Extensions and :: operator
Proposal status
This is an ECMAScript (JavaScript) proposal in stage 1.
Note: The proposal could be seen as the reshape of the "virtual method" part of old bind operator proposal, see tc39/proposal-bind-operator#56.
Simple examples
Example of ad-hoc extension methods and accessors
// define two extension methods
const ::toArray = function () { return [...this] }
const ::toSet = function () { return new Set(this) }
// define a extension accessor
const ::allDivs = {
get() { return this.querySelectorAll('div') }
}
// reuse built-in prototype methods and accessors
const ::flatMap = Array.prototype.flatMap
const ::size = Object.getOwnPropertyDescriptor(Set.prototype, 'size')
// Use extension methods and accesors to calculate
// the count of all classes of div element.
let classCount = document::allDivs
::flatMap(e => e.classList::toArray())
::toSet()::sizeroughly equals to:
// define two extension methods
const $toArray = function () { return [...this] }
const $toSet = function () { return new Set(this) }
// define a extension accessor
const $allDivs = {
get() { return this.querySelectorAll('div') }
}
// reuse built-in prototype method and accessor
const $flatMap = Array.prototype.flatMap
const $size = Object.getOwnPropertyDescriptor(Set.prototype, 'size')
// Use extension methods and accesors to calculate
// the count of all classes of div element.
let $
$ = $allDivs.get.call(document)
$ = $flatMap.call($, e => $toArray.call(e.classList))
$ = $toSet.call($)
$ = $size.get.call($)
let classCount = $Example of using constructors or namespace object as extensions
// util.js
export const toArray = iterable => [...iterable]
export const toSet = iterable => new Set(iterable)import * as util from './util.js'
const ::allDivs = {
get() { return this.querySelectorAll('div') }
}
let classCount = document::allDivs
::Array:flatMap(
e => e.classList::util:toArray())
::util:toSet()
::Set:sizeroughly equals to:
import * as util from './util.js'
const $allDivs = {
get() { return this.querySelectorAll('div') }
}
let $
$ = $allDivs.get.call(document)
$ = Array.prototype.flatMap.call($,
e => util.toArray(e.classList))
$ = util.toSet($)
$ = Object.getOwnPropertyDescriptor(Set.prototype, 'size').get.call($)
let classCount = $Changes of the old bind operator proposal
- keep
obj::foo()syntax for extension methods - repurpose
obj::fooas extension getters and addobj::foo =as extension setters - separate namespace for ad-hoc extension methods and accessors, do not pollute normal binding names
- add
obj::ext:namesyntax - change operator precedence to same as
. - remove
::obj.foo(use cases can be solved by custom extension + library, or other proposals)