Transformer

Transformer is responsible for replacing value within ${} with the actual value. The value to be replaced can be either an environment variable or a path from exiting config file with dot notation.

Referencing env variable

app:
    database:
        password: ${DB_PASSWORD}

After loading and transforming, the above config is changed to following

{
    "app": {
        "database": {
            "password": "password" // assuming process.env.DB_PASSWORD=password
        }
    }
}

Referencing existing config path

auth:
   base-path: https://auth-server
tenant:
   tenant-a: ${auth.base-path}/tenant-a

After loading and transforming, the above config is converted to following

{
    "auth": {
        "basePath": "https://auth-server"
    },
    "tenant": {
        "tenantA": "https://auth-server/tenant-a"
    }
}

TIP

The key in dot notation can be either in kebab-case, came-case or SCREAMING_SNAKE_CASE irrespective of the original key case. So, auth.basePath or AUTH_BASE-PATH or any other possible combination can be used although auth.basePath/auth.base-path is recommended.