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ECMAScript: latest and upcoming features Axel Rauschmayer @rauschma HolyJS Moscow, 11 December 2016 Slides: speakerdeck.com/rauschma

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Dr. Axel Rauschmayer @rauschma Overview • JavaScript: • What’s new? • What does the future bring? • Background: • TC39: who designs JavaScript? • TC39 process: how is JavaScript evolved? You’ll see lots of code! 2

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Evolving JavaScript: TC39 and the TC39 process © by Bryan Wright

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Dr. Axel Rauschmayer @rauschma JavaScript vs. ECMAScript • Meaning: • JavaScript: the language • ECMAScript: the standard for the language • ECMAScript 6 etc.: language versions • Ecma: standards organisation hosting ECMAScript 4

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Dr. Axel Rauschmayer @rauschma TC39 • Ecma Technical Committee 39 (TC39): the committee evolving JavaScript • Members – strictly speaking: companies (all major browser vendors etc.) • Bi-monthly meetings of delegates and invited experts 5

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Dr. Axel Rauschmayer @rauschma Ecma Technical Committee 39 (TC39) github.com/hemanth/tc39-members { "members": { "Ordinary": [ "Adobe", "AMD", "eBay", "Google", "HewlettPackard", "Hitachi", "IBM", "Intel", "KonicaMinolta" ],
 "Associate": [ "Apple", "Canon", "Facebook", "Fujitsu", "JREastMechatronics", "Netflix", "NipponSignal", "NXP", "OMRONSocialSolutions", "Ricoh", "Sony", "Toshiba", "Twitter" ], ... 6

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Dr. Axel Rauschmayer @rauschma Timeline of ECMAScript • ECMAScript 1 (June 1997): first version • ECMAScript 2 (June 1998): keep in sync with ISO standard • ECMAScript 3 (December 1999): many core features – “[…] regular expressions, better string handling, new control statements [do-while, switch], try/catch exception handling, […]” • ECMAScript 4 (abandoned in July 2008) • ECMAScript 5 (December 2009): minor improvements (standard library and strict mode) • ECMAScript 5.1 (June 2011): keep in sync with ISO standard • ECMAScript 6 (June 2015): many new features 7

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Dr. Axel Rauschmayer @rauschma The TC39 process Problems with infrequent, large releases (such as ES6): • Features that are ready sooner have to wait. • Features that are not ready are under pressure to get finished. • Next release would be a long time away. • They may delay the release. 8

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Dr. Axel Rauschmayer @rauschma The TC39 process New TC39 process: • Manage features individually (vs. one monolithic release). • Per feature: proposal that goes through maturity stages, numbered 0 (strawman) – 4 (finished). • Introduce features gradually. • Once a year, there is a new ECMAScript version. • Only features that are ready (=stage 4) are added. 9

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Dr. Axel Rauschmayer @rauschma Stage 0: strawman What is it? • First sketch • Submitted by TC39 member or
 registered TC39 contributor What’s required? • Review at TC39 meeting 10

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Dr. Axel Rauschmayer @rauschma Stage 1: proposal What is it? • Actual proposal of a feature • TC39 is willing to help with designing the feature What’s required? • Identify champion(s), one of them a TC39 member • Spec: prose, examples, API, semantics and algorithms • Implementation: polyfills and demos What’s next? • Major changes are still expected 11

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Dr. Axel Rauschmayer @rauschma Stage 2: draft What is it? • First version of what will be in the spec • Eventual standardisation is likely What’s required? • Formal description of syntax and semantics (gaps are OK) • Two experimental implementations (incl. one transpiler) What’s next? • Only incremental changes are expected 12

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Dr. Axel Rauschmayer @rauschma Stage 3: candidate What is it? • Proposal is mostly finished, now needs feedback from implementations What’s required? • Spec text is complete • Signed off by reviewers and ES spec editor • At least two spec-compliant implementations What’s next? • Changes only in response to critical issues. 13

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Dr. Axel Rauschmayer @rauschma Stage 4: finished What is it? • Proposal ready to be included in the ES specification What’s required? • Test 262 acceptance tests • Two spec-compliant shipping implementations that pass the tests • Significant practical experience with the implementations • ECMAScript spec editor must sign off on the spec text What’s next? • Proposal will be added to spec as soon as possible • When spec is next ratified, the proposal becomes a standard 14

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Dr. Axel Rauschmayer @rauschma Think in proposals & stages, not in ES versions • Before stage 4: proposals may be withdrawn. • Object.observe(): withdrawn at stage 2 • Stage 4: proposal will certainly become a part of ECMAScript. • But: don’t know when exactly. Tip: Ignore features before stage 3. 15

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Dr. Axel Rauschmayer @rauschma Kinds of features 16

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Dr. Axel Rauschmayer @rauschma Kinds of features 17

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Dr. Axel Rauschmayer @rauschma Kinds of features 18

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ECMAScript 2016 Standardised in June 2016 © by Ben Mason

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Dr. Axel Rauschmayer @rauschma New features in ES2016 ES2016 has only two new features: • Array.prototype.includes (Domenic Denicola, Rick Waldron) • Exponentiation Operator (Rick Waldron) 20

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Dr. Axel Rauschmayer @rauschma Array.prototype.includes > ['a', 'b', 'c'].includes('a') true > ['a', 'b', 'c'].includes('d') false > [NaN].includes(NaN) true > [NaN].indexOf(NaN) >= 0 false 21

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Dr. Axel Rauschmayer @rauschma Exponentiation operator // x ** y is same as Math.pow(x, y) const squared = 3 ** 2; // 9 let num = 3; num **= 2; // same: num = num ** 2 console.log(num); // 9 22

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ECMAScript 2017+ © by JD Hancock

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Features accepted for ES2017

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Dr. Axel Rauschmayer @rauschma Async Functions function fetchJsonViaPromises(url) { return fetch(url) // browser API, returns Promise .then(request => request.text()) // returns Promise .then(text => { return JSON.parse(text); }) .catch(error => { console.log(`ERROR: ${error.stack}`); }); } async function fetchJsonAsync(url) { try { const request = await fetch(url); const text = await request.text(); return JSON.parse(text); } catch (error) { console.log(`ERROR: ${error.stack}`); } } 25

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Dr. Axel Rauschmayer @rauschma Async Functions Variants: // Async function declaration async function foo() {} // Async function expression const foo = async function () {}; // Async arrow function const foo = async () => {}; // Async method definition (in classes, too) const obj = { async foo() {} }; 26

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Dr. Axel Rauschmayer @rauschma Fulfilling the Promise of an async function async function asyncFunc() { return 123; } asyncFunc() .then(x => console.log(x)); // 123 27

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Dr. Axel Rauschmayer @rauschma Rejecting the Promise of an async function async function asyncFunc() { throw new Error('Problem!'); } asyncFunc() .catch(err => console.log(err)); // Error: Problem! 28

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Dr. Axel Rauschmayer @rauschma Handling rejected Promises inside async functions async function asyncFunc() { try { await otherAsyncFunc(); } catch (err) { console.error(err); } } // Equivalent to: function asyncFunc() { return otherAsyncFunc() .catch(err => { console.error(err); }); } 29

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Dr. Axel Rauschmayer @rauschma Async functions and Node.js Node.js 7.2.1: $ node --harmony_async_await > async function f() { return 123 } > f().then(x => console.log(x)) 123 30

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Dr. Axel Rauschmayer @rauschma Object.entries() Object.entries() returns an Array of [key,value] pairs: > Object.entries({ one: 1, two: 2 }) [ [ 'one', 1 ], [ 'two', 2 ] ] 31

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Dr. Axel Rauschmayer @rauschma Object.entries() Easier to iterate over properties: const obj = { one: 1, two: 2 }; for (const [k, v] of Object.entries(obj)) { console.log(k, v); } // Output // "one" 1 // "two" 2 32

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Dr. Axel Rauschmayer @rauschma Object.values() Complements Object.keys() and Object.entries(): > Object.values({ one: 1, two: 2 }) [ 1, 2 ] 33

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Dr. Axel Rauschmayer @rauschma String padding > '1'.padStart(3, '0') '001' > 'x'.padStart(3) ' x' > '1'.padEnd(3, '0') '100' > 'x'.padEnd(3) 'x ' 34

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Dr. Axel Rauschmayer @rauschma String padding Use cases: • Displaying tabular data in a monospaced font. • Adding a count to a file name: 'file 001.txt' • Aligning console output: 'Test 001: ✓' • Printing hexadecimal or binary numbers that have a fixed number of digits: '0x00FF' 35

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Dr. Axel Rauschmayer @rauschma Object. getOwnPropertyDescriptors() const obj = { [Symbol('foo')]: 123, get bar() { return 'abc' }, }; console.log(Object.getOwnPropertyDescriptors(obj)); // Output: // { [Symbol('foo')]: // { value: 123, // writable: true, // enumerable: true, // configurable: true }, // bar: // { get: [Function: bar], // set: undefined, // enumerable: true, // configurable: true } } 36

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Dr. Axel Rauschmayer @rauschma Object. getOwnPropertyDescriptors() Object.assign() is limited: can’t copy getters and setters, etc. // Copying properties const target = {}; Object.defineProperties(target, Object.getOwnPropertyDescriptors(source)); // Cloning objects const clone = Object.create( Object.getPrototypeOf(orig), Object.getOwnPropertyDescriptors(orig)); 37

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Dr. Axel Rauschmayer @rauschma Trailing commas in function parameter lists and calls Trailing commas are legal in object and Array literals: const obj = { first: 'Jane', last: 'Doe', }; const arr = [ 'red', 'green', 'blue', ]; console.log(arr.length); // 3 38

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Dr. Axel Rauschmayer @rauschma Trailing commas in function parameter lists and calls Two benefits: • Rearranging items is simpler (no commas to add or remove) • Version control systems can track what really changed. Versus: // From: [ 'foo' ] // To: [ 'foo', 'bar' ] 39

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Dr. Axel Rauschmayer @rauschma Trailing commas in function parameter definitions and calls The proposal: function foo( param1, param2, ) {} foo( 'abc', 'def', ); 40

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Stage 3 proposals (Maybe included in ES2017)

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Dr. Axel Rauschmayer @rauschma import() So far, ES6 modules can only be loaded statically (in a fixed manner, specified at compile time). Load modules dynamically: import('./dir/someModule.js') .then(someModule => someModule.foo()); An operator, but used like a function. 42

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Dr. Axel Rauschmayer @rauschma import() Use cases: • Code splitting: load parts of your program on demand. • Conditional loading of modules:
 if (cond) { import(···).then(···) } • Computed module specifiers:
 import('module'+count).then(···) 43

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Dr. Axel Rauschmayer @rauschma Rest operator for properties (destructuring) const obj = {foo: 1, bar: 2, baz: 3}; const {foo, ...rest} = obj; // Same as: // const foo = 1; // const rest = {bar: 2, baz: 3}; function f({param1, param2, ...rest}) { // rest console.log('All parameters: ', {param1, param2, ...rest}); // spread return param1 + param2; } 44

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Dr. Axel Rauschmayer @rauschma Spread operator for properties (object literals) > const obj = {foo: 1, bar: 2}; > {...obj, baz: 3} { foo: 1, bar: 2, baz: 3 } 45

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Dr. Axel Rauschmayer @rauschma Spread properties use cases // Cloning objects const clone1 = {...obj}; // Merging objects const merged = {...obj1, ...obj2}; // Filling in defaults const data = {...DEFAULTS, ...userData}; // Non-destructively updating property `foo` const obj = {foo: 'a', bar: 'b'}; const obj2 = {...obj, foo: 1}; // {foo: 1, bar: 'b'} 46

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Dr. Axel Rauschmayer @rauschma global Accessing the global object: • Browsers (main thread): window • Browsers (main thread & workers): self • Node.js: global // In browsers console.log(global === window); // true 47

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Dr. Axel Rauschmayer @rauschma History of concurrency in JavaScript • Single main thread + asynchronicity via callbacks • Web Workers • Originally: copy and send strings • Structured cloning: copy and send structured data • Transferables: move and send structured data • Failed experiment: PJS / River Trail • High-level support for data parallelism (map(), filter(), reduce()) 48

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Dr. Axel Rauschmayer @rauschma Shared Array Buffers New – Shared Array Buffers: • A primitive building block for higher-level concurrency abstractions • Share data between workers • Consequence: “atomic” operations for synchronising between threads 49

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Dr. Axel Rauschmayer @rauschma Shared Array Buffers const sh = new SharedArrayBuffer( Uint32Array.BYTES_PER_ELEMENT * 10); const ta = new Uint32Array(sh); worker.postMessage(ta); // share with worker // Writer Atomics.store(ta, 0, 123); // Reader while (Atomics.load(ta, 0) !== 123) ; 50

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Dr. Axel Rauschmayer @rauschma Shared Array Buffers Atomics.load( typedArray, index ) Atomics.store( typedArray, index, value ) Atomics.exchange( typedArray, index, value ) Atomics.compareExchange( typedArray, index, expectedValue, replacementValue ) Atomics.add( typedArray, index, value ) Atomics.sub( typedArray, index, value ) Atomics.and( typedArray, index, value ) Atomics.or( typedArray, index, value ) Atomics.xor( typedArray, index, value ) Atomics.wait( typedArray, index, value, timeout ) Atomics.wake( typedArray, index, count ) Atomics.isLockFree( size ) 51

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SIMD: single instruction, multiple data Aw Bw Cw Ax Bx Cx + = Ay By Cy Az Bz Cz Aw Bw Cw + = Ax Bx Cx + = Ay By Cy + = Az Bz Cz + = SISD SIMD

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Dr. Axel Rauschmayer @rauschma SIMD.JS • SIMD.JS: SIMD support in JavaScript • Example target: Intel’s SSE (Streaming SIMD Extensions) • Speed-up: up to 4 times (sometimes even more) 53

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Dr. Axel Rauschmayer @rauschma SIMD.JS • Operands – vectors of booleans, ints, floats:
 Bool8x16, Float32x4, Uint32x4 • Operations, e.g.: SIMD.Float32x4.abs(v) SIMD.Float32x4.neg(v) SIMD.Float32x4.sqrt(v) SIMD.Float32x4.add(v, w) SIMD.Float32x4.mul(v, w) SIMD.Float32x4.equal(v, w) 54

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Dr. Axel Rauschmayer @rauschma SIMD.JS const a = SIMD.Float32x4(1.0, 2.0, 3.0, 4.0); const b = SIMD.Float32x4(5.0, 6.0, 7.0, 8.0); const c = SIMD.Float32x4.add(a,b); 55

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Dr. Axel Rauschmayer @rauschma SIMD.JS: data types Always 128 bits: • Booleans: SIMD.Bool8x16, SIMD.Bool16x8, SIMD.Bool32x4, SIMD.Bool64x2 • Signed integers: SIMD.Int8x16, SIMD.Int16x8, SIMD.Int32x4 • Unsigned integers: SIMD.Uint8x16, SIMD.Uint16x8, SIMD.Uint32x4 • Floating point numbers: SIMD.Float32x4, SIMD.Float64x2 56

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Dr. Axel Rauschmayer @rauschma Function.prototype. toString revision Improved spec of toString() for functions: • Return source code whenever possible • Previously: optional • Otherwise: standardised placeholder • Previously: must cause SyntaxError (hard to guarantee!) 57

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Dr. Axel Rauschmayer @rauschma Template literal revision Syntax rules after backslash: • \u starts a Unicode escape, which must look like \u{1F4A4} or \u004B • \x starts a hex escape, which must look like \x4B. • \ plus digit starts an octal escape (such as \141). Octal escapes are forbidden in template literals and strict mode string literals. Illegal: latex`\unicode` windowsPath`C:\uuu\xxx\111` 58

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Dr. Axel Rauschmayer @rauschma Template literal revision function tagFunc(tmplObj, substs) { return { Cooked: tmplObj, Raw: tmplObj.raw, }; } tagFunc`\u{4B}`; // { Cooked: [ 'K' ], Raw: [ '\\u{4B}' ] } 59

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Dr. Axel Rauschmayer @rauschma Template literal revision Solution: tagFunc`\uu ${1} \xx` // { Cooked: [ undefined, undefined ], // Raw: [ '\\uu ', ' \\xx' ] } 60

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Dr. Axel Rauschmayer @rauschma Asynchronous iteration // Synchronous iteration: for (const l of readLinesSync(fileName)) { console.log(l); } Problem: readLinesSync() must be synchronous. 61

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Dr. Axel Rauschmayer @rauschma Asynchronous iteration // Asynchronous iteration: for await (const l of readLinesAsync(fileName)) { console.log(l); } Works inside: • Async functions • Async generators (new, part of proposal) 62

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Dr. Axel Rauschmayer @rauschma Asynchronous iteration async function f() { const aI = createAsyncIterable(['a', 'b']); for await (const x of aI) { console.log(x); } } // Output: // a // b 63

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Dr. Axel Rauschmayer @rauschma Asynchronous generators async function* createAsyncIterable(syncIterable) { for (const elem of syncIterable) { yield elem; } } 64

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Dr. Axel Rauschmayer @rauschma Asynchronous generators async function* id(asyncIterable) { for await (const elem of asyncIterable) { yield elem; } } 65

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Dr. Axel Rauschmayer @rauschma Asynchronous iteration interface AsyncIterable { [Symbol.asyncIterator]() : AsyncIterator; } interface AsyncIterator { next() : Promise; } interface IteratorResult { value: any; done: boolean; } 66

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Dr. Axel Rauschmayer @rauschma Asynchronous iteration I’m skeptical: • Relatively complex iteration protocol • New language complexity: async generators (function declaration, function expression, method definition) • Communicating sequential processes (CSP) may be a simpler solution: github.com/ubolonton/js-csp 67

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Dr. Axel Rauschmayer @rauschma Trying out new ES features 68

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Dr. Axel Rauschmayer @rauschma Trying out new ES features 69

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Dr. Axel Rauschmayer @rauschma Summary 70

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Thanks! Twitter: @rauschma Books by Axel (free online): exploringjs.com Blog post: ES2017 These slides: speakerdeck.com/rauschma