once

What is once?

The 'once' npm package is a utility that allows you to ensure a function can only be called once. It is useful for preventing duplicate initialization, handling setup tasks that should only run a single time, or ensuring a callback is only executed once in response to an event or the resolution of a Promise.

What are once's main functionalities?

Ensuring a function is only called once

This feature is used to wrap a function so that it can only be executed once. Subsequent calls to the function will have no effect, and the original return value will be returned.

const once = require('once');
const myFunction = once(() => {
  console.log('This will only be logged once.');
});

myFunction(); // logs 'This will only be logged once.'
myFunction(); // does nothing

Other packages similar to once

lodash.once

This is a function from the Lodash library that ensures a given function can only be called once. It is similar to 'once' but comes as part of the larger Lodash utility library, which includes a wide range of functions for different purposes.

memoizee

Memoizee is a library for memoizing functions, which can also be used to ensure a function is only called once by caching the result of the first call. It is more complex and feature-rich than 'once', offering fine-grained control over cache management and function memoization.

README

once

Only call a function once.

usage

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:

// 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.

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'))
  })
}

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 would potentially hide logical errors.

In the example below, the greet function has to call the callback only once:

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))