├── .DS_Store ├── test-runner ├── tests.js ├── tests.html └── assert.js ├── test ├── type-declarations.js ├── basic.js └── sub-typing.js ├── LICENSE ├── TypedObject.js └── explainer.md /.DS_Store: -------------------------------------------------------------------------------- https://raw.githubusercontent.com/tschneidereit/proposal-typed-objects/HEAD/.DS_Store -------------------------------------------------------------------------------- /test-runner/tests.js: -------------------------------------------------------------------------------- 1 | import {} from "../test/basic.js"; 2 | import {} from "../test/type-declarations.js"; 3 | import {} from "../test/sub-typing.js"; 4 | -------------------------------------------------------------------------------- /test-runner/tests.html: -------------------------------------------------------------------------------- 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | -------------------------------------------------------------------------------- /test/type-declarations.js: -------------------------------------------------------------------------------- 1 | /*--- 2 | flags: [module] 3 | ---*/ 4 | 5 | import { StructType } from "./../TypedObject.js"; 6 | 7 | const LinkedList = StructType.declare("LinkedList"); 8 | LinkedList.define([{ name: "next", type: LinkedList }], []); 9 | 10 | let list = new LinkedList(); 11 | list.next = new LinkedList(); 12 | 13 | assert(list instanceof LinkedList); 14 | assert(list.next instanceof LinkedList); 15 | assert.sameValue(list.next.next, null); 16 | -------------------------------------------------------------------------------- /test/basic.js: -------------------------------------------------------------------------------- 1 | /*--- 2 | flags: [module] 3 | ---*/ 4 | 5 | import { StructType, int32 } from "./../TypedObject.js"; 6 | 7 | const Point2D = new StructType( 8 | [{ name: "x", type: int32 }, { name: "y", type: int32 }], 9 | [], 10 | "Point2D" 11 | ); 12 | 13 | const Line = new StructType( 14 | [{ name: "start", type: Point2D }, { name: "end", type: Point2D }], 15 | [ 16 | { 17 | name: "length", 18 | get: function length() { 19 | return Math.sqrt((this.end.x - this.start.x) ** 2 + (this.end.y - this.start.y) ** 2); 20 | } 21 | }, 22 | { 23 | name: "moveStart", 24 | value: function moveStart(dX, dY) { 25 | this.start.x += dX; 26 | this.start.y += dY; 27 | } 28 | } 29 | ], 30 | "Line" 31 | ); 32 | 33 | let start = new Point2D(10); 34 | assert.sameValue(start.x, 10); 35 | assert.sameValue(start[0], 10); 36 | assert.sameValue(start.y, 0); 37 | assert.sameValue(start[1], 0); 38 | 39 | let line = new Line(start); 40 | assert.sameValue(line.start, start); 41 | assert.sameValue(line[0], start); 42 | assert.sameValue(line.end, null); 43 | assert.sameValue(line[1], line.end); 44 | 45 | line.end = Point2D.fromObject({ x: "10", y: 20.5 }); 46 | assert.sameValue(line.length, 20); 47 | line.moveStart(10, 20); 48 | assert.sameValue(line.length, 10); 49 | 50 | let end = new Point2D(); 51 | line.end = end; 52 | assert.sameValue(line.end, end); 53 | assert.sameValue(line[1], end); 54 | line[1] = new Point2D(); 55 | assert.notSameValue(line.end, end); 56 | assert.sameValue(line.end, line[1]); 57 | 58 | // TODO: enable these once typed objects really do have fields as own properties 59 | // assert(line.hasOwnProperty(0)); 60 | // assert(line.hasOwnProperty(1)); 61 | assert(!line.hasOwnProperty(2)); 62 | assert(!line.hasOwnProperty("start")); 63 | assert(!line.hasOwnProperty("end")); 64 | 65 | assert.throws(TypeError, () => { 66 | new StructType([{ name: "member", type: int32}, {name: "member", type: int32}], [], "Type"); 67 | }, "Repeating named typed field"); 68 | 69 | assert.throws(TypeError, () => { 70 | new StructType([{ name: "member", type: int32}], [{name: "member", value: 42}], [], "Type"); 71 | }, "Repeating named typed field as untyped member"); 72 | 73 | assert.throws(TypeError, () => { 74 | new StructType([], [{ name: "member", type: 7}, {name: "member", value: 42}], [], "Type"); 75 | }, "Repeating untyped member"); 76 | -------------------------------------------------------------------------------- /test/sub-typing.js: -------------------------------------------------------------------------------- 1 | /*--- 2 | flags: [module] 3 | ---*/ 4 | 5 | import { StructType, int32, float32 } from "./../TypedObject.js"; 6 | 7 | const Point2D = new StructType( 8 | [{ name: "x", type: int32 }, { name: "y", type: int32 }], 9 | [], 10 | "Point2D" 11 | ); 12 | const Point3D = new StructType(Point2D, [{ name: "z", type: int32 }], [], "Point3D"); 13 | 14 | let start = new Point3D(0, 1, 2); 15 | assert(start instanceof Point3D); 16 | assert(start instanceof Point2D); 17 | 18 | const Line = new StructType( 19 | [{ name: "start", type: Point2D }, { name: "end", type: Point2D }], 20 | [], 21 | "Line" 22 | ); 23 | 24 | let line = new Line(start, new Point3D(4, 5, 6)); 25 | assert.sameValue(line.start, start); 26 | assert(line.end instanceof Point3D); 27 | assert(line.end instanceof Point2D); 28 | 29 | assert.throws(TypeError, () => { 30 | const InvalidOverride = new StructType(Point2D, [{ name: "x", type: float32 }], [], "Invalid"); 31 | }, "Overriding value typed field with wrong value type"); 32 | 33 | assert.throws(TypeError, () => { 34 | const InvalidOverride = new StructType(Point2D, [{ name: "x", type: Point2D }], [], "Invalid"); 35 | }, "Overriding value typed field with struct type"); 36 | 37 | assert.throws(TypeError, () => { 38 | const InvalidOverride = new StructType(Line, [{ name: "start", type: float32 }], [], "Invalid"); 39 | }, "Overriding struct type field with value type"); 40 | 41 | assert.throws(TypeError, () => { 42 | const InvalidOverride = new StructType(Point2D, [], [{ name: "x", value: 42 }], "Invalid"); 43 | }, "Overriding typed field with untyped member"); 44 | 45 | assert.throws(TypeError, () => { 46 | const Base = new StructType([], [{ name: "member", value: 42}], "Base"); 47 | const InvalidOverride = new StructType(Base, [{ name: "member", type: int32 }], [], "Invalid"); 48 | }, "Overriding untyped member with typed field"); 49 | 50 | assert.throws(TypeError, () => { 51 | const InvalidOverride = new StructType(Point2D, [{ name: "x", type: int32, readonly: true }], [], "Invalid"); 52 | }, "Overriding readonly typed field with read-write field"); 53 | 54 | assert.throws(TypeError, () => { 55 | const Base = new StructType([{ name: "member", type: int32, readonly: true}], [], "Base"); 56 | const InvalidOverride = new StructType(Base, [{ name: "member", type: int32, readonly: false}], [], "Invalid"); 57 | }, "Overriding read-write typed field with readonly field"); 58 | -------------------------------------------------------------------------------- /test-runner/assert.js: -------------------------------------------------------------------------------- 1 | // Copyright (C) 2017 Ecma International. All rights reserved. 2 | // This code is governed by the BSD license found in the LICENSE file. 3 | /*--- 4 | description: | 5 | Collection of assertion functions used throughout test262 6 | ---*/ 7 | 8 | function assert(mustBeTrue, message) { 9 | if (mustBeTrue === true) { 10 | return; 11 | } 12 | 13 | if (message === undefined) { 14 | message = 'Expected true but got ' + String(mustBeTrue); 15 | } 16 | $ERROR(message); 17 | } 18 | 19 | assert._isSameValue = function (a, b) { 20 | if (a === b) { 21 | // Handle +/-0 vs. -/+0 22 | return a !== 0 || 1 / a === 1 / b; 23 | } 24 | 25 | // Handle NaN vs. NaN 26 | return a !== a && b !== b; 27 | }; 28 | 29 | assert.sameValue = function (actual, expected, message) { 30 | if (assert._isSameValue(actual, expected)) { 31 | return; 32 | } 33 | 34 | if (message === undefined) { 35 | message = ''; 36 | } else { 37 | message += ' '; 38 | } 39 | 40 | message += 'Expected SameValue(«' + String(actual) + '», «' + String(expected) + '») to be true'; 41 | 42 | $ERROR(message); 43 | }; 44 | 45 | assert.notSameValue = function (actual, unexpected, message) { 46 | if (!assert._isSameValue(actual, unexpected)) { 47 | return; 48 | } 49 | 50 | if (message === undefined) { 51 | message = ''; 52 | } else { 53 | message += ' '; 54 | } 55 | 56 | message += 'Expected SameValue(«' + String(actual) + '», «' + String(unexpected) + '») to be false'; 57 | 58 | $ERROR(message); 59 | }; 60 | 61 | assert.throws = function (expectedErrorConstructor, func, message) { 62 | if (typeof func !== "function") { 63 | $ERROR('assert.throws requires two arguments: the error constructor ' + 64 | 'and a function to run'); 65 | return; 66 | } 67 | if (message === undefined) { 68 | message = ''; 69 | } else { 70 | message += ' '; 71 | } 72 | 73 | try { 74 | func(); 75 | } catch (thrown) { 76 | if (typeof thrown !== 'object' || thrown === null) { 77 | message += 'Thrown value was not an object!'; 78 | $ERROR(message); 79 | } else if (thrown.constructor !== expectedErrorConstructor) { 80 | message += 'Expected a ' + expectedErrorConstructor.name + ' but got a ' + thrown.constructor.name; 81 | $ERROR(message); 82 | } 83 | return; 84 | } 85 | 86 | message += 'Expected a ' + expectedErrorConstructor.name + ' to be thrown but no exception was thrown at all'; 87 | $ERROR(message); 88 | }; 89 | -------------------------------------------------------------------------------- /LICENSE: -------------------------------------------------------------------------------- 1 | 2 | Apache License 3 | Version 2.0, January 2004 4 | http://www.apache.org/licenses/ 5 | 6 | TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 7 | 8 | 1. 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We also recommend that a 186 | file or class name and description of purpose be included on the 187 | same "printed page" as the copyright notice for easier 188 | identification within third-party archives. 189 | 190 | Copyright [yyyy] [name of copyright owner] 191 | 192 | Licensed under the Apache License, Version 2.0 (the "License"); 193 | you may not use this file except in compliance with the License. 194 | You may obtain a copy of the License at 195 | 196 | http://www.apache.org/licenses/LICENSE-2.0 197 | 198 | Unless required by applicable law or agreed to in writing, software 199 | distributed under the License is distributed on an "AS IS" BASIS, 200 | WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 201 | See the License for the specific language governing permissions and 202 | limitations under the License. 203 | -------------------------------------------------------------------------------- /TypedObject.js: -------------------------------------------------------------------------------- 1 | // Copyright 2018 Mozilla Foundation 2 | // 3 | // Licensed under the Apache License, Version 2.0 (the "License"); 4 | // you may not use this file except in compliance with the License. 5 | // You may obtain a copy of the License at 6 | // 7 | // http://www.apache.org/licenses/LICENSE-2.0 8 | // 9 | // Unless required by applicable law or agreed to in writing, software 10 | // distributed under the License is distributed on an "AS IS" BASIS, 11 | // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 12 | // See the License for the specific language governing permissions and 13 | // limitations under the License. 14 | 15 | const { freeze, seal, defineProperty } = Object; 16 | 17 | export const uint8 = makeNumericCoercer(Uint8Array, "uint8"); 18 | export const uint16 = makeNumericCoercer(Uint16Array, "uint16"); 19 | export const uint32 = makeNumericCoercer(Uint32Array, "uint32"); 20 | export const int8 = makeNumericCoercer(Int8Array, "int8"); 21 | export const int16 = makeNumericCoercer(Int16Array, "int16"); 22 | export const int32 = makeNumericCoercer(Int32Array, "int32"); 23 | export const float32 = makeNumericCoercer(Float32Array, "float32"); 24 | export const float64 = makeNumericCoercer(Float64Array, "float64"); 25 | 26 | export function any(input) { 27 | return input; 28 | } 29 | 30 | export function string(input) { 31 | return input + ""; 32 | } 33 | 34 | export function object(input) { 35 | if (!(typeof input === "object" || typeof input === "function")) { 36 | throw new TypeError(`${input} is not an object`); 37 | } 38 | return input; 39 | } 40 | 41 | export class Struct { 42 | /** 43 | * Creates a new instance of the Struct type. 44 | * 45 | * Initial values are given as indexed parameters, with `undefined` denoting defaulted arguments. 46 | * Non-defaulted arguments have to be of the right type, which for Structs means either the right struct type, or a sub-class of it. 47 | * For primitive values, it means being numeric, or having a `valueOf` implementation that enables coercion to a number. 48 | * No integral or range checks are performed; instead, automatic coercion to the right integer or floating point type is performed. 49 | * 50 | * @param {...any} fieldValues Initial values for all of the Struct's fields 51 | */ 52 | constructor(fieldValues, subClassSignal) { 53 | if (subClassSignal !== properSubClassSignal) { 54 | throw new TypeError( 55 | "Sub-classes of Struct can only be created using the `StructType` constructor" 56 | ); 57 | } 58 | 59 | const structure = new.target.structure(); 60 | const values = []; 61 | 62 | for (let i = 0; i < structure.length; i++) { 63 | const { name, type } = structure[i]; 64 | let value = fieldValues[i]; 65 | 66 | if (typeof value === "undefined") { 67 | value = defaults.get(type); 68 | } else if (structTypes.has(type)) { 69 | if (!(value instanceof type)) { 70 | throw new TypeError( 71 | `Wrong type for argument ${i}: expected instance of ${type.name}, but got ${value}` 72 | ); 73 | } 74 | } else { 75 | value = type(value); 76 | } 77 | 78 | defineProperty(values, i, { value, writable: true }); 79 | } 80 | 81 | freeze(this); 82 | valuesMap.set(this, values); 83 | } 84 | 85 | toString() { 86 | let result = `struct ${this.constructor.name} {`; 87 | const structure = this.constructor.structure(); 88 | for (let i = 0; i < structure.length; i++) { 89 | const field = structure[i]; 90 | const type = field.type; 91 | result += ` ${type.name}(${this[i]}),`; 92 | } 93 | result += ` }`; 94 | return result; 95 | } 96 | 97 | static fromObject(sourceObject = {}) { 98 | const structure = this.structure(); 99 | const values = []; 100 | 101 | for (let i = 0; i < structure.length; i++) { 102 | const { name, type } = structure[i]; 103 | 104 | if (name === undefined) { 105 | name = i; 106 | } 107 | 108 | if (!(name in sourceObject)) { 109 | throw new TypeError(`Field ${name} not found on source object ${sourceObject}`); 110 | } 111 | 112 | let value = sourceObject[name]; 113 | 114 | if (value !== undefined) { 115 | let coercer = coercersMap.get(type) || type; 116 | value = coercer(value); 117 | } else { 118 | value = defaults.get(type); 119 | } 120 | 121 | values.push(value); 122 | } 123 | 124 | return new this(...values, properSubClassSignal); 125 | } 126 | 127 | static structure() { 128 | return structTypes.get(this); 129 | } 130 | 131 | static define(fields, untypedMembers) { 132 | if (!structTypes.has(this)) { 133 | throw new TypeError( 134 | "Struct.define can only be applied to declared but not defined struct types" 135 | ); 136 | } 137 | 138 | if (structTypes.get(this)) { 139 | throw new TypeError(`Struct type ${this.name} already defined`); 140 | } 141 | 142 | const structure = []; 143 | 144 | const baseStructure = this.__proto__.structure(); 145 | for (let i = 0; i < baseStructure.length; i++) { 146 | defineProperty(structure, i, { value: baseStructure[i] }); 147 | } 148 | 149 | structTypes.set(this, structure); 150 | addFields(this, structure, fields); 151 | addUntypedMembers(this, structure, untypedMembers); 152 | 153 | freeze(structure); 154 | seal(this); 155 | } 156 | } 157 | 158 | freeze(Struct); 159 | 160 | export const StructType = function( 161 | baseOrFields, 162 | untypedMembersOrFields, 163 | nameOrUntypedMembers, 164 | name = undefined 165 | ) { 166 | let base = Struct; 167 | let fields; 168 | let untypedMembers; 169 | 170 | if (name !== undefined) { 171 | // If `name` is given, the overload must be 172 | // 0: Base class 173 | // 1: Typed fields 174 | // 2: Untyped fields and methods 175 | // 3: Name 176 | base = baseOrFields; 177 | fields = untypedMembersOrFields; 178 | untypedMembers = nameOrUntypedMembers; 179 | } else { 180 | // Otherwise, it must be 181 | // 1: Typed fields 182 | // 2: Untyped fields and methods 183 | // 2: Name 184 | fields = baseOrFields; 185 | untypedMembers = untypedMembersOrFields; 186 | name = nameOrUntypedMembers; 187 | } 188 | 189 | const def = createStructType(base, name); 190 | def.define(fields, untypedMembers); 191 | 192 | return def; 193 | }; 194 | 195 | StructType.declare = function(nameOrBase, name = undefined) { 196 | let base = nameOrBase; 197 | 198 | if (name === undefined) { 199 | name = nameOrBase; 200 | base = Struct; 201 | } 202 | 203 | return createStructType(base, name); 204 | }; 205 | 206 | function createStructType(base, name) { 207 | class def extends base { 208 | // TODO: properly propagate the subclassing signal through the inheritance chain. 209 | constructor(...fieldValues) { 210 | if (base === Struct) { 211 | super(fieldValues, properSubClassSignal); 212 | } else { 213 | super(...fieldValues); 214 | } 215 | } 216 | } 217 | 218 | defineProperty(def, "name", { value: name }); 219 | 220 | defaults.set(def, null); 221 | coercersMap.set(def, makeStructCoercer(def)); 222 | structTypes.set(def, null); 223 | namedStructMembers.set(def, { __proto__: namedStructMembers.get(base) }); 224 | 225 | return def; 226 | } 227 | 228 | const valuesMap = new WeakMap(); 229 | const coercersMap = new WeakMap(); 230 | 231 | const structTypes = new WeakMap(); 232 | structTypes.set(Struct, freeze([])); 233 | 234 | const namedStructMembers = new WeakMap(); 235 | namedStructMembers.set(Struct, freeze({ __proto__: null })); 236 | 237 | const properSubClassSignal = Symbol("TypedObject struct sub-class"); 238 | 239 | function addFields(typeDefinition, structure, fields) { 240 | let i = structure.length; 241 | for (const { name, type, readonly = false } of fields) { 242 | if (!(structTypes.has(type) || defaults.has(type))) { 243 | throw new TypeError(`Invalid type ${type} for field ${name}`); 244 | } 245 | 246 | // TODO: remove name 247 | const field = freeze({ __proto__: null, name, type, readonly }); 248 | defineProperty(structure, structure.length, { value: field }); 249 | addField(typeDefinition, i, field); 250 | 251 | i++; 252 | } 253 | } 254 | 255 | function addUntypedMembers(typeDefinition, structure, untypedMembers) { 256 | for (const descriptor of untypedMembers) { 257 | addNamedMember(typeDefinition, descriptor.name, descriptor); 258 | } 259 | } 260 | 261 | function addField(typeDefinition, index, { name, type, readonly }) { 262 | function getter() { 263 | return valuesMap.get(this)[index]; 264 | } 265 | 266 | let setter; 267 | if (structTypes.has(type)) { 268 | setter = function(value) { 269 | if (!(value instanceof type)) { 270 | throw new TypeError( 271 | `Wrong type for field "${index}": expected instance of ${type.name}, but got ${value}` 272 | ); 273 | } 274 | valuesMap.get(this)[index] = value; 275 | }; 276 | } else { 277 | setter = function(value) { 278 | valuesMap.get(this)[index] = type(value); 279 | }; 280 | } 281 | 282 | if (readonly) { 283 | setter = function(_) { 284 | throw new TypeError("Can't set readonly field '" + index + "'"); 285 | }; 286 | } 287 | 288 | defineProperty(typeDefinition.prototype, index, { __proto__: null, get: getter, set: setter }); 289 | 290 | if (name) { 291 | addNamedMember(typeDefinition, name, { 292 | __proto__: null, 293 | get: getter, 294 | set: setter, 295 | type, 296 | readonly 297 | }); 298 | } 299 | } 300 | 301 | function addNamedMember(typeDefinition, name, descriptor) { 302 | if (typeof name === "string") { 303 | if (isIndexKey(name)) { 304 | throw new TypeError(`Invalid member name ${name}: member names cannot be valid index keys`); 305 | } 306 | } else if (typeof name !== "symbol") { 307 | throw new TypeError(`Invalid member name ${name}: member names must be strings or symbols`); 308 | } 309 | 310 | const namedMembers = namedStructMembers.get(typeDefinition); 311 | validateOverride(namedMembers, name, descriptor); 312 | defineProperty(namedMembers, name, { value: descriptor }); 313 | 314 | defineProperty(typeDefinition.prototype, name, descriptor); 315 | } 316 | 317 | function validateOverride(namedMembers, name, descriptor) { 318 | const existingMember = namedMembers[name]; 319 | if (!existingMember) { 320 | return; 321 | } 322 | 323 | if (existingMember.readonly) { 324 | throw new TypeError(`Can't override readonly field ${name}`); 325 | } else if (descriptor.readonly) { 326 | throw new TypeError(`Can't override read-write field ${name} with readonly field`); 327 | } 328 | 329 | if (existingMember.type) { 330 | if (!descriptor.type) { 331 | throw new TypeError(`Can't override typed field ${name} with untyped field`); 332 | } else if (existingMember.type !== descriptor.type) { 333 | throw new TypeError(`Can't override typed field ${name}: types must match exactly. Expected ${existingMember.type.name}, got ${descriptor.type.name}`); 334 | } 335 | } else if (descriptor.type) { 336 | throw new TypeError(`Can't override untyped field ${name} with typed field`); 337 | } 338 | } 339 | 340 | function isIndexKey(name) { 341 | return name >>> (0 + "") === name; 342 | } 343 | 344 | const defaults = new WeakMap([ 345 | [uint8, 0], 346 | [uint16, 0], 347 | [uint32, 0], 348 | [int8, 0], 349 | [int16, 0], 350 | [int32, 0], 351 | [float32, 0], 352 | [float64, 0], 353 | [any, undefined], 354 | [string, ""], 355 | [object, null] 356 | ]); 357 | 358 | function makeStructCoercer(structType) { 359 | const structCoercer = input => { 360 | if (input instanceof structType) { 361 | return input; 362 | } 363 | 364 | if (input == null) { 365 | return null; 366 | } 367 | 368 | return structType.fromObject(input); 369 | }; 370 | 371 | defineProperty(structCoercer, "name", { value: structType.name }); 372 | return structCoercer; 373 | } 374 | 375 | function makeNumericCoercer(arrayType, name) { 376 | const typedArray = new arrayType(1); 377 | const numericCoercer = input => { 378 | typedArray[0] = input; 379 | return typedArray[0]; 380 | }; 381 | defineProperty(numericCoercer, "name", { value: name }); 382 | return numericCoercer; 383 | } 384 | -------------------------------------------------------------------------------- /explainer.md: -------------------------------------------------------------------------------- 1 | # Explainer for Typed Objects 2 | 3 | ## Outline 4 | 5 | The explainer proceeds as follows: 6 | 7 | - [Explainer for Typed Objects](#explainer-for-typed-objects) 8 | - [Outline](#outline) 9 | - [Overview](#overview) 10 | - [Characteristics of Struct types](#characteristics-of-struct-types) 11 | - [Types](#types) 12 | - [Value Types](#value-types) 13 | - [Struct Types](#struct-types) 14 | - [Typed field definitions](#typed-field-definitions) 15 | - [Struct Type references](#struct-type-references) 16 | - [Struct Type forward declaration](#struct-type-forward-declaration) 17 | - [Instantiation](#instantiation) 18 | - [Instantiating Struct Types](#instantiating-struct-types) 19 | - [Struct type details](#struct-type-details) 20 | - [Memory layout](#memory-layout) 21 | - [Typed fields](#typed-fields) 22 | - [Reading from typed fields](#reading-from-typed-fields) 23 | - [Writing to typed fields](#writing-to-typed-fields) 24 | - [Immutable typed fields](#immutable-typed-fields) 25 | - [Named typed fields](#named-typed-fields) 26 | - [Inheritance](#inheritance) 27 | - [Layout of subtypes](#layout-of-subtypes) 28 | - [Type-checking for Struct Type references](#type-checking-for-struct-type-references) 29 | - [Overriding named fields](#overriding-named-fields) 30 | - [Exotic behavior of Struct type instances](#exotic-behavior-of-struct-type-instances) 31 | - [Prototypes](#prototypes) 32 | - [Read-only `[[Prototype]]`](#read-only-prototype) 33 | - [Exotic behavior of prototypes](#exotic-behavior-of-prototypes) 34 | 35 | ## Overview 36 | 37 | Typed Objects add a new type of objects to JavaScript: objects with pre-defined storage for member fields with equally pre-defined types. This proposal focuses on mutable Struct types, but the concept is applicable to Value types, too. 38 | 39 | ### Characteristics of Struct types 40 | 41 | Struct Types have these characteristics: 42 | - Fixed layout: a Struct's layout is fixed during construction, i.e. it is sealed during its entire lifetime. 43 | - Indexed typed member fields: a Struct has as own members an indexed list of typed fields, as given in its [definition](#struct-type-definitions). 44 | - Possible field types: typed fields can hold values as described in the [section on Value Type definitions](#value-type-definitions), including references to other Struct type instances. 45 | - Named aliases as `prototype` accessors: typed fields can optionally be given a—String or Symbol—name, in which case an accessor is installed on the `prototype`. 46 | - Support for recursive types: Struct types can be forward-declared and filled in later, enabling support for—directly or indirectly—recursive types. 47 | - Inheritance: Struct types can extend other Struct types (but not other JS classes/constructor functions). Additional typed fields are appended to the end of the parent type's indexed list. 48 | - Prototypes are exotic objects which forbid the definition of properties that shadow typed fields, or have numeric index names. 49 | - Immutable prototype chain—`[[SetPrototypeOf]]` throws when applied to any members of the `Struct` prototype chain. 50 | 51 | See individual sections for more details on these characteristics. 52 | 53 | ## Types 54 | 55 | The central part of the Typed Objects specification are [*Value Types*](#value-types) and [*Struct Types*](#struct-types). Instances of Value Types are immutable and don't have internal structure or object identity: they are primitive values similar to numeric values like `10.5` or `42`, or strings like `"foo"`. Instances of Struct Types are objects like `{ answer: 42 }`, except they have a fixed layout in memory, and each field is defined such that it can hold an instance of a Value Type (and only that, instead of arbitrary values). 56 | 57 | ## Value Types 58 | 59 | The following common Value Types are provided by this spec: 60 | 61 | uint8 int8 any 62 | uint16 int16 string 63 | uint32 int32 float32 object 64 | uint64 int64 float64 ref(T) 65 | 66 | They're exposed as *Function* objects with the internal methods `[[Read]]` and `[[Write]]`, which convert between `Value` and internal representations for the type. `[[Write]]` additionally performs a type-specific type check. Calling the Value Type object as a function executes the steps of the internal `[[Write]]` method followed by those of the internal `[[Read]]` method. I.e., it applies the steps for checking and converting from `Value` into the type's internal representation, and then back to `Value`. 67 | 68 | Host environments can provide additional kinds of Value Types, with their own implementations of `[[Read]]` and `[[Write]]`. 69 | 70 | For all Value Types provided by this specification, `[[Read]]` converts from the internal representation to a value and returns it. The behavior of `[[Write]]` for all types provided by this specification is described below. 71 | 72 | For the numeric types and the `string` type, the implementations of `[[Write]]` apply coercions: they ensure that the given value is of the right type by coercing it. For numeric types, the coercion is identical to [that applied when writing to an element in a Typed Array](https://tc39.github.io/ecma262/#sec-numbertorawbytes). For `string`, it's identical to that applied when coercing a value to string by other means, e.g. when appending it to an existing string: `"existing string" + value`. 73 | 74 | For `object`, a type check without coercion is performed: 75 | 76 | ```js 77 | object("foo") // throws 78 | object({}) // returns the object {} 79 | object(null) // returns null 80 | ``` 81 | 82 | For [`ref(T)`](#struct-type-references), the same kind of type check without coercion is performed, but additionally the object is required to match `T`, in a manner [described below](#type-checking-for-struct-type-references). 83 | 84 | For `any`, the coercion is a no-op, because any kind of value is acceptable: 85 | 86 | ```js 87 | any(x) === x 88 | ``` 89 | 90 | ## Struct Types 91 | 92 | Struct Types are defined using the `StructType` constructor, which has two overloads: 93 | 94 | ```js 95 | function StructType(typedFields, name = undefined) 96 | function StructType(baseType, typedFields, name = undefined) 97 | ``` 98 | 99 | The difference between the two overloads is whether the type inherits from the base Struct Type, `Struct`, or a more specialized type. 100 | 101 | Parameters: 102 | - `baseType` - A `constructor function` whose instances are `instanceof Struct`, i.e. `Struct` or a sub-class of `Struct`. 103 | - `typedFields` - An `Iterable` list of [typed field definitions](#typed-field-definitions). 104 | - `name` [optional] - A `string` used as the type's name. 105 | 106 | ### Typed field definitions 107 | 108 | A typed field definition is an `object` definining the characteristics of a `Struct`'s typed field. It has the following members: 109 | - `type` - A [Value Type](#value-types), specifying the field's type. 110 | - `name` [optional] - A `string` or `symbol` used as an optional name for the field. If given, an accessor is created that allows reading and, if the field is writable, writing the field using a name in addition to its index. 111 | - `readonly` [optional] - A `boolean`. If `true`, the field can only be set via the type's constructor and is immutable afterwards. 112 | 113 | ### Struct Type references 114 | 115 | Just as other JS objects, Struct Type instances are passed by reference. These references are instances of [Value Types](#value-types). Defining a Struct Type using the `StructType` constructor actually defines two types: the Struct Type itself, and an accompanying Value Type for references to instances of the Struct Type. Just as other Value Types, it can be used as the type of a Struct Type's fields. This type is exposed as the `ref` property on the Struct Type's constructor: 116 | ```js 117 | const Point = new StructType([{ name: "x", type: float64 }, { name: "y", type: float64 }]); 118 | const Line = new StructType([{ name: "from", type: Point.ref }, { name: "to", type: Point.ref }]); 119 | ``` 120 | 121 | ### Struct Type forward declaration 122 | 123 | To enable recursive types, it's possible to declare a Struct Type without defining it. Declaration is done using `StructType.declare`, which has two overloads: 124 | ```js 125 | StructType.declare(name) 126 | StructType.declare(baseType, name) 127 | ``` 128 | 129 | The first overload declares a Struct Type that extends `Struct`, the second creates a Struct type that extends the given `baseType`. 130 | 131 | The type can then be defined using its `define` method: 132 | ```js 133 | const LinkedList = StructType.declare("LinkedList"); 134 | LinkedList.define([{ name: "next", type: LinkedList.ref }]); 135 | ``` 136 | 137 | `define` takes a single parameter, `typedFields`, and performs the same steps for defining the type's fields as `new StructType` does. 138 | 139 | 140 | ## Instantiation 141 | 142 | ### Instantiating Struct Types 143 | 144 | You can create an instance of a Struct Type using the `new` operator: 145 | 146 | ```js 147 | const Point = new StructType([{ name: "x", type: float64 }, { name: "y", type: float64 }]); 148 | 149 | let from = new Point(); 150 | console.log(from[0], from.x); // logs "0, 0" 151 | ``` 152 | 153 | The resulting object is called a *typed object*: it will have the fields specified in `Line`. 154 | If no parameters are passed, each field will be initialized to its type's default value: 155 | - `0` for numeric types 156 | - `''` for `string` 157 | - `null` for `object` and Struct Type references 158 | - `undefined` for `any` 159 | 160 | Any parameters passed are used as initial values for the type's typed fields: 161 | 162 | ```js 163 | const Point = new StructType([{ name: "x", type: float64 }, { name: "y", type: float64 }]); 164 | const Line = new StructType([{ name: "from", type: Point.ref }, { name: "to", type: Point.ref }]); 165 | 166 | let from = new Point(42, 7); 167 | let line = new Line(from); 168 | console.log(line.from === from, line[0] === line.from, line.from.x); //logs "true, true, 42" 169 | ``` 170 | 171 | ## Struct type details 172 | 173 | Struct types are specified as a new kind of [Built-in Exotic Object](https://tc39.github.io/ecma262/#sec-built-in-exotic-object-internal-methods-and-slots). They override some internal methods, much like [Integer-Indexed Exotic Objects](https://tc39.github.io/ecma262/#sec-integer-indexed-exotic-objects) do. 174 | 175 | ### Memory layout 176 | 177 | Struct type objects have the following internal slots: 178 | - `[[FieldTypes]]` — A list containing references to each of the type's field's types. 179 | 180 | Struct type instance objects have the following internal slots: 181 | - `[[StructType]]` — An immutable reference to the instance's type. 182 | - `[[Values]]` — A list of the abstract values for all the fields produce by the fields' associated Value Type `[[Write]]` method 183 | 184 | For the `Struct` type itself, `[[FieldTypes]]` is set to an empty list. 185 | 186 | ### Typed fields 187 | 188 | Struct types have their property-access related internal methods overridden to perform type checking and coercion, based on the internal slots described above. 189 | 190 | #### Reading from typed fields 191 | 192 | Reading from a typed field returns the result of converting the internal representation of the field's contents to a value representation, using the field's type's `[[Read]]` method. 193 | 194 | In slightly more detail, a `[[Get]]` operation on a Struct Type instance `O` with property key `P`, and receiver `receiver` performs the following steps: 195 | 1. If [Type](https://tc39.github.io/ecma262/#sec-ecmascript-data-types-and-values)(P) is `String`, then 196 | 1. Let `numericIndex` be ! [CanonicalNumericIndexString](https://tc39.github.io/ecma262/#sec-canonicalnumericindexstring)(P) . 197 | 2. If `numericIndex` is not *undefined*, then 198 | 1. Let `fieldType` be `O.[[StructType]].[[FieldTypes]][numericIndex]`. 199 | 2. Let `TV` be `O.[[Values]][numericIndex]`. 200 | 3. Return ! `fieldType.[[Read]](TV)`. 201 | 2. Return ? [OrdinaryGet](https://tc39.github.io/ecma262/#sec-ordinaryget)(O, P, Receiver). 202 | 203 | #### Writing to typed fields 204 | 205 | Writing to a typed field stores the result of converting the given value `V` to an internal representation as the field's contents, using the field's type's `[[Write]]` method. 206 | 207 | In slightly more detail, a `[[Set]]` operation on a Struct Type instance `O` with property key `P`, value `V`, and receiver `receiver` performs the following steps: 208 | 1. If [Type](https://tc39.github.io/ecma262/#sec-ecmascript-data-types-and-values)(P) is `String`, then 209 | 1. Let `numericIndex` be ! [CanonicalNumericIndexString](https://tc39.github.io/ecma262/#sec-canonicalnumericindexstring)(P) . 210 | 2. If `numericIndex` is not *undefined*, then 211 | 1. Let `fieldType` be `O.[[StructType]].[[FieldTypes]][numericIndex]`. 212 | 2. Let `TV` be ? `fieldType.[[Write]](V)`. 213 | 3. Set `O.[[Values]][numericIndex]` to `TV`. 214 | 4. Return `V`. 215 | 2. Return ? [OrdinaryGet](https://tc39.github.io/ecma262/#sec-ordinaryget)(O, P, Receiver). 216 | 217 | #### Immutable typed fields 218 | 219 | Typed fields can be marked as `readonly`, in which case any attempt to write to them after the Struct's construction throws an error. 220 | 221 | #### Named typed fields 222 | 223 | When defining a Struct type, its typed fields can optionally be given names. These names can be strings or symbols, with the only restriction being that a string name can't be a valid numeric index, i.e. [CanonicalNumericIndexString](https://tc39.github.io/ecma262/#sec-canonicalnumericindexstring) must return `undefined`. 224 | 225 | If a typed field is named, an accessor with that name will be added to the Struct type's `prototype`. That accessor simply forwards `[[Get]]` and `[[Set]]` operations to the field's numeric index. 226 | 227 | ### Inheritance 228 | 229 | Struct types can extend other Struct types. If no base type is given, a Struct type will extend `Struct`, which is itself a Struct type without any typed fields. 230 | 231 | #### Layout of subtypes 232 | 233 | When initializing a Struct Type object `O` as a subtype of a Struct Type `P`, the supertype's fields are copied and additional fields are appended by extending `O`'s `[[FieldOffsets]]` and `[[FieldTypes]]` internal slots. Instances of the new Struct Type have their `[[Values]]` buffer extended accordingly. 234 | 235 | ### Type-checking for Struct Type references 236 | 237 | The type check for Struct Type references ensures that the given value is an instance of either the specified type itself, or of a sub-type. The applied test has to be stronger than `instanceof`, since that is easily falsifiable, removing the guarantees around the struct's layout and behavior. 238 | 239 | Instead, to facilitate type-checks, Struct type instances have an internal slot `[[StructType]]`, containing a reference to the associated Struct type constructor. Struct type constructors, in turn, have an internal slot `[[BaseType]]`, containing a reference to the type this type extends. For the `Struct` constructor, the value of this field is `null`. 240 | 241 | Given a value `V`, the `[[Write]]` internal method of Struct Type reference types performs the following steps to verify type compatibility for the given value: 242 | 1. Let `O` be `? ToObject(V)`. 243 | 2. If `O` does not have the internal slot `[[StructType]]`, throw a `TypeError` exception. 244 | 3. Let `type` be `O.[[StructType]]`. 245 | 4. Repeat, while `type` is not `null` 246 | 1. If `type` is equal to *expectedType*, return *true*. 247 | 2. Let `type` be `type.[[BaseType]]`. 248 | 249 | *Note: in practice, implementations don't need to, and aren't expected to, perform this expensive test. Well-established [fast subtying checks that are equivalent to this check exist](https://www.researchgate.net/publication/221552851/download).* 250 | 251 | #### Overriding named fields 252 | 253 | Named fields can be overridden, but some restrictions apply: 254 | - A field can't change from mutable to immutable and vice-versa. 255 | - The type of mutable fields is invariant—the field's type must be exactly the same as the overridden field's type. 256 | - The type of immutable fields is covariant—the field's type can be a subtype of the overridden field's type. 257 | 258 | #### Exotic behavior of Struct type instances 259 | 260 | Struct type instances are exotic objects in two ways: 261 | 262 | 1. They are [immutable prototype exotic objects](https://tc39.github.io/ecma262/#sec-immutable-prototype-exotic-objects). See the section on [read-only prototypes](#read-only-prototype) below. 263 | 2. They override the same set of internal methods as [Integer-Indexed Exotic Objects](https://tc39.github.io/ecma262/#sec-integer-indexed-exotic-objects) to provide special handling of integer-index property keys, as [described above](#typed-fields). 264 | 265 | ### Prototypes 266 | 267 | Prototypes are set up the same way as for classes: the constructor function form one prototype chain, starting with `Struct`, and instances have another prototype chain, starting with `Struct.prototype`. 268 | 269 | E.g., for the Struct type `Point` 270 | - the `[[Prototype]]` is set to `Struct` 271 | - the `[[Prototype]]` of instances of `Point` is set to `Point.prototype` 272 | - The `[[Prototype]]` of `Point.prototype` is set to `Struct.prototype` 273 | 274 | #### Read-only `[[Prototype]]` 275 | 276 | The above-described prototype chains are immutable: `[[SetPrototypeOf]]` throws for all objects on the prototype chains. I.e., all these objects are [immutable prototype exotic objects](https://tc39.github.io/ecma262/#sec-immutable-prototype-exotic-objects). 277 | 278 | #### Exotic behavior of prototypes 279 | 280 | The prototypes of Struct instances are exotic objects in two ways: 281 | 1. As described above, they are [immutable prototype exotic objects](https://tc39.github.io/ecma262/#sec-immutable-prototype-exotic-objects). 282 | 2. Their overrides of `[[DefineOwnProperty]]` and `[[Set]]` prevent shadowing of [named typed fields](#named-typed-fields), and defining any integer index property key-named properties. 283 | 284 | Combined, these rules enable the same degree of strong reasoning about a Struct type instance's fields whether it's accessed by its integer index property key, or by its—optional—string or symbol name. This also enables easier and more stable optimization in engines, leading to higher and more predictable performance. 285 | --------------------------------------------------------------------------------