/var/runtime/node_modules/@aws-sdk/node_modules/once
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# once Only call a function once. ## usage ```javascriptvar 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 oncerequire('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 ascomplicated as necessary. To check whether you function has been called, use `fn.called`. Once thefunction is called for the first time the return value of the originalfunction is saved in `fn.value` and subsequent calls will continue toreturn this value. ```javascriptvar 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 wouldpotentially hide logical errors. In the example below, the `greet` function has to call the callback only once: ```javascriptfunction 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 missedgreet(null, once(msg)) // once.strict will print 'Hello anonymous' and throw an error when the callback will be called the second timegreet(null, once.strict(msg))```