let, the new var

Real 

locally scoped

variables

Example

    function Foo() {
      //...
      for(var i=0; i<10; i+=1) {
        let tmp = i;
        console.log(tmp);  // 0, 1, 2...
      }
      //...
      console.log(i);  // 10
      console.log(tmp);  // undefined
    }  
    

Bits, Bytes, Blobs

C-like

Structured types

Example

    var Point2D = new StructType({ x: uint32, y: uint32 });
    var Color = new StructType({ r: uint8, g: uint8, b: uint8 });
    let Pixel = new StructType({ point: Point2D, color: Color });
    let Line = new ArrayType(Pixel, 2);
    let line = new Line({
     { 
       point: { x: 10, y: 10},
       color: { r: 255, g: 255, b: 255}
     },
     {
       point: { x: 200, y: 30},
       color: { r: 0, g: 100, b: 255 }
     }
    });
    

Destructuring

for 

Python-Ruby-PHP 

Example

    var { r, g, b } = line.color;
    let [x, y] = circle.center;
    let [a, b] = [b, a];
    

Rest, Spread

default params

Default params

    function king_of_the_north(who) {
        return is_the_new_king(who || 'john-snow.gif');
    }
    

    function king_of_the_north(who='john-snow.gif') {
        return is_the_new_king(who);
    }
    

Rest params

    function all_kings_of_the_north() {
        var kings = Array.prototype.slice.call(arguments);
        kings.forEach(function(king){
            king.i_am_the_king();
        });
    }
    

    function all_kings_of_the_north(...kings) {
        kings.forEach(function(king){
            king.i_am_the_king();
        });
    }
    

Comprehension

Example

    [1,2,3].map(function(i){
        return i*i;
    });
    

        
    [ for (i of [1,2,3]) i*i ]
    

Generators

Run

To

Completion...

...Run

Stop

Run!

Example

    function* fibonacci() {
        let [prev, curr] = [0, 1];
        for (;;) {
            [prev, curr] = [curr, prev + curr];
            yield curr;
        }
    }

    let seq = fibonacci();
    console.log(seq.next().value);
    console.log(seq.next().value);
    console.log(seq.next().value);

    

Proxies

JavaScript 

Object 

Wrapping

Example

    var handler = {
        get: function(myProxy, name){
            return true;
        }
    };
    var king_of_the_north = Proxy.create(handler);
    king_of_the_north.is_the_king; // true
    king_of_the_north.is_the_king = false; // Error
    

Other methods

Maps, WeakMaps

WeakMap

(or Dictionnary)

Example

    var john_snow = {
        is_king: true,
        really: function() {
            return !john_snow.is_king;
        }
    };

    var kings_of_the_north = new WeakMap();
    kings_of_the_north.set(john_snow, john_snow);

    kings_of_the_north.get(john_snow);
    kings_of_the_north.get(john_snow).is_king;
    kings_of_the_north.get(john_snow).really();
    

Map vs. WeakMap

Modules

CommonJS ❤ AMD

Named exports

        //------ kings.js ------
        var kings = {
            john: {
                is_king: true
            }
        }
        function is_king() {
            return kings.john.is_king;
        }

        export { kings, is_king };
    

        
        //------ main.js ------
        import { is_king, kings } from 'kings';
    

Default exports

    //------ kings.js ------
    var K = {
        kings: {
            john: {
                is_king: true
            }
        },
        is_king() {
            return kings.john.is_king;
        }
    }
    export default K;

    //------ main.js ------
    import K from 'kings';