stage-0 to stage-4 ECMAscript proposals.
TOC:
- Stage 0:
- Defensible Classes
- Relationships
- String.prototype.at
- Reflect.isCallable
- Reflect.isConstructor
- Additional metaproperties
- Function Bind Syntax
- 64-Bit Integer Operations
- Method Parameter Decorators
- Function Expression Decorators
- Zones
- Object enumerables
- Nested import declarations
- is{Type} APIs
- Orthogonal Class Member Syntax
- Pattern Matching Syntax
- Structured cloning and transfer
- WHATWG URL
- Stage 1:
- export v from "mod"; statements
- Observable
- String.prototype.matchAll
- WeakRefs
- Frozen Realms
- Math Extensions
of
andfrom
on collection constructors- Generator arrow functions.
- Date.parse fallback semantics
- Generator arrow functions (=>*)
- Promise.try
of
andfrom
on collection- Optional Chaining
- Math.signbit: IEEE-754 sign bit
- Error Stacks
- do expressions
- Realms
- Stage 2:
- Stage 3:
- global
- Rest and Spread properties
- Async-iteration
- Function.prototype.toString revision
- SIMD APIs
- Lifting Template Literal Restriction
- Shared memory and atomics
- global
- import()
- RegExp Lookbehind Assertions
- Unicode property escapes in RE
- RegExp Named Capture Groups
- s (dotAll) flag for regular expressions
- Asynchronous Iterators
- Promise.prototype.finally
- Class Fields
Stage 0:
Defensible Classes
Stage-0
// const class
const class Point {
constructor(x, y) {
public getX() { return x; }
public getY() { return y; }
}
toString() {
return `<${this.getX()}, ${this.getY()}>`;
}
}
Relationships
Stage-0
x @ r // The object x is in the r relationship with what value?
x @ r = y; // Store that x is in the r relationship with value y.
String.prototype.at
Stage-0
'abc𝌆def'.at(3)
// → '𝌆'
Reflect.isCallable
Stage-0
Reflect.isCallable(argument);
Reflect.isConstructor
Stage-0
Reflect.isConstructor(argument)
Additional metaproperties
Stage-0
function.callee; // function object that is currently being evaluated by the running execution context.
function.count; // number of arguments pass to the function.
function.arguments; // array containing the actual arguments passed to the function.
Function Bind Syntax
Stage-0
// :: which performs this binding and method extraction.
Promise.resolve(123).then(::console.log);
64-Bit Integer Operations
Stage-0
// return the high 32 bit part of the 64 bit addition of (hi0, lo0) and (hi1, lo1)
Math.iaddh(lo0, hi0, lo1, hi1);
// return the high 32 bit part of the 64 bit subtraction of (hi0, lo0) and (hi1, lo1)
Math.isubh(lo0, hi0, lo1, hi1);
// return the high 32 bit part of the signed 64 bit product of the 32 bit numbers a and b
Math.imulh(a, b);
// return the high 32 bit part of the unsigned 64 bit product of the 32 bit numbers a and b
Math.umulh(a, b);
Method Parameter Decorators
Stage-0
//decorators that operate on method and constructor parameters.
class MyComponent {
refresh(@lastRefreshTime timeStamp) { … }
}
export function lastRefreshTime(...) {
// at minimum, the arguments of this function should contain:
// - reference to owner of the parameter (the method)
// - parameter index
// - parameter name
// - is parameter a rest parameter?
// store parameter metadata using the same storage mechanism
// as the one used for methods
}
Function Expression Decorators
Stage-0
scheduleForFrequentReexecution(@memoize function(value) {
value++
});
export function memoize(...) {
// at minimum, the arguments of this function should contain:
// - reference to the decorated function expression
// - arguments passed into the memoize function (if any)
// wrap the decorated function expression memoization implementation and return it
}
Zones
Stage-0
//a primitive for context propagation across multiple logically-connected async operations
class Zone {
constructor({ name, parent });
name;
get parent();
fork({ name });
run(callback);
wrap(callback);
static get current();
}
const loadZone = Zone.current.fork({ name: "loading zone" });
window.onload = loadZone.wrap(e => { ... });
Object enumerables
Stage-0
Object.enumerableKeys(obj); // Ordered list of keys.
Object.enumerableValues(obj); // Ordered list of Values.
Object.enumerableEntries(obj); //Ordered list of key value pairs.
Nested import declarations
Stage-0
describe("fancy feature #5", () => {
import { strictEqual } from "assert";
it("should work on the client", () => {
import { check } from "./client.js";
strictEqual(check(), "client ok");
});
it("should work on the client", () => {
import { check } from "./server.js";
strictEqual(check(), "server ok");
});
it("should work on both client and server", () => {
import { check } from "./both.js";
strictEqual(check(), "both ok");
});
});
is{Type} APIs
Stage-0
Builtin.is(Date, vm.runInNewContext('Date')); // false
Builtin.typeOf([]); // 'Array'
Builtin.typeOf(new ArrayBuffer()); // 'ArrayBuffer'
Builtin.typeOf(async function foo() {});
// So on.
Orthogonal Class Member Syntax
Stage-0
//A kitchen sink example
class Foo {
//instance members
own x=0, y=0; // two data properties
own #secret; // a private field
// initial value undefined
own *[Symbol.iterator](){yield this.#secret}
// a generator method
own #callback(){} //a private instance method
//class constructor members
static #p=new Set(), q=Foo.#p;
// a private field and a property
// of the class constructor
static get p(){return Foo.#p} //accessor method
//prototype methods
setCallback(f){this.#callback=f}
constructor(s){
this.#secret = s;
}
}
Pattern Matching Syntax
Stage-0
let getLength = vector => match (vector) {
{ x, y, z }: Math.sqrt(x ** 2 + y ** 2 + z ** 2),
{ x, y }: Math.sqrt(x ** 2 + y ** 2),
[...]: vector.length,
else: {
throw new Error("Unknown vector type");
}
}
Structured cloning and transfer
Stage-0
StructuredClone(input, transferList, targetRealm)
WHATWG URL
Stage-0
const base = new URL('http://example.org/foo');
const url = new URL('bar', base);
Stage 1:
export v from "mod"; statements
Stage-1
export v, {x, y as w} from "mod";
export v, * as ns from "mod";
Observable
Stage-1
// Observable as a Constructor:
function listen(element, eventName) {
return new Observable(observer => {
// Create an event handler which sends data to the sink
let handler = event => observer.next(event);
// Attach the event handler
element.addEventListener(eventName, handler, true);
// Return a function which will cancel the event stream
return () => {
// Detach the event handler from the element
element.removeEventListener(eventName, handler, true);
};
});
}
// Observable.of creates an Observable of the values provided as arguments
Observable.of("R", "G", "B").subscribe({
next(color) {
console.log(color);
}
});
// Observable.from converts its argument to an Observable.
Observable.from(["R", "G", "B"]).subscribe({
next(color) {
console.log(color);
}
});
String.prototype.matchAll
Stage-1
var str = 'Hello world!!!';
var regexp = /(\w+)\W*/g;
console.log(str.matchAll(regexp));
/*
[
{
0: "Hello ",
1: "Hello"
index: 0,
input: "Hello world!!!"
},
{
0: "world!!!",
1: "world"
index: 6,
input: "Hello world!!!"
}
]
*/
WeakRefs
Stage-1
// Make a new weak reference.
// The target is a strong pointer to the object that will be pointed
// at weakly by the result.
// The executor is an optional argument that will be invoked after the
// target becomes unreachable.
// The holdings is an optional argument that will be provided to the
// executor when it is invoked for target.
makeWeakRef(target, executor, holdings);
Frozen Realms
Stage-1
class Realm {
// From the prior Realm API proposal
const global -> object // access this realm's global object
eval(stringable) -> any // do an indirect eval in this realm
// We expect the rest of earlier proposal to be re-proposed eventually in
// some form, but do not rely here on any of the remainder.
// New with this proposal
static immutableRoot() -> Realm // transitively immutable realm
spawn(endowments) -> Realm // lightweight child realm
}
Math Extensions
Stage-1
// Possible ones:
Math.map
Math.scale
Math.remap
Math.clamp
Math.constrain
Math.toDegrees(double angrad)
Math.toRadians(double angdeg)
of
and from
on collection constructors
Stage-1
Map.of( ...items );
Map.from( ...items );
Set.of( ...items );
Set.from( ...items );
WeakMap.of( ...items );
WeakMap.from( ...items );
WeakSet.of( ...items );
WeakSet.from( ...items );
Generator arrow functions.
Stage-1
let cat = *() => { yield 'meow'; }
Date.parse fallback semantics
Stage-1
// New grammar should be used as the "fallback"
// when date strings do not conform to the
// regular Date Time String Format.
Generator arrow functions (=>*)
Stage-1
// current
x => x * x;
(...) => { statements }
(...) => ( expr )
// proposed generator arrows...
// Irregular
() =*>
// Hostile to ! (async function)
() => * { ...yield... }
// Not good
() => * (yield a, yield b)
// Ok if 1 token
x *=> x * x;
// Bad (ASI)
*() => ...
// Hostile to !
(x) =* {...}
Promise.try
Stage-1
// Promise.try(function() fn) -> Promise
of
and from
on collection
Stage-1
CollectionCreate ( C, source [ , mapfn [ , thisArg ] ] )
Map.of ( ...items )
Set.of ( ...items )
WeakMap.of ( ...items )
WeakSet.of ( ...items )
Map.from ( source [ , mapFn [ , thisArg ] ] )
Set.from ( source [ , mapFn [ , thisArg ] ] )
WeakMap.from ( source [ , mapFn [ , thisArg ] ] )
WeakSet.from ( source [ , mapFn [ , thisArg ] ] )
Optional Chaining
Stage-1
obj?.prop // optional property access
obj?.[expr] // ditto
func?.(...args) // optional function or method call
new C?.(...args) // optional constructor invocation
Math.signbit: IEEE-754 sign bit
Stage-1
Math.signbit(x);
/*
Returns whether the sign bit of x is set.
If n is NaN, the result is false.
If n is -0, the result is true.
If n is negative, the result is true.
Otherwise, the result is false.
*/
Error Stacks
Stage-1
Error.prototype.stack;
System.getStack;
System.getStackString;
Object.getOwnPropertyDescriptor(new Error(), 'stack');
Object.getOwnPropertyDescriptor(Error.prototype, 'stack');
do expressions
Stage-1
let x = do {
let tmp = f();
tmp * tmp + 1
};
let x = do {
if (foo()) { f() }
else if (bar()) { g() }
else { h() }
};
Realms
Stage-1
let realm = new Realm();
let outerGlobal = window;
let innerGlobal = realm.global;
let f = realm.evalScript("(function() { return 17 })");
f() === 17 // true
Reflect.getPrototypeOf(f) === outerGlobal.Function.prototype // false
Reflect.getPrototypeOf(f) === innerGlobal.Function.prototype // true
class EmptyRealm extends Realm {
constructor(...args) { super(...args); }
init() { /* do nothing */ }
}
class FakeWindow extends Realm {
init() {
super.init(); // install the standard primordials
let global = this.global;
global.document = new FakeDocument(...);
global.alert = new Proxy(fakeAlert, { ... });
...
}
}
Stage 2:
Template Literal Revision
Stage-2
// The proposal is about fixing those Illegal token errors, avoid restrictions on escape sequences.
let document = latex`
\newcommand{\fun}{\textbf{Fun!}} // works just fine
\newcommand{\unicode}{\textbf{Unicode!}} // Illegal token!
\newcommand{\xerxes}{\textbf{King!}} // Illegal token!
Breve over the h goes \u{h}ere // Illegal token!
function.sent Meta Property
Stage-2
// Avoid ingnoring the first `next` call.
function *adder(total=0) {
let increment=1;
do {
switch (request = function.sent){
case undefined: break;
case "done": return total;
default: increment = Number(request);
}
yield total += increment;
} while (true)
}
let tally = adder();
tally.next(0.1); // argument no longer ignored
tally.next(0.1);
tally.next(0.1);
let last=tally.next("done");
console.log(last.value); //0.3
Class Property Declarations
Stage-2
// Class instance field
class MyClass {
myProp = 42;
constructor() {
console.log(this.myProp); // Prints '42'
}
}
// Static property
class MyClass {
static myStaticProp = 42;
constructor() {
console.log(MyClass.myStaticProp); // Prints '42'
}
}
// Class Static Properties
class MyClass {
static myStaticProp = 42;
constructor() {
console.log(MyClass.myStaticProp); // Prints '42'
}
}
Class and Property Decorators
Stage-2
class C {
@writable(false)
method() { }
}
function writable(value) {
return function (target, key, descriptor) {
descriptor.writable = value;
return descriptor;
}
}
String.prototype.{trimStart,trimEnd}
Stage-2
" Hey JS!".trimStart(); // "Hey JS!"
" Hey JS! ".trimEnd();// " Hey JS!"
// P.S: trimLeft/trimRight are aliases.
Legacy RegExp features
Stage-2
RegExpAlloc( newTarget );
RegExpBuiltInExec( R, S );
RegExp.input;
RegExp.prototype.compile( pattern, flags ); // modifications
Intl.Segmenter: Unicode segmentation
Stage-2
// Create a segmenter in your locale
let segmenter = Intl.Segmenter("fr", {type: "word"});
// Get an iterator over a string
let iterator = segmenter.segment("Ceci n'est pas une pipe");
// Iterate over it!
for (let {segment, breakType} of iterator) {
console.log(`segment: ${segment} breakType: ${breakType}`);
break;
}
// logs the following to the console:
// index: Ceci breakType: letter
export * as ns from "mod"; statements
Stage-2
export * as ns from "mod"; // Exporting the ModuleNameSpace object as a named export.
Stage 3:
global
Stage-3
// global to rule them all.
var getGlobal = function () {
// the only reliable means to get the global object is
// `Function('return this')()`
// However, this causes CSP violations in Chrome apps.
if (typeof self !== 'undefined') { return self; }
if (typeof window !== 'undefined') { return window; }
if (typeof global !== 'undefined') { return global; }
throw new Error('unable to locate global object');
};
Rest and Spread properties
Stage-3
let { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 }; // Rest.
let n = { x, y, ...z }; // Spread.
Async-iteration
Stage-3
asyncIterator
.next()
.then(({ value, done }) => /* ... */);
Function.prototype.toString revision
Stage-3
// String's parse must contains the same
// function body and parameter list as the original.
O.gOPD({ get a(){} }, "a").get // "function a(){}"
O.gOPD({ set a(b){} }, "a").set // "function a(b){}"
SIMD APIs
Stage-3
/*a meta-variable ranging over all SIMD types:
Float32x4, Int32x4, Int16x8 Int8x16, Uint32x4,
Uint16x8, Uint8x16, Bool32x4, Bool16x8 and Bool8x16. */
Lifting Template Literal Restriction
Stage-3
function tag(strs) {
strs[0] === undefined
strs.raw[0] === "\\unicode and \\u{55}";
}
tag`\unicode and \u{55}`
let bad = `bad escape sequence: \unicode`; // throws early error
Shared memory and atomics
Stage-3
var sab = new SharedArrayBuffer(1024); // 1KiB shared memory
w.postMessage(sab, [sab])
// In the worker:
var sab;
onmessage = function (ev) {
sab = ev.data; // 1KiB shared memory, the same memory as in the parent
}
global
stage 3
typeof global; // object, helps in writing a portable code.
import()
stage 3
import(`./language-packs/${navigator.language}.js`) // import(specifier)
RegExp Lookbehind Assertions
Stage-3
const str = '1947';
// (?<=(\d+)(\d+))$/ => (947) and (1)
// Greediness proceeds from right to left
// match[1] => 947 and match[2] => 1
// Numbering capture groups
// /(?<=\1(.))/
// Referring to capture groups
// /(?<!.)/
// Negative assertions
Unicode property escapes in RE
Stage-3
const regexGreekSymbol = /\p{Script=Greek}/u;
regexGreekSymbol.test('π');
RegExp Named Capture Groups
Stage-3
let {one, two} = /^(?<one>.*):(?<two>.*)$/u.exec('foo:bar');
console.log(`one: ${one}, two: ${two}`); // prints one: foo, two: bar
s (dotAll) flag for regular expressions
Stage-3
const re = /foo.bar/s; // Or, `const re = new RegExp('foo.bar', 's');`.
re.test('foo\nbar');
// → true
re.dotAll
// → true
re.flags
// → 's'
/foo.bar/s.test('foo\nbar');
// → true
Asynchronous Iterators
Stage-3
asyncIterator.next().then(result => console.log(result.value));
for await (let line of readLines(filePath)) {
print(line);
}
async function *readLines(path) {
let file = await fileOpen(path);
try {
while (!file.EOF)
yield file.readLine();
} finally {
await file.close();
}
}
Promise.prototype.finally
Stage-3
somePromise()
.then(() => {})
.catch(() => {})
.finally(() => {})
Class Fields
Stage-3
class Point {
#x = 0; // private fields start with #
myProp = 42; // public field
constructor() {
this.#x; // 0
this.myProp; // 0
}
}