Environment Variables

viper supports reading configuration from environment variables, with higher priority than config files and defaults.

Automatic Environment

Enable automatic environment variable lookup with a prefix:

const v = createViper()

v.setEnvPrefix('APP')
v.automaticEnv()

When automaticEnv() is enabled, any get(key) call will also check for a matching environment variable. The env var name is derived by:

1. Prepending the prefix (if set)
2. Replacing dots with underscores
3. Uppercasing everything

Key	Env Var (prefix: APP)	Env Var (no prefix)
host	APP_HOST	HOST
database.host	APP_DATABASE_HOST	DATABASE_HOST
log.level	APP_LOG_LEVEL	LOG_LEVEL

Explicit Binding

For env vars that don't follow the naming convention, bind them explicitly:

v.bindEnv('database.url', 'DATABASE_URL')
v.bindEnv('redis.host', 'REDIS_HOST', 'CACHE_HOST') // tries each in order

Multiple env var names can be bound to a single key. viper checks them in order and returns the first defined value.

Binding Without Arguments

Calling bindEnv without explicit env var names derives the name automatically (using the prefix):

v.setEnvPrefix('APP')
v.bindEnv('database.host') // binds to APP_DATABASE_HOST

The difference from automaticEnv() is that bindEnv registers a specific key, while automaticEnv applies to all keys.

Live Reads

Environment variables are read from process.env on every get() call. They are never cached, so changes to the environment are reflected immediately:

process.env.APP_HOST = 'first'
v.get('host') // 'first'

process.env.APP_HOST = 'second'
v.get('host') // 'second'

Precedence with Other Sources

Environment variables have higher priority than config files and defaults, but lower than explicit overrides:

v.setDefault('host', 'default-host') // priority 4
// config file sets host to 'config-host'      // priority 3
// APP_HOST=env-host                           // priority 2
v.set('host', 'override-host') // priority 1

v.get('host') // 'override-host'

Remove the override and it falls through to env:

v.get('host') // 'env-host' (from APP_HOST)