This package contains an io-ts decoder combinator that turns a decoder expecting possibly nested objects into a decoder expecting a flat object whose keys are styled in UPPER_SNAKE_CASE, like how environment variables are defined.

This combinator will therefore let you compose an environment variable parser from smaller object decoder units -- perhaps controlling different parts of your application -- and taking care of converting names for you, as long as the property names of your object decoder units consistently follow one of the supported casing conventions.

import { forEnv } from "io-ts-env";
import { string } from "io-ts/lib/Decoder";
import { pipe } from "fp-ts/pipeable";

const decodeEnv = pipe(
  type({
    foo: string,
    barBaz: string,
    nested: type({
      innerProperty: string
    })
  }),
  forEnv()
);

The resulting decoder will first preproces and environment variable object (for instance, process.env, or a parsed .env file using e.g. envfile) looking like this:

{
  FOO: "some value",
  BAR_BAZ: "another value",
  NESTED__INNER_PROPERTY: "nested value"
}

The resulting environment variable decoder will have convert names with words from the original properties (inferred from casing conventions) separated by _ and nested object property names are concatnated separated by __. Both separators can be overridden. For instance, the combinator below will convert the variable NESTED_INNER-PROPERTY=value into { Nested: { InnerProperty: "value" }}

/**
 * This will now match environment variables like:
 * `NESTED_INNER-PROPERTY` and convert it into
 *
 */
forEnv({
  wordSeparator: "-",
  objectSeparator: "_",
  wordCasing: "upperCamelCase"
});

Supported casing conventions

• UpperCamelCase
• lowerCamelCase (default)
• UPPER-KEBAB-CASE
• lower-kebab-case
• UPPER_SNAKE_CASE
• lower_snake_case

Error reporting

The package attempts to map back key errors from io-ts into the environment variable name, so that you won't have to remember the convention you set up. So a property fooBar should appear as FOO_BAR in the error report.

However, there are limitations. By design of io-ts Decoder which swallows all internal definitions and exposes only the decoder method, it is not possible to get the generated full path of the environment variable that is missing in a nested object, because the type decoder will bail out as soon as an outer key is missing. So the variable NESTED_INNER-PROPERTY would first fail saying that the key NESTED is missing, but once you have that, it would complain that NESTED_INNER-PROPERTY is missing. This also means that it is not possible to list all environment variables from the decoder, since the information of its properties cannot be accessed.

Issues like these could be circumvented by using io-ts Type or Codec, where property definitions are still accessible. A combinator for them may be implemented in future.