outvariant

Type-safe implementation of invariant with positionals.

Motivation

$3

This implementation asserts the given predicate expression so it's treated as non-nullable after the invariant call:

``ts
// Regular invariant:
invariant(user, 'Failed to fetch')
user?.firstName // "user" is possibly undefined

// Outvariant:
invariant(user, 'Failed to fetch')
user.firstName // OK, "invariant" ensured the "user" exists
`

$3

This implementation uses rest parameters to support dynamic number of positionals:

`js
invariant(predicate, 'Expected %s but got %s', 'one', false)
`

What is this for?

Invariant is a shorthand function that asserts a given predicate and throws an error if that predicate is false.

Compare these two pieces of code identical in behavior:

`js
if (!token) {
throw new Error(Expected a token to be set but got ${typeof token})
}
`

`js
import { invariant } from 'outvariant'

invariant(token, 'Expected a token to be set but got %s', typeof token)
`

Using invariant reduces the visual nesting of the code and leads to cleaner error messages thanks to formatted positionals (i.e. the %s (string) positional above).

Usage

$3

`sh
npm install outvariant


or


yarn add outvariant
`

> You may want to install this library as a dev dependency (-D) based on your usage.

$3

`js
import { invariant } from 'outvariant'

invariant(user, 'Failed to load: expected user, but got %o', user)
`

Positionals

The following positional tokens are supported:

| Token | Expected value type |
| --------- | ------------------------------------------------------- |
| %s | String |
| %d/%i | Number |
| %j | JSON (non-stringified) |
| %o | Arbitrary object or object-like (i.e. a class instance) |

Whenever present in the error message, a positional token will look up the value to insert in its place from the arguments given to invariant.

`js
invariant(
false,
'Expected the "%s" property but got %j',
// Note that positionals are sensitive to order:
// - "firstName" replaces "%s" because it's first.
// - {"id":1} replaces "%j" because it's second.
'firstName',
{
id: 1,
}
)
`

Polymorphic errors

It is possible to throw a custom Error instance using invariant.as:

`js
import { invariant } from 'outvariant'

class NetworkError extends Error {
constructor(message) {
super(message)
}
}

invariant.as(NetworkError, res.fulfilled, 'Failed to handle response')
`

Note that providing a custom error constructor as the argument to invariant.as requires the custom constructor's signature to be compatible with the Error class constructor.

If your error constructor has a different signature, you can pass a function as the first argument to invariant.as that creates a new custom error instance.

`js
import { invariant } from 'outvariant'

class NetworkError extends Error {
constructor(statusCode, message) {
super(message)
this.statusCode = statusCode
}
}

invariant.as(
(message) => new NetworkError(500, message),
res.fulfilled,
'Failed to handle response'
)
`

Abstract the error into helper functions for flexibility:

`js
function toNetworkError(statusCode) {
return (message) => new NetworkError(statusCode, message)
}

invariant.as(toNetworkError(404), res?.user?.id, 'User Not Found')
invariant.as(toNetworkError(500), res.fulfilled, 'Internal Server Error')
``

Contributing

Please open an issue or submit a pull request if you wish to contribute. Thank you.