Why is devDependencies' pruning skipped even if NPM_CONFIG_PRODUCTION is true?

Question

When trying to push a Node.js project to Heroku, I can find between the log lines the following one:

Pruning devDependencies
remote:        Skipping because NPM_CONFIG_PRODUCTION is 'true'

Now, I was expecting pruning is skipped at the opposite condition ( NPM_CONFIG_PRODUCTION=false ).

Any thoughts about?

Answer 1

The default behaviour of Heroku's Node.js buildpack is to install dependencies and devDependencies , build the slug, and then strip out the devDependencies . They'll be available during the build process but not afterwards.

Setting NPM_CONFIG_PRODUCTION to true prevents devDependencies from being installed in the first place :

You can direct Heroku to only install dependencies by setting environment variables NPM_CONFIG_PRODUCTION=true or YARN_PRODUCTION=true .

Since devDependencies are never installed there is no need to prune them. There's nothing to strip out.

(Technically, the buildpack skips pruning if NPM_CONFIG_PRODUCTION is non-null ; its actual value doesn't matter for this part of the logic.)