small-invariant 🔬💥

small-invariant is a fork of Alex Reardon's tiny-invariant that does not swallow error messages in production. tiny-invariant is great and deservedly popular, but if your production logs your error messages, eg to Sentry, then you want something more substantial than "Invariant failed".

For more information, view the readme for tiny-invariant.

The TL;DR is:

What is invariant?

An invariant function takes a value, and if the value is falsy then the invariant function will throw. If the value is truthy, then the function will not throw.

import invariant from 'small-invariant'

invariant(truthyValue, 'This should not throw!')

invariant(falsyValue, 'This will throw!')
// Error('Invariant failed: This will throw!');

Type narrowing

small-invariant is useful for correctly narrowing types for typescript

const value: Person | null = { name: 'Alex' } // type of value == 'Person | null'
invariant(value, 'Expected value to be a person')
// type of value has been narrowed to 'Person'

API

(condition: any, message?: string | (() => string)) => void

• condition is required and can be anything
• message optional string or a function that returns a string (() => string)

Installation

# yarn
yarn add small-invariant

# npm
npm install small-invariant --save

Builds

• We have a es (EcmaScript module) build
• We have a cjs (CommonJS) build
• We have a umd (Universal module definition) build in case you needed it

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