Templates

Reference one environment variable from inside another, and build strings from keys with the resolve tagged template.

An environment value can reference another with ${VAR}. envapt expands those references when you read the value, so you compose values in your .env file instead of repeating them.

DB_HOST=localhost
DB_PORT=5432
DATABASE_URL=postgres://${DB_HOST}:${DB_PORT}/app

const  = .('DATABASE_URL');
// "postgres://localhost:5432/app"

Expansion runs on read, through every reader (get, getNumber, getUsing, and the rest), not when the file loads.

Nested references

A referenced variable can itself contain references. envapt resolves them transitively.

PROTOCOL=https
HOST=api.example.com
BASE_URL=${PROTOCOL}://${HOST}
HEALTHCHECK=${BASE_URL}/health

Reading HEALTHCHECK returns https://api.example.com/health.

Unresolved references

When a referenced variable is missing or empty, envapt leaves the literal ${VAR} text in place.

GREETING=hello ${NAME}

Reading GREETING with no NAME set returns hello ${NAME}. Enable debug mode to log each unresolved reference. Under strict mode, an unresolved reference throws EnvaptError with code MissingEnvValue instead of being left as text.

WARNING

In non-strict mode the placeholder reaches your consumer: the value still carries the literal ${VAR} text rather than a resolved value. Turn on strict mode if a missing reference should be a hard failure instead.

Cycle protection

A variable that references itself, directly or through a chain, does not loop. envapt detects the cycle and leaves the looping reference as its literal text.

A=${B}
B=${A}

Reading A returns ${B} rather than recursing forever.

Syntax limits

The only recognized form is ${NAME}, where NAME is letters, digits, and underscores.

NOTE

There is no default-value syntax. ${VAR:-fallback} is not parsed as a default, the whole token is treated as the variable name and left unresolved if it has no match. Pass a fallback to the reader instead, e.g. Envapter.get('VAR', 'fallback').

There is also no escape sequence. A $ that is not followed by {...} is left untouched, so price: $5 stays price: $5.

The resolve tagged template

resolve builds a string from keys at the call site, and expands any ${VAR} references inside the resolved values. Each ${...} interpolation in the template is an env var name, not a JavaScript value.

// API_HOST=api.example.com, API_PORT=8080
const endpoint = .`https://${'API_HOST'}:${'API_PORT'}`;        
const endpoint: string

resolve returns a string. Under strict mode, a missing or empty key throws EnvaptError with code MissingEnvValue; otherwise the missing key resolves to an empty string.