io-ts-env
v0.4.1
Published
An io-ts decoder combinator that adapts a decoder to receive a flattened environment variable object.
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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.