Heroku node_modules not cached - heroku

Question

I have toplevel package.json where I set cacheDirectories as per https://devcenter.heroku.com/articles/nodejs-support#custom-caching
"scripts": {
"start": "npm --prefix frontend start && npm --prefix mockserver start",
},
"dependencies": {
},
"devDependencies": {
},
"cacheDirectories": ["frontend/node_modules", "mockserver/node_modules"],
...
So no dependencies in toplevel, but I have frontend/package.json and mockserver/package.json with a lot of dependencies that get installed in their respective node_modules. This is how the respective package.jsons look like (for example mockserver):
"scripts": {
"start": "npm install && npm run build && node dist/server.js",
},
dependencies...
devdependencies...
This works fine and deploys, but it does not cache the the two node_modules defined to be cached.
Here is the output from heroku build:
-----> Creating runtime environment
NPM_CONFIG_LOGLEVEL=error
NPM_CONFIG_PRODUCTION=true
NODE_VERBOSE=false
NODE_ENV=demo
NODE_MODULES_CACHE=true
npm scripts will see NODE_ENV=production (not 'demo')
https://docs.npmjs.com/misc/config#production
-----> Installing binaries
engines.node (package.json): 7.10.0
engines.npm (package.json): unspecified (use default)
Downloading and installing node 7.10.0...
Using default npm version: 4.2.0
-----> Restoring cache
Loading 2 from cacheDirectories (package.json):
- frontend/node_modules (not cached - skipping)
- mockserver/node_modules (not cached - skipping)
-----> Building dependencies
Installing node modules (package.json)
-----> Caching build
Clearing previous node cache
Saving 2 cacheDirectories (package.json):
- frontend/node_modules (nothing to cache)
- mockserver/node_modules (nothing to cache)
-----> Build succeeded!
Just to note that the two node_modules were installed in the previous build but it says (not cached - skipping).
Thanks in advance!

Answer 1

I got the answer here: https://github.com/heroku/heroku-buildpack-nodejs/issues/435
In summary, I should have used heroku-postbuild instead of start script to install custom dependencies. And use start only as application starting command.
Top level package.json
"scripts": {
"heroku-postbuild": "npm --prefix frontend run installAndCompile && npm --prefix mockserver run installAndCompile",
"start": "npm --prefix mockserver start"
},
Subproject's package.json
"scripts": {
"installAndCompile": "npm install && npm run compile"
},