PreviewDrop for Express

Deploy previews for Express & Node.js apps

Push a branch, get a live preview URL. PreviewDrop installs your dependencies, runs your Node server as a real process, and posts the link to the pull request, usually within a couple of minutes.

TL;DR

Connect your repo, push. PreviewDrop detects Node.js, runs npm ci and boots node server.js behind a TLS-terminated subdomain. Because it’s an always-on process, your websockets, SSE streams, in-memory caches and persistent DB pools all behave exactly as they do locally.

What the build needs to do

You don't write this. PreviewDrop detects Expressfrom your repo and builds the container for you — a Dockerfile in your repo is not used. The reference below shows the install, build and start steps a working Express image needs. If detection gets one wrong, set the matching Install, Build or Start command in Project Settings. Your app must listen on $PORT.

Reference (not read by PreviewDrop)
FROM node:22-slim
WORKDIR /app
COPY package*.json ./
RUN npm ci --omit=dev
COPY . .
# If you have a build step (TypeScript / bundler), run it here:
# RUN npm run build  ->  then CMD ["node", "dist/server.js"]
EXPOSE 3000
CMD ["node", "server.js"]

Environment variables

Set these in the Variables tab of your PreviewDrop project. They're encrypted at rest and injected into every preview container at start.

Variable	Example value	Note
PORT	3000	PreviewDrop injects this. Read it with process.env.PORT and listen on it.
NODE_ENV	production	Enables Express production optimisations and disables verbose error pages.
DATABASE_URL	postgres://user:pass@host:5432/db	Point at a shared dev DB, or omit for stateless previews.

Common gotchas

Listen on 0.0.0.0 and read process.env.PORT

Use app.listen(process.env.PORT || 3000, '0.0.0.0'). Omitting the host can bind to localhost inside the container, which the proxy can’t reach — the preview will 502.

Commit your lockfile

npm ci requires a committed package-lock.json and fails without one. If you don’t keep a lockfile, set the Install command in Project Settings to npm install --omit=dev. (pnpm / yarn work too — use their install + lockfile.)

Don’t commit node_modules

Keep node_modules in .gitignore so the build installs fresh dependencies. Native modules (bcrypt, sharp, better-sqlite3) compile per-platform — a macOS/Windows node_modules will crash inside the Linux container.

Run your build step

If you ship TypeScript or a bundler, set the Build command to npm run build and point the Start command at the compiled output (node dist/server.js). Don’t rely on ts-node in previews — it’s slower and pulls dev deps you excluded with --omit=dev.

Why not Vercel or Netlify?

Vercel runs Express by wrapping the whole app in a serverless function: every request cold-boots the app, websockets and long-lived SSE connections break, and you can’t hold an in-memory cache or a persistent database pool between requests. PreviewDrop runs node server.js as a genuine always-on process — the same way it runs in production.

Preview your Express app in 5 minutes

Connect a repo, push a branch, get a URL. No credit card, no trial clock.

Start free