znv

Parse your environment with Zod.

Pass in a schema and your process.env. Get back a validated, type-safe,
read-only environment object that you can export for use in your app. You can
optionally provide defaults (which can be matched against NODE_ENV values like
production or development), as well as help strings that will be included in
the error thrown when an env var is missing.

Features

- No dependencies
- Fully type-safe
- Compatible with serverless environments (import znv/compat instead of znv)

Status

Unstable: znv has not yet hit v1.0.0, and per semver there may be breaking
changes in minor versions before the v1.0.0 release. Any (known) breaking
changes will be documented in release notes. znv is used in production in
several services at the primary author's workplace. Feedback and suggestions
about final API design are welcome.

Contents

- Quickstart
- Motivation
- Usage
- parseEnv
- Extra schemas
- Coercion rules
- Comparison to other libraries
- Complementary tooling
- How do I pronounce znv?

Quickstart

``bash
npm i znv zod


or

pnpm add znv zod

or

yarn add znv zod
`

Create a file named something like env.ts:

`ts
import { parseEnv } from "znv";
import { z } from "zod";

export const { NICKNAME, LLAMA_COUNT, COLOR, SHINY } = parseEnv(process.env, {
NICKNAME: z.string().min(1),
LLAMA_COUNT: z.number().int().positive(),
COLOR: z.enum(["red", "blue"]),
SHINY: z.boolean().default(true),
});

console.log([NICKNAME, LLAMA_COUNT, COLOR, SHINY].join(", "));
`

Let's run this with ts-node:

`
$ LLAMA_COUNT=huge COLOR=cyan ts-node env.ts
`

Oops! Let's fix those issues:

`
$ LLAMA_COUNT=24 COLOR=red NICKNAME=coolguy ts-node env.ts
`

Now we see the expected output:

`
coolguy, 24, red, true
`

Since parseEnv didn't throw, our exported values are guaranteed to be defined.
Their TypeScript types will be inferred based on the schemas we used — COLOR
will be even be typed to the union of literal strings 'red' | 'blue' rather
than just string.

---

A more elaborate example:

`ts
// znv re-exports zod as 'z' to save a few keystrokes.
import { parseEnv, z, port } from "znv";

export const { API_SERVER, HOST, PORT, EDITORS, POST_LIMIT, AUTH_SERVER } =
parseEnv(process.env, {
// you can provide defaults with .default(). these will be validated
// against the schema.
API_SERVER: z.string().url().default("https://api.llamafy.biz"),

// specs can also be more detailed.
HOST: {
schema: z.string().min(1),

// the description is handy as in-code documentation, but is also printed
// to the console if validation for this env var fails.
description: "The hostname for this service.",

// instead of specifying defaults as part of the zod schema, you can pass
// them in the defaults object. a default will be matched based on the
// value of NODE_ENV.
defaults: {
production: "my-cool-llama.website",
test: "cool-llama-staging.cloud-provider.zone",

// "_" is a special token that can be used in defaults. its value will
// be used if NODE_ENV doesn't match any other provided key.
_: "localhost",
},
},

// znv provides helpers for a few very common environment var types not
// covered by zod. these can have further refinements chained to them:
PORT: port().default(8080),

// using a zod array() or object() as a spec will make znv attempt to
// JSON.parse the env var if it's present.
EDITORS: z.array(z.string().min(1)),

// optional values are also supported and provide a way to benefit from the
// validation and static typing provided by zod even if you don't want to
// error out on a missing value.
POST_LIMIT: z.number().optional(),

// use all of the expressiveness of zod, including enums and post-processing.
AUTH_SERVER: z
.enum(["prod", "staging"])
.optional()
.transform((prefix) =>
prefix ? http://auth-${prefix}.cool-llama.app : "http://localhost:91",
),
});
`

If any env var fails validation, parseEnv() will throw. All failing specs will
be aggregated in the error message, with each showing the received value, the
reason for the failure, and a hint about the var's purpose (if description was
provided in the spec).

Motivation

Environment variables are one way to pass runtime configuration into your
application. As promoted by the Twelve-Factor App
methodology, this helps keep config (which can
vary by deployment) cleanly separated from code, encouraging maintainable
practices and better security hygiene. But passing in configuration via env vars
can often turn into an ad-hoc affair, with access and validation scattered
across your codebase. At worst, a misconfigured environment will launch and run
without apparent error, with issues only making themselves apparent later when a
certain code path is hit. A good way to avoid this is to **declare and validate
environment variables in one place** and export the validated result, so that
other parts of your code can make their dependencies on these vars explicit.

Env vars represent one of the _boundaries_ of your application, just like file
I/O or a server request. In TypeScript, as in many other typed languages, these
boundaries present a challenge to maintaining a well-typed app.
Zod does an excellent job at parsing and
validating poorly-typed data at boundaries into clean, well-typed values. znv
facilitates its use for environment validation.

$3

znv is a small module that works hand-in-hand with Zod. Since env vars, when
defined, are _always strings_, Zod schemas like z.number() will fail to parse
them out-of-the-box. Zod allows you to use a preprocess
schema to handle coercions, but
peppering your schemas with preprocessors to this end is verbose, error-prone,
and clunky. znv wraps each of the Zod schemas you pass to parseEnv in a
preprocessor that tries to coerce a string to a type the schema expects.

These preprocessors don't do any validation of their own — in fact, they try to
do as little work as possible and defer to your schema to handle the validation.
In practice, this should be pretty much transparent to you, but you can check
out the coercion rules if you'd like more info.

> Since v3.20, Zod provides
> z.coerce for
> primitive coercion, but this is often too naive to be useful. For example,
> z.coerce.boolean() will parse "false" into true, since the string "false"
> is _truthy_ in JavaScript. znv will coerce "false" into false, which is
> probably what you expect.

znv also makes it easy to define defaults for env vars based on your
environment. Zod allows you to add a default value for a schema, but making a
given default vary by environment or only act as a fallback in certain
environments is not straightforward.

Usage

$3

Parse the given environment using the given schemas. Returns a read-only
object that maps the keys of the schemas object to their respective parsed
values.

Throws if any schema fails to parse its respective env var. The error aggregates
all parsing failures for the schemas.

Optionally, you can pass a custom error reporter as the third parameter to
parseEnv to customize how errors are displayed. The reporter is a function
that receives error details and returns a string. Alternately, you can pass an
object of _token formatters_ as the third parameter to parseEnv; this can be
useful if you want to retain the default error reporting format but want to
customize some aspects of it (for example, by redacting secrets).

#### environment: Record

You usually want to pass in process.env as the first argument.

> It is not recommended to use znv for general-purpose schema validation —
> just use Zod (with
> preprocessors to handle
> coercion, if necessary).

#### schemas: Record

Maps env var names to validators. You can either use a Zod schema directly, or
pass a DetailedSpec object that has the following fields:

- schema: ZodType

The Zod validator schema.

- description?: string

Optional help text that will be displayed when this env var is missing or
fails to validate.

- defaults?: Record

An object that maps from NODE_ENV values to values that will be passed as
input to the schema if this var isn't present in the environment. For example:

`ts
const schemas = {
FRUIT: {
schema: z.string().min(1),
defaults: {
production: "orange",
development: "banana",
},
},
};

// FRUIT wll have value "banana".
const { FRUIT } = parseEnv({ NODE_ENV: "development" }, schemas);

// FRUIT wll have value "orange".
const { FRUIT } = parseEnv({ NODE_ENV: "production" }, schemas);

// FRUIT wll have value "fig".
const { FRUIT } = parseEnv({ NODE_ENV: "production", FRUIT: "fig" }, schemas);

// FRUIT wll have value "apple".
const { FRUIT } = parseEnv({ FRUIT: "apple" }, schemas);

// this will throw, since NODE_ENV doesn't match "production" or "development".
const { FRUIT } = parseEnv({}, schemas);
`

defaults accepts a special token as a key: _. This is like the default
clause in a switch case — its value will be used if NODE_ENV doesn't match
any other key in defaults.

> (As an aside, it is not recommended to use staging as a possible value
> for NODE_ENV. Your staging environment should be as similar to your
> production environment as possible, and NODE_ENV=production has special
> meaning for several tools and libraries. For example,
> npm install and
> yarn install
> by default won't install devDependencies if NODE_ENV=production;
> Express
> and React will also
> behave differently depending on whether NODE_ENV is production or not.
> Instead, your staging environment should also set NODE_ENV=production, and
> you should define your own env var(s) for any special configuration that's
> necessary for your staging environment.)

Caveats aside, _ lets you express a few interesting scenarios:

`ts
// one default for production, and one for all other environments, including
// development and testing.
{ production: "prod default", _: "dev default" }

// default for all non-production environments, but require the var to be
// passed in for production.
{ production: undefined, _: "dev default" }

// unconditional default. equivalent to adding .default("some default")
// to the zod schema, but this might be more stylistically consistent with
// your other specs if they use the defaults field.
{ _: "unconditional default" }
`

Some testing tools like Jest set NODE_ENV to test,
so you can also use defaults to provide default env vars for testing.

parseEnv doesn't restrict or validate NODE_ENV to any particular values,
but you can add NODE_ENV to your schemas like any other env var. For
example, you could use
NODE_ENV: z.enum(["production", "development", "test", "ci"]) to enforce
that NODE_ENV is always defined and is one of those four expected values.

#### reporterOrFormatters?: Reporter | TokenFormatters

An optional error reporter or object of error token formatters, for customizing
the displayed output when a validation error occurs.

- Reporter: (errors: ErrorWithContext[], schemas: Schemas) => string

A reporter is a function that takes a list of errors and the schemas you
passed to parseEnv and returns a string. Each error has the following
format:

`ts
{
/* The env var name. /
key: string;
/* The actual value present in process.env[key], or undefined. /
receivedValue: unknown;
/* ZodError if Zod parsing failed, or Error if a preprocessor threw. /
error: unknown;
/* If a default was provided, whether the default value was used. /
defaultUsed: boolean;
/* If a default was provided, the given default value. /
defaultValue: unknown;
}
`

- TokenFormatters

An object with the following structure:

`ts
{
/* Formatter for the env var name. /
formatVarName?: (key: string) => string;

/* For parsed objects with errors, formatter for object keys. /
formatObjKey?: (key: string) => string;

/* Formatter for the actual value we received for the env var. /
formatReceivedValue?: (val: unknown) => string;

/* Formatter for the default value provided for the schema. /
formatDefaultValue?: (val: unknown) => string;

/* Formatter for the error summary header. /
formatHeader?: (header: string) => string;
}
`

For example, if you want to redact value names, you can invoke parseEnv like
this:

`ts
export const { SOME_VAL } = parseEnv(
process.env,
{ SOME_VAL: z.number().nonnegative() },
{ formatReceivedValue: () => "" },
);
`

$3

znv exports a very small number of extra schemas for common env var types.

#### port()

port() is an alias for z.number().int().nonnegative().lte(65535).

#### deprecate()

deprecate() is an alias for
z.undefined().transform(() => undefined as never). parseEnv will throw if a
var using the deprecate() schema is passed in from the environment.

Coercion rules

znv tries to do as little work as possible to coerce env vars (which are always
strings when they're present) to the input
types of your schemas.
If the env var doesn't look like the input type, znv will pass it to the
validator as-is with the assumption that the validator will throw. For example,
if your schema is z.number(), znv will test it against a numeric regex first,
rather than unconditionally wrap it in Number() or parseFloat() (and thus
coerce it to NaN).

By modifying as little as possible, znv tries to get out of Zod's way and let it
do the heavy lifting of validation. This also lets us produce less confusing
error messages: if you pass the string "banana" to your number schema, it should
be able to say "you gave me 'banana' instead of a number!" rather than "you gave
me NaN instead of a number!"

Coercions only happen at the top level of a schema. If you define an object
with nested schemas, no coercions will be applied to the keys.

Some notable coercion mechanics:

- If your schema's input is a boolean, znv will coerce "true", "yes" and
"1" to true, and "false", "no" and "0" to false. All other values
will be passed through.

> Some CLI tool conventions dictate that a variable simply being present in
> the environment (even with no value, eg. setting MY_VALUE= with no
> right-hand side) should be interpreted as true. However, this convention
> doesn't seem to be in widespread use in Node, probably because it causes the
> var to evaluate to the empty string (which is falsy). znv demands a little
> more specificity by default, while still hedging a bit for some common
> true/false equivalents. If you want the "any defined value" behaviour, you
> can use
> z.string().optional().transform(v => v === undefined ? false : true).

- If your schema's input is an object or array (or record or tuple), znv will
attempt to JSON.parse the input value if it's not undefined or the empty
string.

> Remember, with great power comes great responsibility! If you're using
> an object or array schema to pass in dozens or hundreds of kilobytes of data
> as an env var, you may be doing something wrong. (Certain platforms also
> impose limits on environment variable
> length.)

- If your schema's input is a Date, znv will call new Date() with the input
value. This has a number of pitfalls, since the Date() constructor is
excessively forgiving. The value is passed in as a string, which means
trying to pass a Unix epoch will yield unexpected results. (Epochs need to
be passed in as number: new Date() with an epoch as a string will either
give you invalid date or a completely nonsensical date.) _You should only
pass in ISO 8601 date strings_, such as those returned by
Date.prototype.toISOString().
Improved validation for Date schemas could be added in a future version.

- Zod defines "nullable" as distinct from "optional". If your schema is
nullable, znv will coerce undefined to null. Generally it's preferred to
simply use optional.

Comparison to other libraries

$3

Envalid is a nice library that inspired znv's API design. Envalid is written in
TypeScript and performs some inference of the return value based on the
validator schema you pass in, but won't do things like narrow enumerated types
(str({ choices: ['a', 'b'] })) to a union of literals. Expressing defaults is
more limited (you can't have different defaults for test and development
environments, for example). Defaults are not passed through validators.

Envalid's validators are built-in and express a handful of types with limited
options and no ability to perform postprocessing. For other use cases you have
to write your own custom
validators.

Envalid wraps its return value in a proxy, which can't be opted out of and has
some surprising effects.

$3

Joi is the Cadillac of schema validation libraries. Its default of coercing
strings to the target type makes it easy to adopt for environment validation.
Unfortunately, Joi is written in JavaScript and its type definitions support a
very limited form of inference when they work at all.

$3

Hey, what's Zod doing here? Doesn't znv use Zod?

If you just want to parse some values against a certain schema, **you might not
need znv**. Just use Zod directly.

znv is best-suited for _environment validation_: it automatically wraps your Zod
schemas in preprocessors that coerce env vars, which are always strings, into
the appropriate type. This is different from Zod's built-in z.coerce, which is
often too naive to be useful. For example, z.coerce.boolean() will parse
"false" into true, since the string "false" is _truthy_ in JavaScript. znv
will coerce "false" into false, which is probably what you expect. Check the
section on coercion rules for more information.

Complementary tooling

The eslint-plugin-n rule
no-process-env
is recommended to restrict usage of process.env` outside of the module that
parses your schema.

znv also works great with dotenv.

How do I pronounce znv?

If you usually pronounce "z" as "zed," then you could say "zenv." If you usually
pronounce "z" as "zee," you could say "zee en vee."

Or do your own thing. I'm not the boss of you.