requiredEnv(name: string): string {
  const value = process.env[name]
  if (!value) throw new Error(`Missing environment variable: ${name}`)
  return value
}

export const config = {
  databaseUrl: requiredEnv("DATABASE_URL"),
  apiSecret: requiredEnv("API_SECRET"),
}

Fail fast when config is invalid.