3 Only call a function once.
8 var once = require('once')
10 function load (file, cb) {
13 loader.once('load', cb)
14 loader.once('error', cb)
18 Or add to the Function.prototype in a responsible way:
21 // only has to be done once
22 require('once').proto()
24 function load (file, cb) {
27 loader.once('load', cb)
28 loader.once('error', cb)
32 Ironically, the prototype feature makes this module twice as
33 complicated as necessary.
35 To check whether you function has been called, use `fn.called`. Once the
36 function is called for the first time the return value of the original
37 function is saved in `fn.value` and subsequent calls will continue to
41 var once = require('once')
45 var stream = createStream()
46 stream.once('data', cb)
47 stream.once('end', function () {
48 if (!cb.called) cb(new Error('not found'))
53 ## `once.strict(func)`
55 Throw an error if the function is called twice.
57 Some functions are expected to be called only once. Using `once` for them would
58 potentially hide logical errors.
60 In the example below, the `greet` function has to call the callback only once:
63 function greet (name, cb) {
64 // return is missing from the if statement
65 // when no name is passed, the callback is called twice
66 if (!name) cb('Hello anonymous')
74 // this will print 'Hello anonymous' but the logical error will be missed
75 greet(null, once(msg))
77 // once.strict will print 'Hello anonymous' and throw an error when the callback will be called the second time
78 greet(null, once.strict(msg))