38 lines
1.6 KiB
Markdown
38 lines
1.6 KiB
Markdown
# 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)
|