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# invariant
[](https://travis-ci.org/zertosh/invariant)
A mirror of Facebook's `invariant` (e.g. [React](https://github.com/facebook/react/blob/v0.13.3/src/vendor/core/invariant.js), [flux](https://github.com/facebook/flux/blob/2.0.2/src/invariant.js)).
A way to provide descriptive errors in development but generic errors in production.
## Install
With [npm](http://npmjs.org) do:
```sh
npm install invariant
```
## `invariant(condition, message)`
```js
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](https://github.com/substack/node-browserify), it'll use `browser.js` (instead of `invariant.js`) and the [envify](https://github.com/hughsk/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 `process.env`. See [Server rendering is slower with npm react #812](https://github.com/facebook/react/issues/812)
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