source: trip-planner-front/node_modules/once/README.md@ 76712b2

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1# once
2
3Only call a function once.
4
5## usage
6
7```javascript
8var once = require('once')
9
10function load (file, cb) {
11 cb = once(cb)
12 loader.load('file')
13 loader.once('load', cb)
14 loader.once('error', cb)
15}
16```
17
18Or add to the Function.prototype in a responsible way:
19
20```javascript
21// only has to be done once
22require('once').proto()
23
24function load (file, cb) {
25 cb = cb.once()
26 loader.load('file')
27 loader.once('load', cb)
28 loader.once('error', cb)
29}
30```
31
32Ironically, the prototype feature makes this module twice as
33complicated as necessary.
34
35To check whether you function has been called, use `fn.called`. Once the
36function is called for the first time the return value of the original
37function is saved in `fn.value` and subsequent calls will continue to
38return this value.
39
40```javascript
41var once = require('once')
42
43function load (cb) {
44 cb = once(cb)
45 var stream = createStream()
46 stream.once('data', cb)
47 stream.once('end', function () {
48 if (!cb.called) cb(new Error('not found'))
49 })
50}
51```
52
53## `once.strict(func)`
54
55Throw an error if the function is called twice.
56
57Some functions are expected to be called only once. Using `once` for them would
58potentially hide logical errors.
59
60In the example below, the `greet` function has to call the callback only once:
61
62```javascript
63function 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')
67 cb('Hello ' + name)
68}
69
70function log (msg) {
71 console.log(msg)
72}
73
74// this will print 'Hello anonymous' but the logical error will be missed
75greet(null, once(msg))
76
77// once.strict will print 'Hello anonymous' and throw an error when the callback will be called the second time
78greet(null, once.strict(msg))
79```
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