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79 lines
1.7 KiB
79 lines
1.7 KiB
# once |
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Only call a function once. |
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## usage |
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```javascript |
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var once = require('once') |
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function load (file, cb) { |
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cb = once(cb) |
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loader.load('file') |
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loader.once('load', cb) |
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loader.once('error', cb) |
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} |
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``` |
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Or add to the Function.prototype in a responsible way: |
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```javascript |
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// only has to be done once |
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require('once').proto() |
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function load (file, cb) { |
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cb = cb.once() |
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loader.load('file') |
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loader.once('load', cb) |
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loader.once('error', cb) |
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} |
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``` |
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Ironically, the prototype feature makes this module twice as |
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complicated as necessary. |
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To check whether you function has been called, use `fn.called`. Once the |
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function is called for the first time the return value of the original |
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function is saved in `fn.value` and subsequent calls will continue to |
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return this value. |
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```javascript |
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var once = require('once') |
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function load (cb) { |
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cb = once(cb) |
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var stream = createStream() |
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stream.once('data', cb) |
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stream.once('end', function () { |
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if (!cb.called) cb(new Error('not found')) |
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}) |
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} |
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``` |
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## `once.strict(func)` |
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Throw an error if the function is called twice. |
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Some functions are expected to be called only once. Using `once` for them would |
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potentially hide logical errors. |
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In the example below, the `greet` function has to call the callback only once: |
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```javascript |
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function greet (name, cb) { |
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// return is missing from the if statement |
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// when no name is passed, the callback is called twice |
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if (!name) cb('Hello anonymous') |
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cb('Hello ' + name) |
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} |
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function log (msg) { |
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console.log(msg) |
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} |
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// this will print 'Hello anonymous' but the logical error will be missed |
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greet(null, once(msg)) |
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// once.strict will print 'Hello anonymous' and throw an error when the callback will be called the second time |
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greet(null, once.strict(msg)) |
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```
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