Run npm from subdirectories on Heroku

I have a project which contains 2 subprojects:

• First is the API
• Second is the client

And both of these projects have their own dependencies mapped in their own packages.json files, which is placed in each subdir.

So the question is how to run npm install from sub directories on heroku?

I tried putting something like this in the main npm file

"scripts": {
    "postinstall": "cd my_subdir; npm install"
}

But it doesn't work, showing can't cd to my_subdir