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once

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Stage 1
Classification: API Change
Human Validated: KW
Title: Function once
Authors: J. S. Choi
Champions: J. S. Choi
Last Presented: March 2022
Stage Upgrades:
Stage 1: 2022-03-31
Stage 2: NA
Stage 2.7: NA
Stage 3: NA
Stage 4: NA
Last Commit: 2022-07-10
Topics: functions others async
Keywords: callback standardization promise
GitHub Link: https://github.com/tc39/proposal-function-once
GitHub Note Link: https://github.com/tc39/notes/blob/HEAD/meetings/2022-03/mar-29.md#functionprototypeonce-for-stage-1

Proposal Description:

Function.prototype.once for JavaScript

ECMAScript Stage-1 Proposal. 2022.

Co-champions: Hemanth HM; J. S. Choi.

Rationale

It is often useful to ensure that callbacks execute only once, no matter how many times are those callbacks called. To do this, developers frequently use “once” functions, which wrap around those callbacks and ensure they are called at most once. This proposal would standardize such a once function in the language core.

Description

The Function.prototype.once method would create a new function that calls the original function at most once, no matter how much the new function is called. Arguments given in this call are passed to the original function. Any subsequent calls to the created function would return the result of its first call.

function f (x) { console.log(x); return x * 2; }
 
const fOnce = f.once();
fOnce(3); // Prints 3 and returns 6.
fOnce(3); // Does not print anything. Returns 6.
fOnce(2); // Does not print anything. Returns 6.

Real-world examples

The following code was adapted to use this proposal.

From execa@6.1.0:

export function execa (file, args, options) {
  /* … */
  const handlePromise = async () => { /* … */ };
  const handlePromiseOnce = handlePromise.once();
  /* … */
  return mergePromise(spawned, handlePromiseOnce);
});

From glob@7.2.1:

function Glob (pattern, options, cb) {
  /* … */
  if (typeof cb === 'function') {
    cb = cb.once();
    this.on('error', cb);
    this.on('end', function (matches) {
      cb(null, matches);
    })
  } /* … */
});

From Meteor@2.6.1:

// “Are we running Meteor from a git checkout?”
export const inCheckout = (function () {
  try { /* … */ } catch (e) { console.log(e); }
  return false;
}).once();

From cypress@9.5.2:

cy.on('command:retry', (() => { /* … */ }).once());

From jitsi-meet 1.0.5913:

this._hangup = (() => {
  sendAnalytics(createToolbarEvent('hangup'));
  /* … */
}).once();

Precedents and web compatibility

There is a popular NPM library called once that allows monkey patching, which may raise concerns about Function.prototype.once’s web compatibility.

However, since its first public version, the once library’s monkey patching has been opt-in only. The monkey patching is not conditional, and there is no actual web-compatibility risk from this library.

// The default form exports a function.
once = require('once');
fOnce = once(f);
 
// The opt-in form monkey patches Function.prototype.
require('once').proto();
fOnce = f.once();

Other popular once functions from libraries (e.g., lodash.once, Underscore and onetime) also do not use conditional monkey patching.

A code search for !Function.prototype.once (as in if (!Function.prototype.once) { /* monkey patching */ }) also gave no results in any indexed open-source code. It is unlikely that any production code on the web is conditionally monkey patching a once method into Function.prototype.


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