var invariant = require('invariant');
invariant(someTruthyVal, 'This will not throw');
// No errors
invariant(someFalseyVal, 'This will throw an error withthis 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.
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.
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