outvariant
Type-safe implementation of invariant with positionals.
Motivation
Type-safely
This implementation asserts the given predicate expression so it's treated as non-nullable after the invariant
call:
invariant(user, 'Failed to fetch')
user?.firstName
invariant(user, 'Failed to fetch')
user.firstName
Positionals support
This implementation uses rest parameters to support dynamic number of positionals:
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:
if (!token) {
throw new Error(`Expected a token to be set but got ${typeof token}`)
}
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
Install
npm install outvariant
yarn add outvariant
You may want to install this library as a dev dependency (-D
) based on your usage.
Write an assertion
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
.
invariant(
false,
'Expected the "%s" property but got %j',
'firstName',
{
id: 1,
}
)
Polymorphic errors
It is possible to throw a custom Error
instance using invariant.as
:
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.
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:
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.