ECMAScript Optional Static Typing Proposal
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ECMAScript Proposal: Optional Static Typing

Current status of this proposal is -1. It's in a theoretical state at the moment to better understand how types could function in Javascript and the long-term future benefits or complications they could cause to future proposals.


With TypedArrays and classes finalized, ECMAScript is in a good place to finally discuss types again. The demand for types as a different approach to code has been so strong in the past few years that separate languages have been created to deal with the perceived shortcomings. Types won't be an easy discussion, nor an easy addition, since they touch a large amount of the language; however, they are something that needs rigorous discussion.

The types described below bring ECMAScript in line or surpasses the type systems in most languages. For developers it cleans up a lot of the syntax, as described later, for TypedArrays, SIMD, and working with number types (floats vs signed and unsigned integers). It also allows for new language features like function overloading and a clean syntax for operator overloading. For implementors, added types offer a way to better optimize the JIT when specific types are used. For languages built on top of Javascript this allows more explicit type usage and closer matching to hardware.

Types Proposed

Since it would be potentially years before this would be implemented this proposal includes a new keyword enum for enumerated types and the following types:

int8, int16, int32, int64
uint8, uint16, uint32, uint64
float16, float32, float64, float80, float128
decimal32, decimal64, decimal128
boolean8x16, boolean16x8, boolean32x4, boolean64x2, boolean8x32, boolean16x16, boolean32x8, boolean64x4
int8x16, int16x8, int32x4, int64x2, int8x32, int16x16, int32x8, int64x4
uint8x16, uint16x8, uint32x4, uint64x2, uint8x32, uint16x16, uint32x8, uint64x4
float32x4, float64x2, float32x8, float64x4

These types once imported behave like a const declaration and cannot be reassigned.

Variable Declaration With Type

This syntax is taken from ActionScript and other proposals over the years. It's subjectively concise, readable, and consistent throughout the proposal.

var a:Type = value;
let b:Type = value;
const c:Type = value;

typeof Operator

One of the first complications with types is typeof's behavior. All of the above types would return their string conversion.

let a:uint8 = 0; // typeof a == "uint8"
let b:uint8? = 0; // typeof b == "uint8?"
let c:uint8[] = []; // typeof c == "object"
let d:(uint8):uint8 = x => x * x; // typeof d == "function"

instanceof Operator


if (a instanceof uint8) {}

Also this would be nice for function signatures.

if (a instanceof (uint8):uint8) {}

That would imply Object.getPrototypeOf(a) === ((uint8):uint8).prototype.

I'm not well versed on if this makes sense though, but it would be like each typed function has a prototype defined by the signature.

Nullable Types

All types except any are non-nullable. The syntax below creates a nullable uint8 typed variable:

let a:uint8? = null; // typeof a == "uint8?"

any Type

Using any? would result in a syntax error since any already includes nullable types. As would using any[] since it already includes array types. Using just [] would be the type for arrays that can contain anything. For example:

let a:[];

Variable-length Typed Arrays

let a:uint8[]; // []
a.push(0); // [0]
let b:uint8[] = [0, 1, 2, 3];
let c:uint8[]?; // null

The index operator doesn't perform casting just to be clear so array objects even when typed still behave like objects.

let a:uint8[] = [0, 1, 2, 3];
a['a'] = 0;
'a' in a; // true
delete a['a'];

Fixed-length Typed Arrays

let a:uint8[4]; // [0, 0, 0, 0]
// a.push(0); TypeError: a is fixed-length
// a.pop(); TypeError: a is fixed-length
a[0] = 1; // valid
// a[a.length] = 2; Out of range
let b:uint8[4] = [0, 1, 2, 3];
let c:uint8[4]?; // null

Typed arrays would be zero-ed at creation. That is the allocated memory would be set to all zeroes.

Mixing Variable-length and Fixed-length Arrays

function F(c:boolean):uint8[] // default case, return a resizable array
    let a:uint8[4] = [0, 1, 2, 3];
    let b:uint8[6] = [0, 1, 2, 3, 4, 5];
    return c ? a : b;

function F(c:boolean):uint8[6] // Resizes a if c is true
    let a:uint8[4] = [0, 1, 2, 3];
    let b:uint8[6] = [0, 1, 2, 3, 4, 5];
    return c ? a : b;

Any Typed Array

let a:[]; // Using any[] is a syntax error as explained before
let b:[]? = null; // nullable array

Deleting a typed array element results in a type error:

const a:uint8[] = [0, 1, 2, 3];
// delete a[0]; TypeError: a is fixed-length

Array length Type And Operations

Valid types for defining the length of an array are as follows:


By default length is uint32.


let a:uint8[:int8] = [0, 1, 2, 3, 4];
let bar = a.length; // length is type int8
let a:uint8[5:uint64] = [0, 1, 2, 3, 4];
let b = a.length; // length is type uint64 with value 5
let n = 5;
let a:uint8[n:uint64] = [0, 1, 2, 3, 4];
let b = a.length; // length is type uint64 with value 5

Setting the length reallocates the array truncating when applicable.

let a:uint8[] = [0, 1, 2, 3, 4];
a.length = 4; // [0, 1, 2, 4]
a.length = 6; // [0, 1, 2, 4, 0, 0]
let a:uint8[5] = [0, 1, 2, 3, 4];
// a.length = 4; TypeError: a is fixed-length

Array views:

Like TypedArray views this array syntax allows any array, even arrays of typed objects to be viewed as different objects. Stride would have performance implications but would allow for a single view to access elements with padding between elements.

let view = Type[](buffer [, byteOffset [, byteLength [, byteStride]]]);
let a:uint64[] = [1];
let b = uint32[](a, 0, 8);

Take special note of the lack of new in this syntax. Adding new in the above case would pass the arguments to the constructor for each element.

Multidimensional and Jagged Array Support Via User-defined Index Operators

Rather than defining index functions for various multidimensional and jagged array implementations the user is given the ability to define their own. Any lambda parameter passed to the "index constructor" creates an indexing function. More than one can be defined as long as they have unique signatures. The signature (x:string) is reserved for keys and can't be used.

An example of a user-defined index to access a 16 element grid with (x, y) coordinates:

let grid = new uint8[16:uint32, (x:uint32, y:uint32) => y * 4 + x];
// grid[0] = 10; Error, invalid arguments
grid[2, 1] = 10;
let grid = new uint8[16:uint32, i => i, (x:uint32, y:uint32) => y * 4 + x];
grid[0] = 10;
grid[2, 1] = 10;

For a variable-length array it works as expected where the user drops the length:

var grid = new uint8[(x, y, z) => z * 4 * 4 + y * 4 + x];
var grid2 = new uint8[uint64, (x, y, z) => z * 4 * 4 + y * 4 + x];

Views also work as expected allowing one to apply custom indexing to existing arrays:

var gridView = uint32[(x, y, z) => z * 4 * 4 + y * 4 + x](grid);

Implicit Casting

The default numeric type Number would convert implicitly with precedence given to decimal128/64/32, float128/80/64/32/16, uint64/32/16/8, int64/32/16/8. (This is up for debate). Examples are shown later with class constructor overloading.

function F(a:float32) { }
function F(a:uint32) { }
F(1); // float32 called
F(uint32(1)); // uint32 called

Explicit Casting

let a = uint8(65535); // Cast taking the lowest 8 bits so the value 255, but note that a is still typed as any

Many truncation rules have intuitive rules going from larger bits to smaller bits or signed types to unsigned types. Type casts like decimal to float or float to decimal would need to be clear.

Function signatures with constraints

function F(a:int32, b:string, c:bigint[], callback:(boolean, string) = (b, s = 'none') => b ? s : ''):int32 { }

Typed Arrow Functions

let a:(int32, string):string; // hold a reference to a signature of this type
let b:(); // void is the default return type for a signature without a return type
let c = (s:string, x:int32) => s + x; // implicit return type of string
let d = (x:uint8, y:uint8):uint16 => x + y; // explicit return type
let e = x:uint8 => x + y; // single parameter

Like other types they can be made nullable. An example showing an extreme case where everything is made nullable:

let a:(uint32?)?:uint32? = null;

This can be written also using the interfaces syntax, which is explained later:

let a:{ (:uint32?):uint32; }? = null;

Integer Binary Shifts

let a:int8 = -128;
a >> 1; // -64, sign extension
let b:uint8 = 128;
b >> 1; // 64, no sign extension as would be expected with an unsigned type

Integer Division

let a:int32 = 3;
a /= 2; // 1

Expanding Representable Numbers

In ECMAScript currently the following values are equal:

let a = 2 ** 53;
a == a + 1; // true

In order to bypass this behavior a variable must be explicitly typed.

let a:uint64 = 2 ** 53;
a == a + 1; // false

Ideally statements like the following will work as expected with the type propagating to the right hand side:

let a:uint64 = 9007199254740992 + 9007199254740993; // 18014398509481985

In the following case the type propagates to the arguments.

function F(a:uint64) {}
F(9007199254740992 + 9007199254740993); // 18014398509481985

Consider where the literals are not directly typed. In this case they are typed as Number:

function F(a:uint64) {}
var a = 9007199254740992 + 9007199254740993;
F(a); // 18014398509481984

Destructuring Assignment Casting

Array destructuring with default values:

[a:uint32 = 1, b:float32 = 2] = F();

Object destructuring with default values:

{ (a:uint8) = 1, (b:uint8) = 2 } = { a: 2 };

Object destructuring with default value and new name:

let { (a:uint8): b = 1 } = { a: 2 }; // b is 2

Assigning to an already declared variable:

let b:uint8;
({ a: b = 1 } = { a: 2 }); // b is 2

Destructuring with functions:

(({ (a:uint8): b = 0, (b:uint8): a = 0}, [c:uint8]) =>
    // a = 2, b = 1, c = 0
})({a: 1, b: 2}, [0]);

Nested/deep object destructuring:

const { a: { (a2:uint32): b, a3: [, c:uint8] } } = { a: { a2: 1, a3: [2, 3] } }; // b is 1, c is 3

Destructuring objects with arrays:

const { (a:uint8[]) } = { a: [1, 2, 3] } }; // a is [1, 2, 3] with type uint8[]

Typed return values for destructuring

Basic array destructuring:

function F():[uint8, uint32]
    return [1, 2];
const [a, b] = F();

Array defaults

function F():[uint8, uint32 = 10]
    return [1];
const [a, b] = F(); // a is 1 and b is 10

Basic object destructuring:

function F():{ a:uint8; b:float32; }
    return { a: 1, b: 2 };
const { a, b } = F();

Object defaults:

function F():{ a:uint8; b:float32 = 10; }
    return { a: 1 };
const { a, b } = F(); // { a: 1, b: 10 }

Overloaded example for the return type:

function F():[int32]
    return [1];
function F():[int32, int32]
    return [2, 3];
function F():{ a:uint8; b:float32; }
    return { a: 1, b: 2 };
const [a] = F(); // a is 1
const [b, ...c] = F(); // b is 2 and c is [3]
const { a:d, b:e } = F(); // d is 1 and e is 2

TODO: Define the behavior when defining the same signature for a function.

Explicitly selecting an overload:

function F():[int32]
    return [1];
function F():[float32]
    return [2.0];
const [a:int32] = F();
const [a:float32] = F();

TypeError example:

function F():[int32, float32]
    return [1, 2];
    // return [1]; // TypeError, expected float32 in returned array for second element


Interfaces can be used to type objects, arrays, and functions. This allows users to remove redundant type information that is used in multiple places such as in destructuring calls. In addition, interfaces can be used to define contracts for classes and their required properties.

Object Interfaces

interface IExample
    ?c:any; // Optional property. A default value can be assigned like:
    // ?c:any = [];
function F():IExample
    return { a: 'a', b: x => x };

The syntax for optional properties could have the question mark before or after the property as this does not conflict with the type. It's unclear which is more consistent or ideal.

Array Interfaces

interface IExample
    ?string // Optional item. A default value can be assigned like:
    // ?string = 10
function F():IExample
    return ['a', 1];

An optional nullable item would look like ?uint32? which looks odd, but is why the question mark is before the type.

Function Interfaces

With function overloading an interface can place multiple function constraints.

interface IExample
    (:string, :uint32):void;
    ?(:string, :string):void; // Optional overload. A default value can be assigned like:
    // ?(:string, :string):void = (x, y) => x + y;
function F(a:IExample)
    a('a', 1);
    // a('a'); // TypeError: No matching signature for (string).

Signature equality checks ignore renaming:

interface IExample
function F(a:IExample)
    a({a:1}); // 1
F(({(a:uint32):b}) => b); // This works since the signature check ignores any renaming

An example of taking a typed object:

interface IExample

Argument names in function interfaces are optional, but if they are included then the implementing function must match the signature exactly. This is done in case named parameters are added later. Note that if an interface is used then the name can be changed in the passed in function. For example:

interface IExample
    (:string = 5, named:uint32):void;
function F(a:IExample)
    a(named: 10); // 10
F((a, b) => b);

The interface in this example defines the mapping for "named" to the second parameter.

It might not be obvious at first glance, but there are two separate syntaxes for defining function type constraints. One without an interface, for single non-overloaded function signatures, and with interface, for either constraining the parameter names or to define overloaded function type constraints.

function (a:(uint32, uint32):void) {} // Using non-overloaded function signature
function (a:{ (:uint32, :uint32):void; }) {} // Identical to the above using Interface syntax

Most of the time users will use the first syntax, but the latter can be used if a function uses multiple overloaded signatures:

function (a:{ (:uint32):void; (:string):void; })

The other use case as explained is to apply strict parameter names:

var a:{ (b:uint32):void; };
// a = c => 1; // TypeError: First parameter name must be "b".

Nested Interfaces

interface IA { a:uint32; }
interface IB { (:IA):void }
// interface IB { (:{ a:uint32; }):void }

Extending Interfaces


Implementing Interfaces

TODO: Class examples

Typed Assignment

A variable by default is typed any meaning it's dynamic and its type changes depending on the last assigned value. As an example one can write:

let a = new MyType();
a = 5; // a is type any and is 5

If one wants to constrain the variable type they can write:

let a:MyType = new MyType();
// a = 5; // Equivelant to using implicit casting: a = MyType(5);

This redundancy in declaring types for the variable can be removed with a typed assignment:

let a := new MyType(); // a is type MyType
// a = 5; // Equivelant to using implicit casting: a = MyType(5);

This new form of assignment is useful with both var and let declarations. With const it has no uses:

const a = new MyType(); // a is type MyType
const b:MyType = new MyType(); // Redundant, b is type MyType even without explicitly specifying the type
const c := new MyType(); // Redundant, c is type MyType even without explicitly specifying the type
const d:MyType = 1; // Calls a matching constructor
const e:uint8 = 1; // Without the type this would have been typed Number
class A {}
class B extends A {}
const f:A = new B(); // This might not even be useful to allow

This assignment also works with destructuring:

let { a, b } := { a:uint8: 1, b:uint32: 2 }; // a is type uint8 and b is type uint32

Function Overloading

function F(x:int32[]) { return "int32"; }
function F(s:string[]) { return "string"; }
F(["test"]); // "string"

Up for debate is if accessing the separate functions is required. Functions are objects so using a key syntax with a string isn't ideal. Something like F["(int32[])"] wouldn't be viable. It's possible Reflect could have something added to it to allow access.

Signatures must match for a typed function:

function F(a:uint8, b:string) {}
// F(1); // TypeError: Function F has no matching signature

Adding a normal untyped function acts like a catch all for any arguments:

function F() {} // untyped function
function F(a:uint8) {}
F(1, 2); // Calls the untyped function

If the intention is to created a typed function with no arguments then setting the return value is sufficient:

function F():void {}
// F(1); // TypeError: Function F has no matching signature

Duplicate signatures are not allowed:

function F(a:uint8) {}
// function F(a:uint8, b:string = `b`) {} // TypeError: A function declaration with that signature already exists

Async Functions and overloading

async does not create a unique signature. Consider the following:

async function F() {}
// function F() { return new Promise(resolve => {}); } // TypeError: "A function with that signature already exists"
await F();

While async functions and synchronous functions can overload the same name, they must have unique signatures.

Generator Overloading

var o = {};
o[Symbol.iterator] =
    function* ():int32
        yield* [1, 2, 3];
    function* ():[int32, int32]
        yield* [[0, 1], [1, 2], [2, 3]];

[...o:int32]; // [1, 2, 3] Explicit selection of the generator return signature
for (const a:int32 of o) {} // Type is optional in this case
[...o:[int32, int32]]; // [[0, 1], [1, 2], [2, 3]]
for (const [a:int32, b:int32] of o) {} // Type is optional in this case

Object Typing


let o = { (a:uint8): 1 };

This syntax is used because like destructuring the grammar cannot differentiate the multiple cases where types are included or excluded resulting in an ambiguous grammar. The parenthesis cleanly solves this.

let a = [];
let o = { a };
o = { a:[] };
o = { (a:uint8[]) }; // cast a to uint8[]
o = { (a:uint8[]):[] }; // new object with property a set to an empty array of type uint8[]

This syntax works with any arrays:

let o = { a:[] }; // Normal array syntax works as expected
let o = { (a:[]): [] }; // With typing this is identical to the above

Object.defineProperty and Object.defineProperties have a type property in the descriptor that accepts a type or string representing a type:

Object.defineProperty(o, 'a', { type: uint8 }); // using the type
Object.defineProperty(o, 'b', { type: 'uint8' }); // using a string representing the type
        type: uint8,
        value: 0,
        writable: true
        type: string,
        value: 'a',
        writable: true

The type information is also available in the property descriptor accessed with Object.getOwnPropertyDescriptor or Object.getOwnPropertyDescriptors:

const o = { a:uint8 };
const descriptor = Object.getOwnPropertyDescriptor(o, 'a');
descriptor.type; // uint8

const descriptors = Object.getOwnPropertyDescriptors(o);
descriptors.a.type; // uint8

Note that the type descriptor is the actual type and not a string.

Constructor Overloading

class MyType
    x:float32; // Able to define members outside of the constructor
        this.x = x;
        this.x = float32(y) * 2;

Implicit casting using the constructors:

let t:MyType = 1; // float32 constructor call
let t:MyType = uint32(1); // uint32 constructor called

Constructing arrays all of the same type:

let t = new MyType[5](1);

parseFloat and parseInt For Each New Type

For integers (including bigint) the parse function would have the signature parse(string, radix = 10).

let a:uint8 = uint8.parse('1', 10);
let b:uint8 = uint8.parse('1'); // Same as the above with a default 10 for radix
let c:uint8 = '1'; // Calls parse automatically making it identical to the above

For floats, decimals, and rational the signature is just parse(string).

let a:float32 = float32.parse('1.2');

TODO: Define the expected inputs allowed. (See: Also should a failure throw or return NaN if the type supports it. I'm leaning toward throwing in all cases where erroneous values are parsed. It's usually not in the program's design that NaN is an expected value and parsing to NaN just created hidden bugs.

Implicit SIMD Constructors

Going from a scalar to a vector:

let a:float32x4 = 1; // Equivalent to let a = float32x4(1, 1, 1, 1);

Implicit Array Cast

let a:MyType[] = [1, 2, 3, uint32(1)];

Initializer List for Array of Class Instances

Implicit array casting already exists for single variables as defined above. It's possible one might want to compactly create instances. The following syntax is proposed:

let a = new MyType[] = [(10, 20), (30, 40), 10];

This would be equivalent to:

let a = new MyType[] = [new MyType(10, 20), new MyType(30, 40), 10];

Due to the very specialized syntax it can't be introduced later. In ECMAScript the parentheses have defined meaning such that [(10, 20), 30] is [20, 30] when evaluated. This special syntax takes into account that an array is being created requiring more grammar rules to specialize this case.

Initializer lists work well with SIMD to create compact arrays of vectors:

let a = new float32x4[] =
    (1, 2, 3, 4), (1, 2, 3, 4), (1, 2, 3, 4),
    (1, 2, 3, 4), (1, 2, 3, 4), (1, 2, 3, 4),
    (1, 2, 3, 4), (1, 2, 3, 4), (1, 2, 3, 4)

The syntax also has to work with typed functions intuitively:

function F(a:float32x4)

F([(1, 2, 3, 4)]);


Types would function exactly like you'd expect with decorators, but with the addition that they can be overloaded.

function AlwaysReturnValue(value:uint32)
    return function (target, name, descriptor)
        descriptor.get = () => value;
        return descriptor;
function AlwaysReturnValue(value:float32) { /* ... */ }

Classes and Operator Overloading

The following symbols can be used to define operator overloading.


In addition, a compact syntax is proposed with signatures. These can be overloaded to work with various types. Note that the unary operators have no parameters which differentiates them from the binary operators.

class A
  operator +=(rhs) { }
  operator -=(rhs) { }
  operator *=(rhs) { }
  operator /=(rhs) { }
  operator %=(rhs) { }
  operator **=(rhs) { }
  operator <<=(rhs) { }
  operator >>=(rhs) { }
  operator >>>=(rhs) { }
  operator &=(rhs) { }
  operator ^=(rhs) { }
  operator |=(rhs) { }
  operator +(rhs) { }
  operator -(rhs) { }
  operator *(rhs) { }
  operator /(rhs) { }
  operator %(rhs) { }
  operator **(rhs) { }
  operator <<(rhs) { }
  operator >>(rhs) { }
  operator >>>(rhs) { }
  operator &(rhs) { }
  operator |(rhs) { }
  operator ^(rhs) { }
  operator ~() { }
  operator <(rhs) { }
  operator <=(rhs) { }
  operator >(rhs) { }
  operator >=(rhs) { }
  operator &&(rhs) { }
  operator ||(rhs) { }
  operator !() { }
  operator ++() { }
  operator --() { }
  operator -() { }
  operator +() { }


class Vector2d
    constructor(x:float32 = 0, y:float32 = 0)
        this.x = x;
        this.y = y;
        return Math.sqrt(x * x + y * y); // uses Math.sqrt(v:float32):float32 due to input and return type
    get X():float64 // return implicit cast
        return this.x;
    set X(x:float64)
        this.x = x / 2;
    operator +(v:Vector2d) // Same as [Symbol.addition](v:Vector2d)
        return new vector2d(this.x + v.x, this.y + v.y);
    operator ==(v:Vector2d)
        // equality check between this and v
var a = { b: 0 };
a[Symbol.additionAssignment] = function(value)
  this.b += value;
a += 5; // a.b is 5

Class Extension

Example defined in say MyClass.js defining extensions to Vector2d defined above:

class Vector2d
    operator ==(v:MyClass)
        // equality check between this and MyClass
    operator +(v:MyClass)
        return v + this; // defined in terms of the MyClass operator

Note that no members may be defined in an extension class. The new methods are simply appended to the existing class definition.

SIMD Operators

All SIMD types would have operator overloading added when used with the same type.

let a = uint32x4(1, 2, 3, 4) + uint32x4(5, 6, 7, 8); // uint32x4
let b = uint32x4(1, 2, 3, 4) < uint32x4(5, 6, 7, 8); // boolean32x4

It's also possible to overload class operators to work with them, but the optimizations would be implementation specific if they result in SIMD instructions.

enum Type

Enumerations with enum that support any type including functions.

enum Count { Zero, One, Two }; // Starts at 0
let c:Count = Count.Zero;

enum Count { One = 1, Two, Three }; // Two is 2 since these are sequential
let c:Count = Count.One;

enum Count:float32 { Zero, One, Two };

enum Counter:(float32):float32 { Zero = x => 0, One = x => x + 1, Two = x => x + 2 }

Custom sequential functions for numerical and string types (these aren't closures):

enum Count:float32 { Zero = (index, name) => index * 100, One, Two }; // 0, 100, 200
enum Count:string { Zero = (index, name) => name, One, Two = (index, name) => name.toLowerCase(), Three }; // "Zero", "One", "two", "three"
enum Flags:uint32 { None = 0, Flag1 = (index, name) => 1 << (index - 1), Flag2, Flag3 } // 0, 1, 2, 4

Index operator:

enum Count { Zero, One, Two };
Count[0]; // Count.Zero
Count['Zero']; // Count.Zero

Get enum value as string:

Count.toString(Count.Zero); // 'Zero'

It seems like there needs to be an expression form also. Something akin to Function or GeneratorFunction which allows the construction of features with strings. It's not clear to me if this is required or beneficial, but it could be. I guess the syntax would look like:

new enum('a', 0, 'b', 1);
new enum(':uint8', 'a', 0, 'b', 1);
new enum(':string', 'None', 'none', 'Flag1', '(index, name) => name', 'Flag2', 'Flag3'); // This doesn't make much sense though since the value pairing is broken. Need a different syntax

Similar to Array there would be a number of reserved functions:

enum.prototype.keys() // Array Iterator with the string keys
enum.prototype.values() // Array Iterator with the values
enum.prototype.entries() // Array Iterator with [key, value]
enum.prototype.forEach((key, value, enumeration) => {})
enum.prototype.filter((key, value, enumeration) => {}) // returns an Array, value, enumeration) => {}) // returns an Array

Iteration would work like this:

enum Count { Zero, One, Two };

for (let [key, value] of Count)
    // key = 'Zero', value = 0

Enum values can reference previous values:

enum E { A = 0, B = A + 5 };

Rest Parameters

function F(a:string, ...args:uint32) {}
F('a', 0, 1, 2, 3);

Rest parameters are valid for signatures:

let a:(...:uint8);

Multiple rest parameters can be used:

function F(a:string, ...args:uint32, ...args2:string, callback:()) {}
F('a', 0, 1, 2, 'a', 'b', () => {});

Dynamic types have less precedence than typed parameters:

function F(...args1, callback:(), ...args2, callback:()) {}
F('a', 1, 1.0, () => {}, 'b', 2, 2.0, () => {});

Try Catch

Catch clauses can be typed allowing for minimal conditional catch clauses.

    // Statement that throws
catch (e:TypeError)
    // Statements to handle TypeError exceptions
catch (e:RangeError)
    // Statements to handle RangeError exceptions
catch (e:EvalError)
    // Statements to handle EvalError exceptions
catch (e)
    // Statements to handle any unspecified exceptions

Placement New

Arbitrary arrays can be allocated into using the placement new syntax. This works with both a single instance and array of instances.

let a = new(buffer, byteOffset) MyType(20);
let a = new(buffer, byteOffset, byteStride) MyType[10](20);

Control Structures

if else

A table should be included here with every type and which values evaluate to executing. At first glance it might just be 0 and NaN do not execute and all other values do. SIMD types probably would not implicitly cast to boolean and attempting to would produce a TypeError indicating no implicit cast is available.


The variable when typed in a switch statement must be integral or a string type. Specifically int8/16/32/64, uint8/16/32/64, number, and string. Most languages do not allow floating point case statements unless they also support ranges. (This could be considered later without causing backwards compatability issues).

Enumerations can be used dependent on if their type is integral or string.

let a:uint32 = 10;
switch (a)
    case 10:
    case `baz`: // TypeError unexpected string literal, expected uint32 literal
let a:float32 = 1.23;
//switch (a) // TypeError float32 cannot be used in the switch variable

Member memory alignment and offset

Two new keys would be added to the Object.defineProperty called align and offset. For consistency between codebases two reserved decorators would be created called @align and @offset that would set the underlying keys with byte values. Align defines the memory address to be a multiple of a given number. On some software architectures specialized move operations and cache boundaries can use these for small advantages. Offset is always defined as the number of bytes from the start of the instance allocation in memory. It's possible to create a union by defining overlapping offsets.

Along with the member decorators, two class reserved properties would be created, align and size. These would control the allocated memory alignment of the instances and the allocated size of the instances.

@align(16) // A.align = 16; Defines the class memory alignment to be 16 byte aligned
@size(32) // A.size = 32; Defines the class as 32 bytes. Pads with zeros when allocating
class A
    x:float32; // Aligned to 16 bytes because of the class alignment and offset by 2 bytes because of the property alignment
    y:float32x4; // 2 (from the offset above) + 4 (for x) is 6 bytes and we said it has to be aligned to 4 bytes so 8 bytes offset from the start of the allocation. Instead of @align(4) we could have put @offset(8)

These language features only apply if all the properties in a class are typed along with the complete prototype chain. Adding properties later with Object.defineProperty is allowed, but new properties are appended to the end. Modifying properties in a super class would not be allowed. It's likely that one would need to remove and readd all the properties if the goal is to change the structure. Or offset could be modified with Object.defineProperty to rearrange the properties.

WIP: The behavior of modifying plain old data classes that are already used in arrays needs to be well-defined.

An example of overlapping properties using offset creating a union where both properties map to the same memory:

class A // 16 bytes

Global Objects

The following global objects could be used as types:

DataView, Date, Error, EvalError, InternalError, Map, Promise, Proxy, RangeError, ReferenceError, RegExp, Set, SyntaxError, TypeError, URIError, WeakMap, WeakSet

Undecided Topics

Import Types

This has been brought up before, but possible solutions due to compatability issues would be to introduce special imports. Brenden once suggested something like:

import {int8, int16, int32, int64} from "@valueobjects";
//import "@valueobjects";

Overview of Future Considerations and Concerns


This section is to show that a generic syntax can be seamlessly added with no syntax issues. A generic function example:

function A<T>(a:T):T
    let b:T;

A generic class example:

class Vector2d<T>
    constructor(x:T = 0, y:T = 0) // T could be inferred, but that might be asking too much. In any case T must have a constructor supporting a parameter 0 if this is a class.
        this.x = x;
        this.y = y;

Generic constraints aren't defined here but would need to be. TypeScript has their extends type syntax. Being able to constrain T to an interface seems like an obvious requirement. Also being able to constrain to a list of specific types or specifically to numeric, floating point, or integer types. Another consideration is being able to support a default type. Also generic specialization for specific types that require custom definitions. There's probably more to consider, but those are the big ideas for generics.

Typedefs or aliases for types are a requirement. Not sure what the best syntax is for proposing these. There's a lot of ways to approach them. TypeScript has a system, but I haven't seen alternatives so it's hard for me to judge if it's the best or most ideal syntax.


Taken from TypeScript an interface allows for structural subtyping, essentially structure matching.


Value Type Classes

I left value type classes out of this discussion since I'm still not sure how they'll be proposed. Doesn't sound like they have a strong proposal still or syntax.

Union Types

Having function overloading removes most use cases for TypeScript's union types and optional parameters. Future proposals can try to justify them since none of their syntax conflicts with anything proposed.

let a:string|uint32 = `hello`;

Numeric Literals

Many languages have numeric literal suffixes to indicate a number is a specific data type. This isn't necessary if explicit casts are used. Since there are so many types, the use of suffixes would not be overly readable. The goal was to keep type names to a minimum number of characters also so that literals are less needed.

Private, Public, and Static types and methods

Unlike other proposals adding types allows for robust type checking that allows for private, public, and static member and method checks. It can be added like this:

Partial Class

partial class MyType

Partial classes are when you define a single class into multiple pieces. When using partial classes the ordering members would be undefined. What this means is you cannot create views of a partial class using the normal array syntax and this would throw an error.

Switch ranges

If case ranges were added and switches were allowed to use non-integral and non-string types then the following syntax could be used in future proposals without conflicting since this proposal would throw a TypeError restricting all cases of its usage keeping the behavior open for later ideas.

let a:float32 = 1 / 5;
switch (a)
    case 0..0.99:


These are incredibly niche. That said I've had at least one person mention them to me in an issue. C itself has many rules like that property order is undefined allowing properties to be rearranged more optimally meaning the memory layout isn't defined. This would all have to be defined probably since some view this as a benefit and others view it as a design problem. Controlling options with decorators might be ideal. Packing and other rules would also need to be clearly defined.

Would allow the following types to define bit lengths.

class Vector2d
    x:uint8(4); // 4 bits
    y:uint8(4); // 4 bits

Exception filters


A very compact syntax can be used later for exception filters:

catch (e:Error => e.message == `a`)


catch (e:Error => console.log(e.message))

This accomplishes exception filters without requiring a keyword like "when". That said it would probably not be a true lambda and instead be limited to only expressions.

Named Arguments

Named arguments comes up once in a while as a compact way to skip default parameters.

function F(a:uint8, b:string = 0, ...args:string) {}
F(8, args:`a`, `b`);

function G(option1:string, option2:string) {}
// G(option2: `a`); Error no signature for G matches (option2:string)

The above syntax is probably what would be used and it has no obvious conflicts with types.


Packet bit writer/reader

Previous discussions

Current Mailing List Thread:

Second Thread:
Original thread:
This one contains a lot of my old thoughts:

Required Changes to the Specification:

6.1 ECMAScript Language Types

Would need to include the types listed above. Probably in a more verbose view than a list.

6.1.3 The Boolean Type

boolean is an alias for Boolean when boolean is imported.

6.1.4 The String Type

string is an alias for String when string is imported.

6.1.5 The Symbol Type

symbol is an alias for Symbol when symbol is imported.

6.1.6 The Number Type

number is an alias for Number when number is imported.

6.1.7 The Object Type

object is an alias for Object when object is imported.

6.1.8 Integral Types Signed

int8, int16, int32, int64

int8.parse(string, radix = 10) Unsigned

uint8, uint16, uint32, uint64

uint8.parse(string, radix = 10)

6.1.9 Big Integer


bigint.parse(string, radix = 10)

6.1.10 Float

float16, float32, float64, float80, float128

TODO: Requirements in the spec? Is referring to the specs for each sufficient?

6.1.11 Decimal

decimal32, decimal64, decimal128

6.1.12 SIMD

boolean8x16, boolean16x8, boolean32x4, boolean64x2, boolean8x32, boolean16x16, boolean32x8, boolean64x4
int8x16, int16x8, int32x4, int64x2, int8x32, int16x16, int32x8, int64x4
uint8x16, uint16x8, uint32x4, uint64x2, uint8x32, uint16x16, uint32x8, uint64x4
float32x4, float64x2, float32x8, float64x4

6.1.13 Rational


6.1.14 Complex


6.1.15 Any


6.1.16 Void


Used solely in function signatures to denote that a return value does not exist.

7.2.12 SameValueNonNumber ( x, y )


The internal comparison abstract operation SameValueNonNumber(x, y), where neither x nor y are Number values, produces true or false. Such a comparison is performed as follows:

Assert: Type(x) is not Number.
Assert: Type(x) is the same as Type(y).
If Type(x) is Undefined, return true.
If Type(x) is Null, return true.
If Type(x) is String, then
    If x and y are exactly the same sequence of code units (same length and same code units at corresponding indices), return true; otherwise, return false.
If Type(x) is Boolean, then
    If x and y are both true or both false, return true; otherwise, return false.
If Type(x) is Symbol, then
    If x and y are both the same Symbol value, return true; otherwise, return false.
If x and y are the same Object value, return true. Otherwise, return false. 

7.2.14 Abstract Equality Comparison

The comparison x == y, where x and y are values, produces true or false. Such a comparison is performed as follows:

  1. If Type(x) is the same as Type(y), then a. Return the result of performing Strict Equality Comparison x === y.
  2. If Type(x) has an implicit cast to Type(y), then a.
  3. If x is null and y is undefined, return true.
  4. If x is undefined and y is null, return true.
  5. If Type(x) is Number and Type(y) is String, return the result of the comparison x == ! ToNumber(y).
  6. If Type(x) is String and Type(y) is Number, return the result of the comparison ! ToNumber(x) == y.
  7. If Type(x) is Boolean, return the result of the comparison ! ToNumber(x) == y.
  8. If Type(y) is Boolean, return the result of the comparison x == ! ToNumber(y).
  9. If Type(x) is either String, Number, or Symbol and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).
  10. If Type(x) is Object and Type(y) is either String, Number, or Symbol, return the result of the comparison ToPrimitive(x) == y.
  11. Return false.

Move enum from to Static Semantics: MV

This needs to be changed to work with more than Number. Ideally this operation is delayed until the type is determined. As an example the following should be intuitively legal without any Number conversion done.

let a:uint64 = 0xffffffffffffffff;

The same could be true for bigint support.

12.5.6 The typeof Operator

The table needs to be updated with all the new types and nullable type explanation.

12.10 Relational Operators

12.11 Equality Operators

Not sure if something needs to be changed in these.

12.5.7 to 12.11

Theese contain the operator definitions. Would probably need to include at least a brief change to explain the behavior of all the types. SIMD ones would require the most explanation.

12.14.5 Destructuring Assignment

Type casting syntax described above would need to be included.

13.3.2 Variable Statement

Would need to cover the optional typing syntax and grammar.

13.15 The try Statement

CatchParameter needs to be modified to allow type constraints.

14 ECMAScript Language: Functions and Classes

Function overloading would need to be included and the expected operations for matching a list of arguments to parameters and types. This will also need to cover cases like ambiguous function overloading.

14.2 Arrow Function Definitions

The grammar rule ArrowFunction needs an optional return type and ArrowParameters needs optional type information per each parameter.

14.3 Method Definitions

Grammar requires typing information as defined above. Specifically MethodDefinition's first rule and the get and set ones.

14.5 Class Definitions

Grammar requires typing information for members and methods. Specifically ClassElement and MethodDefinition. ConstructorMethod is referenced as being a MethodDefinition so it should be fine after the MethodDefinition changes.

19.2 Function Objects

Needs to support type information in the constructor. Examples from the documentation with typed examples:

new Function("a:string", "b:uint8", "c:int32", "return a + b + c;")
new Function("a:string, b:uint8, c:int32", "return a + b + c;") 
new Function("a:string,b:uint8", "c:int32", "return a + b + c;") 

Syntax to define a return type:

new Function("a:string", "b:uint8[]", "c:int32", ":string", "return a + b + c;")

New Sections for Each Type

As described before each type needs a parse function to turn a string into the type. Also for float and decimal .EPSILON needs defined in their section. Also for all the integer (not bigint), float, and decimal types need MIN_VALUE and MAX_VALUE defined in their section.

20.2 The Math Object

All the math operations need to be overloaded to work with the integer, float, and decimal types. Meaning if they take in the type they should return the same type.

25.2.2 Properties of the GeneratorFunction Constructor

Similar to Function the constructor needs to be changed to allow types. For example:

new GeneratorFunction("a:float32", ":float32", "yield a * 2;");

EnumDeclaration :
    enum Identifier ColonTypeopt { EnumElementList }

EnumElementList :
    EnumElementList , EnumElement

EnumElement :
    Identifier = Literal
    Identifier = ArrowFunction

A.2 Expressions

BindingIdentifier[Yield, Await] :
Identifier ColonTypeopt

PropertyDefinition[Yield, Await] :
    IdentifierReference[?Yield, ?Await]
    CoverInitializedName[?Yield, ?Await]
    PropertyName[?Yield, ?Await] ColonTypeopt : AssignmentExpression[+In, ?Yield, ?Await]
    MethodDefinition[?Yield, ?Await]

ColonIntegralType :
    : IntegralType

TypedArrayIndexList :
    ArrowFunction , TypedArrayIndexList

ColonTypedArrayIndexList :
    , TypedArrayIndexList

TypedArrayIndexParameters :
    AssignmentExpressionopt ColonIntegralTypeopt ColonTypedArrayIndexListopt

PlacementNew[Yield, Await] :
    ( AssignmentExpression[?Yield, ?Await] )
    ( AssignmentExpression[?Yield, ?Await] , AssignmentExpression[?Yield, ?Await] )
    ( AssignmentExpression[?Yield, ?Await] , AssignmentExpression[?Yield, ?Await] , AssignmentExpression[?Yield, ?Await] )

MemberExpression[Yield, Await] :
    PrimaryExpression[?Yield, ?Await]
    MemberExpression[?Yield, ?Await] [ Expression[+In, ?Yield, ?Await] ]
    MemberExpression[?Yield, ?Await] . IdentifierName
    MemberExpression[?Yield, ?Await] TemplateLiteral[?Yield, ?Await, +Tagged]
    SuperProperty[?Yield, ?Await]
    new MemberExpression[?Yield, ?Await] Arguments[?Yield, ?Await]
    new PlacementNew[?Yield, ?Await] opt Idenitifier [ TypedArrayIndexParameters ]

A.3 Statements

BindingProperty[Yield, Await] :     SingleNameBinding[?Yield, ?Await]
    PropertyName[?Yield, ?Await] ColonTypeopt : BindingElement[?Yield, ?Await]

SingleNameBinding[Yield, Await] :
    BindingIdentifier[?Yield, ?Await] Initializer[+In, ?Yield, ?Await] opt

BindingRestElement[Yield, Await] :
    ... BindingIdentifier[?Yield, ?Await] ColonTypeopt
    ... BindingPattern[?Yield, ?Await]

CatchParameter[Yield, Await] :
    BindingIdentifier[?Yield, ?Await] ColonTypeopt
    BindingPattern[?Yield, ?Await]

A.4 Functions and Classes

FunctionDeclaration[Yield, Await, Default] :
    function BindingIdentifier[?Yield, ?Await] ( FormalParameters[~Yield, ~Await] ) ColonTypeopt { FunctionBody[~Yield, ~Await] }
    [+Default]function ( FormalParameters[~Yield, ~Await] ) ColonTypeopt { FunctionBody[~Yield, ~Await] }

FunctionExpression :
    function BindingIdentifier[~Yield, ~Await] opt ( FormalParameters[~Yield, ~Await] ) ColonTypeopt { FunctionBody[~Yield, ~Await] }

ClassElement[Yield, Await] :
    MemberDefinition[?Yield, ?Await]
    MethodDefinition[?Yield, ?Await]
    staticMethodDefinition[?Yield, ?Await]

MemberDefinition :
    Identifier ColonTypeopt ;