Typescript: builds fine locally, but fails in Heroku

I’m writing the post I wish I found when I googled the title you are currently seeing.

The context:

• a Typescript express.js app that builds (`tsc`) fine locally but fails when deployed via Heroku
• the errors were complaining about missing modules (that are dev dependencies)

The cause: How Heroku prunes dependencies. What you should know is that by the time that “start” is run, devDependencies are pruned. That meant, my scripts that looked like this:

 "scripts": {
   "build": "tsc",
   "start": "npm run build && node ./dist/index.js",
   "dev": "nodemon index.ts",
   "test": "echo \"Error: no test specified\" && exit 1"
 }

Would result in errors complaining about type packages that could not be found, such as @types/express. This is because when Typescript compiles, it would need access to the packages such as @types/express , which in this situation it doesn’t, because Heroku already pruned all the devDependencies.

The fix:

"scripts": {
  "build": "tsc",
  "heroku-postbuild": "npm run build",
  "start": "node ./dist/index.js",
  "dev": "nodemon index.ts",
  "test": "echo \"Error: no test specified\" && exit 1"
}

The command for heroku-postbuild will be run by Heroku right after installing all dependencies (i.e. running npm install) but before the pruning of devDependencies.

Hope that helped!