1 | # invariant
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3 | [![Build Status](https://travis-ci.org/zertosh/invariant.svg?branch=master)](https://travis-ci.org/zertosh/invariant)
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4 |
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5 | 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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7 | A way to provide descriptive errors in development but generic errors in production.
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8 |
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9 | ## Install
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10 |
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11 | With [npm](http://npmjs.org) do:
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12 |
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13 | ```sh
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14 | npm install invariant
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15 | ```
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16 |
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17 | ## `invariant(condition, message)`
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18 |
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19 | ```js
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20 | var invariant = require('invariant');
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21 |
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22 | invariant(someTruthyVal, 'This will not throw');
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23 | // No errors
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24 |
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25 | invariant(someFalseyVal, 'This will throw an error with this message');
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26 | // Error: Invariant Violation: This will throw an error with this message
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27 | ```
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28 |
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29 | **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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30 |
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31 | ### Browser
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32 |
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33 | 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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35 | ### Node
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36 |
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37 | 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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