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Extensions proposal for ECMAScript
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tc39/proposal-extensions
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This is an ECMAScript (JavaScript) proposal in stage 1.
Note: The proposal could be seen as the reshape of the "virtual method" part of oldbind operator proposal, seetc39/proposal-bind-operator#56.
// define two extension methodsconst ::toArray=function(){return[...this]}const ::toSet=function(){returnnewSet(this)}// define a extension accessorconst ::allDivs={get(){returnthis.querySelectorAll('div')}}// reuse built-in prototype methods and accessorsconst ::flatMap=Array.prototype.flatMapconst ::size=Object.getOwnPropertyDescriptor(Set.prototype,'size')// Use extension methods and accesors to calculate// the count of all classes of div element.letclassCount=document::allDivs::flatMap(e=>e.classList::toArray())::toSet()::size
roughly equals to:
// define two extension methodsconst$toArray=function(){return[...this]}const$toSet=function(){returnnewSet(this)}// define a extension accessorconst$allDivs={get(){returnthis.querySelectorAll('div')}}// reuse built-in prototype method and accessorconst$flatMap=Array.prototype.flatMapconst$size=Object.getOwnPropertyDescriptor(Set.prototype,'size')// Use extension methods and accesors to calculate// the count of all classes of div element.let$$=$allDivs.get.call(document)$=$flatMap.call($,e=>$toArray.call(e.classList))$=$toSet.call($)$=$size.get.call($)letclassCount=$
// util.jsexportconsttoArray=iterable=>[...iterable]exportconsttoSet=iterable=>newSet(iterable)
import*asutilfrom'./util.js'const ::allDivs={get(){returnthis.querySelectorAll('div')}}letclassCount=document::allDivs::Array:flatMap(e=>e.classList::util:toArray())::util:toSet()::Set:size
roughly equals to:
import*asutilfrom'./util.js'const$allDivs={get(){returnthis.querySelectorAll('div')}}let$$=$allDivs.get.call(document)$=Array.prototype.flatMap.call($,e=>util.toArray(e.classList))$=util.toSet($)$=Object.getOwnPropertyDescriptor(Set.prototype,'size').get.call($)letclassCount=$
- 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)
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Extensions proposal for ECMAScript