| # invariant |
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| [](https://travis-ci.org/zertosh/invariant) |
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| 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)). |
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| A way to provide descriptive errors in development but generic errors in production. |
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| ## Install |
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| With [npm](http://npmjs.org) do: |
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| ```sh |
| npm install invariant |
| ``` |
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| ## `invariant(condition, message)` |
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| ```js |
| var invariant = require('invariant'); |
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| invariant(someTruthyVal, 'This will not throw'); |
| // No errors |
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| invariant(someFalseyVal, 'This will throw an error with this message'); |
| // Error: Invariant Violation: This will throw an error with this message |
| ``` |
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| **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. |
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| ### Browser |
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| 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`. |
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| ### Node |
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| 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) |
