EcmaScript Sixth Edition

• 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);

• proxies: let obj = Proxy.create(handler, proto). Long story short: ~ operator overloading with object-like elements.

• weak map: let map = new WeakMap. Use it if you have circular references in it.

• 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.

• array and generator comprehension: [a+b for (a in A) for (b in B)] (array comprehension), (x for (x of generateValues()) if (x.color === 'blue')) (generator expression).

• binary data: const Pixel = new StructType({x:uint32, y:uint32, color:Color}) (if Color is itself a StructType), new ArrayType(Pixel, 3).

• class syntax, with extends, prototype, and super:

  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]; }
  }
  

• 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));
  

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

  // The following regexp spans multiple lines.
  re`line1: (words )*
  line2: \w+`
  
  
  // It desugars to:
  re({raw:'line1: (words )*\nline2: \w+',
      cooked:'line1: (words )*\nline2: \w+'})
  

Specification: 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.