error: Cannot find module '/app/app,js' heroku

January 5, 2022

My web works perfectly on my local machine and all that, but in Heroku I deployed my app and nothing is working, and when:
$ heroku logs --tail

State changed from crashed to starting
Starting process with command `node app.js`
internal/modules/cjs/loader.js:883
throw err;
^

Error: Cannot find module 'express'
Require stack:
- /app/app.js
at Function.Module._resolveFilename (internal/modules/cjs/loader.js:880:15)
at Function.Module._load (internal/modules/cjs/loader.js:725:27)
at Module.require (internal/modules/cjs/loader.js:952:19)
at require (internal/modules/cjs/helpers.js:88:18)
at Object.<anonymous> (/app/app.js:1:17)
at Module._compile (internal/modules/cjs/loader.js:1063:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:1092:10)
at Module.load (internal/modules/cjs/loader.js:928:32)
at Function.Module._load (internal/modules/cjs/loader.js:769:14)
at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:72:12) {
code: 'MODULE_NOT_FOUND',
requireStack: [ '/app/app.js' ]
}

directory structure:

-app
 -css
 -files
 -js
 -logo
 -views
 -index.html
-app.js
-composer.json
-package-lock.json
-package.json
-Procfile

Procfile:

web: node app.js

package.json:

{
  "name": "app",
  "version": "1.0.0",
  "engines": {
    "node": "14.15.3"
  },
  "private": "true",
  "main": "app.js",
  "scripts": {
    "start": "node app.js",
    "dev": "nodemon app.js"
  },
  "keywords": [],
  "author": "me",
  "repository": "appRep",
  "license": "ISC",
  "devDependencies": {
    "chokidar": "^3.5.2",
    "express": "^4.17.2",
    "fs": "0.0.1-security",
    "nodemailer": "^6.7.2",
    "nodemon": "^2.0.15",
    "path": "^0.12.7"
  }
}

.env: (even before the .env file everything was the same)

PORT = 8081

Do you guys have any idea?
Thank you in advance!!

>Solution :

Your express package is listed in devDependencies in package.json, try moving it to dependencies like so

"dependencies": {
  "express": "^4.17.2"
}

devDependencies should list only packages which are not essential for app deployed in production (Heroku). It might be necessary to move more packages to dependencies not only express.