In a npm configuration file any environment variable (specified by $VARIABLE_NAME or ${VARIABLE_NAME}) is being filtered/replaced by the env varibale value. If the filtering process can't find the env varibale an error message is logged in the build console.

However, in some cases it would be desirable to keep the "${VARIABLE_NAME}" string in the used npm config file so that npm itself replaces this. One scenario is for authentication, like this:

registry=http://nexus.acme.org/repository/npm-group/
//nexus.acme.org/repository/npm-releases/:_authToken=${NPM_TOKEN}

In this case, for security reasons, you don't want the env variable to be expanded/filtered during the copying of the config file but rather have npm fetching the env variable value during the "npm publish" execution.