npm install @epic-web/invariant

The Problem

Your application has boundaries. Network requests/responses, file system reads, etc. When you're working with these boundaries, you need to be able to handle errors that may occur, even in TypeScript.

TypeScript will typically make these boundaries much more obvious because it doesn't like not knowing what the type of something is. For example:

const formData = new FormData(formElement)
const name = await formData.get('name')
// name is `File | string | null`

Often it's a good idea to use a proper parsing library for situations like this, but for simple cases that can often feel like overkill. But you don't want to just ignore TypeScript because:

TypeScript is that brutally honest friend you put up with because they save you from making terrible mistakes. – @kentcdodds

So you check it:

const formData = new FormData(formElement)
const name = await formData.get('name')
// name is `File | string | null`
if (typeof name !== 'string') {
    throw new Error('Name must be a string')
}
// now name is `string` (and TypeScript knows it too)

You're fine throwing a descriptive error here because it's just very unlikely this will ever happen and even if it does you wouldn't really know what to do about it anyway.

It's not a big deal, but there's a tiny bit of boilerplate that would be nice to avoid. Especially when you find yourself doing this all over the codebase. This is the problem @epic-web/invariant solves.

The Solution

Here's the diff from what we had above:

const formData = new FormData(formElement)
const name = await formData.get('name')
// name is `File | string | null`
- if (typeof name !== 'string') {
- 	throw new Error('Name must be a string')
- }
+ invariant(typeof name === 'string', 'Name must be a string')
// now name is `string` (and TypeScript knows it too)

It's pretty simple. But honestly, it's nicer to read, it throws a special InvariantError object to distinguish it from other types of errors, and we have another useful utility for throwing Response objects instead of Error objects which is handy in Remix.

Usage

invariant

The invariant function is used to assert that a condition is true. If the condition is false, it throws an error with the provided message.

Basic Usage

import { invariant } from '@epic-web/invariant'

const creature = { name: 'Dragon', type: 'Fire' }
invariant(creature.name === 'Dragon', 'Creature must be a Dragon')

Throwing a Response on False Condition

import { invariant } from '@epic-web/invariant'

const creature = { name: 'Unicorn', type: 'Magic' }
invariant(creature.type === 'Fire', 'Creature must be of type Fire')
// Throws: InvariantError: Creature must be of type Fire

Using Callback for Error Message

import { invariantResponse } from '@epic-web/invariant'

const creature = { name: 'Elf', type: 'Forest' }
invariant(creature.type === 'Water', () => 'Creature must be of type Water')
// Throws: InvariantError: Creature must be of type Water

invariantResponse

The invariantResponse function works similarly to invariant, but instead of throwing an InvariantError, it throws a Response object.

Basic Usage

import { invariantResponse } from '@epic-web/invariant'

const creature = { name: 'Phoenix', type: 'Fire' }
invariantResponse(creature.type === 'Fire', 'Creature must be of type Fire')

Throwing a Response on False Condition

import { invariantResponse } from '@epic-web/invariant'

const creature = { name: 'Griffin', type: 'Air' }
invariantResponse(creature.type === 'Water', 'Creature must be of type Water')
// Throws: Response { status: 400, body: 'Creature must be of type Water' }

The response status default if 400 (Bad Request), but you'll find how to change that below.

Using Callback for Response Message

import { invariantResponse } from '@epic-web/invariant'

const creature = { name: 'Mermaid', type: 'Water' }
invariantResponse(
    creature.type === 'Land',
    () => `Expected a Land creature, but got a ${creature.type} creature`,
)

Throwing a Response with Additional Options

import { invariantResponse } from '@epic-web/invariant'

const creature = { name: 'Cerberus', type: 'Underworld' }
invariantResponse(
    creature.type === 'Sky',
    JSON.stringify({ error: 'Creature must be of type Sky' }),
    { status: 500, headers: { 'Content-Type': 'text/json' } },
)

Differences from invariant

There are three main differences. With @epic-web/invariant:

1. Error messages are the same in dev and prod
2. It's typesafe
3. We support the common case (for Remix anyway) of throwing Responses as well with invariantResponse.

License

MIT