ECMAScript 2015

• let, const (define block-local vars), function in block scope

• Destructuring: let {x, y} = pt; let [s, v, o] = triple(); (assuming let pt = {x:2, y:-5}, for instance).

• Parameter default values: function f(x, y=1, z=0) {⋯}

• Rest: function g(i, j, ...r) { return r.slice(i, j); } (instead of using arguments like a madman).

• Spread: let a = [0,1,2,3], o = new Something(...a);. Also works in array literals: [1, ...array, 4].

• Shorthand object literals: let one = 1; { one, func_one() {return 1;}, ['key ' + one]: 1 }.

• Fat arrow: (a) => a * a is the same as (function(a) { return a * a; }).bind(this)

• Map, Set: let m = new Map(); m.set(key, value); m.has(key); m.get(key). Also has .clear(), .delete(), .forEach(), .keys().

• Weak map: let map = new WeakMap(). Use it if you have circular references in it. You also have new WeakSet().

• Promise: new Promise((resolve, reject) => {…}).

  • resolve(valueOrPromise) returns the promised value (or a new promise, to chain them) in promise.then(value => {…}).
  • reject(new Error(…)) breaks the promise in promise.then(…).then(…).catch(error => {…}).
  • Quick promise creation: Promise.resolve(value), Promise.reject(error).
  • Iterables: Promise.all(listOfPromises).then(listOfValues => …), Promise.race(listOfPromises).then(valueThatResolvedFirst => …)

• Proxies: let obj = new Proxy(proto, handler). Long story short: ~ operator overloading with object-like elements. (Can also catch all property accesses on an object.)

• Generators: function* gen() { yield 1; yield 2; } Actually, gen() returns ~ an object with a next() function property.

• Iterators: for (var [key, val] of items(x)) { alert(key + ',' + val); }. Iterators can be generators or proxies.

• Class syntax, with extends, super, and static:

  class Point extends Base {
    constructor(x,y) {
      super();
      this[px] = x, this[py] = y;
      this.r = function() { return Math.sqrt(x*x + y*y); }
    }
    get x() { return this[px]; }
    get y() { return this[py]; }
    proto_r() { return Math.sqrt(this[px] * this[px] +
        this[py] * this[py]); }
    equals(p) { return this[px] === p[px] &&
        this[py] === p[py]; }
  }
  

• Symbols generate unique inaccessible keys, useful in maps and classes (private members).

  let a = Map();
  {
    let k = Symbol();
    a.set(k, 'value');
    // Here, we can get and reset 'value' as a.get(k).
  }
  // Here, a.get(k) is invalid, a.size is 1, but the key cannot be seen.
  

• Modules:

  module math {
    export function sum(x, y) {
      return x + y;
    }
    export var pi = 3.141593;
  }
  
  import {sum, pi} from math;
  alert(sum(pi,pi));
  

• Template strings: multiline, substitution-ready strings with extensibility. `You are ${age} years old.`.

  let table = 'email', column = 'sender'
  sql`SELECT count(*) FROM ${table}
  WHERE ${table}.${column} LIKE '%\n%';`
  

  // Implementation: function sql(portions, ...inlineParams) { portions[0] === 'SELECT count(*) FROM ' portions[1] === '\nWHERE ' portions[3] === " LIKE '%\n%';" // ← An actual newline! portions.raw[3] === " LIKE '%\n%';" // ← An escaped newline. inlineParams[0] === inlineParams[1] === 'email' inlineParams[2] === 'sender' }

• Typed Array

Specification: http://people.mozilla.org/~jorendorff/es6-draft.html

Proposals: http://wiki.ecmascript.org/doku.php?id=harmony:proposals

Thanks Brendan Eich, Allen Wirfs-Brock, Douglas Crockford, Mark Miller, Tom Van Cutsem, Alex Russell, and so many others.