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# once
Only call a function once.
## usage
```javascript
var once = require('once')
function load (file, cb) {
cb = once(cb)
loader.load('file')
loader.once('load', cb)
loader.once('error', cb)
}
```
Or add to the Function.prototype in a responsible way:
```javascript
// only has to be done once
require('once').proto()
function load (file, cb) {
cb = cb.once()
loader.load('file')
loader.once('load', cb)
loader.once('error', cb)
}
```
Ironically, the prototype feature makes this module twice as
complicated as necessary.
To check whether you function has been called, use `fn.called`. Once the
function is called for the first time the return value of the original
function is saved in `fn.value` and subsequent calls will continue to
return this value.
```javascript
var once = require('once')
function load (cb) {
cb = once(cb)
var stream = createStream()
stream.once('data', cb)
stream.once('end', function () {
if (!cb.called) cb(new Error('not found'))
})
}
```
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