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Only call a function once.


```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’)) }) } ```


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)) ```