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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'))
})
}
```
## `once.strict(func)`
Throw an error if the function is called twice.
Some functions are expected to be called only once. Using `once` for them would
potentially hide logical errors.
In the example below, the `greet` function has to call the callback only once:
```javascript
function greet (name, cb) {
// return is missing from the if statement
// when no name is passed, the callback is called twice
if (!name) cb('Hello anonymous')
cb('Hello ' + name)
}
function log (msg) {
console.log(msg)
}
// this will print 'Hello anonymous' but the logical error will be missed
greet(null, once(msg))
// once.strict will print 'Hello anonymous' and throw an error when the callback will be called the second time
greet(null, once.strict(msg))
```