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.