invariant

What is invariant?

The invariant npm package is a utility that can be used to provide invariant checks. An invariant is a condition that can be checked to be true at certain points during the execution of a program. If the condition is false, invariant will throw an error with a message provided by the developer. This is particularly useful for catching programming errors and ensuring that certain assumptions hold true during the execution of a program.

What are invariant's main functionalities?

Basic invariant check

This feature allows developers to assert conditions within their code and throw meaningful errors if those conditions are not met. It's particularly useful for validating arguments to functions or ensuring state consistency.

const invariant = require('invariant');

function divide(a, b) {
  invariant(b !== 0, 'Cannot divide by zero.');
  return a / b;
}

divide(10, 0); // This will throw an error with the message 'Cannot divide by zero.'

Other packages similar to invariant

assert

The 'assert' module is a part of Node.js core and provides a simple set of assertion tests that can be used to test invariants. Unlike 'invariant', which is designed for production use, 'assert' is primarily intended for testing purposes. 'assert' provides a wider range of assertion types, such as deep equality checks, but does not allow for custom error messages in the same way 'invariant' does.

check-types

The 'check-types' npm package offers a rich set of assertions for type checking and is more focused on type validation rather than general invariants. While 'invariant' is used to assert any condition and throw an error if it fails, 'check-types' provides more granular type-specific assertions, such as checking if a value is a string, number, etc. This makes 'check-types' more suitable for validating input types rather than enforcing general program invariants.

README

invariant

A mirror of Facebook's invariant (e.g. React, flux).

Install

With npm do:

npm install invariant

invariant(condition, message)

var invariant = require('invariant');

invariant(someTruthyVal, 'This will not throw');
// No errors

invariant(someFalseyVal, 'This will throw an error with this message');
// Error: Invariant Violation: This will throw an error with this message

Note: When process.env.NODE_ENV is not production, the message is required. If omitted, invariant will throw regardless of the truthiness of the condition. When process.env.NODE_ENV is production, the message is optional – so they can be minified away.

Browser

When used with browserify, it'll use browser.js (instead of invariant.js) and the envify transform will inline the value of process.env.NODE_ENV.

Node

The node version is optimized around the performance implications of accessing process.env. The value of process.env.NODE_ENV is cached, and repeatedly used instead of reading proces.env. See Server rendering is slower with npm react #812