Default Parameters

Define your parameter defaults in the function signature itself.

No more doing this:

if (!myMissingParameter)
    myMissingParameter = 0;
                        

Default Parameters

Just write your code like this instead:

function myFunc(a, b = 1, c = 0) {
    // ... function body is here
}
                            

Default Parameters

Or some expressions as well.

function myFunc(a = 0, b = a*a, c = b*a) {
    return [a, b, c];
}

// myFunc(5); evaluates to [5, 25, 125]
                            

Rest Parameters

Handle indefinite number of parameters?

function myFunc(a, b, ...r) {
    console.log(Array.isArray(r));
    return r.concat([a, b]);
}
                            

Rest Parameters

> myFunc(1, 2);
true
[1, 2]
                            

Rest Parameters

> myFunc(1, 2, 3, 4, 5);
true
[3, 4, 5, 1, 2]
                            

Spread

The dual of the rest parameters.

> var a = [3, 4, 5];
> [1, 2, ...a];
[1, 2, 3, 4, 5]
                            

Spread

You can even use it in expressions as well!

function f(...r) {
    return r;
}

function g(a) {
    return f(...a);
}
                            

Sets

The name says it. Its a set of arbitrary values.

> var s = new Set([1, true, "three"]);
> s.has(true);
true
> s.has("true");
false
> s.size();
3
                            

Sets

Additional and deletion of elements is also easy.

> s.delete("three");
true
> s.has("three");
false
> s.add("two");
> s.size()
3
                            

Maps

Key/Value pairs the JS way. Your keys aren't converted to strings implicitly.

var data = {val: "age"};
var m = new Map([[name, "sankha"], [data, 20]]);

> m.get(name);
"sankha"
> m.get(data);
20
                            

WeakMap

Cycles between keys and values can tie up space in a Map.
Weakmap comes to rescue!

> var objkey1 = {toString: function(){return "objkey1"}};
> var objkey2 = {toString: function(){return "objkey2"}};
> var wm = new WeakMap();
> wm.set(objkey1, objkey2);
> wm.set(objkey2, objkey1);
> wm.get(objkey1);
({toString:(function (){return "objkey2"})})
                            

Iterators

for-of loops.

> for (var v of [1, 2, 3]) {
  console.log(v);
}
1
2
3
                            

Iterators

Iterables from Sets and Maps.

> for (var e of set) {
  console.log(e);
}

> for (var [k, v] of map) {
  console.log(k, v);
}
                            

Iterators

Iterate over keys, values and items.

> var s = new Set([1, 2, 3]);
> var it = set.keys();
> it = set.values();
> it = set.entries();
> it.next();
[1, 1]
> it.next();
[2, 2]
                            

Generators

Better way to create iterators. Each call to next() executes the function body till the next yield.

function myGenerator() {
  for (var i = 0; i < 3; i++)
    yield i * 2;
}

var g = myGenerator();
g.next(); // returns 0
g.next(); // returns 2
g.next(); // returns 4
g.next(); // throws StopIteration
                            

Proxy

To enable ES programmers to represent virtualized objects (proxies).

> var MethodSink = Proxy({}, {
  has: function(target, name) { return true; },
  get: function(target, name, receiver) {
    if (name in Object.prototype) {
      return Object.prototype[name];
    }
    return function(...args) {
      return receiver.__noSuchMethod__(name, args);
    }
  }
});
                            

Proxy

Lets create a default __noSuchMethod__.

> Object.defineProperty(
  Object.prototype,
  '__noSuchMethod__',
  {
    configurable: true,
    writable: true,
    value: function(name, args) {
      throw new TypeError(name + " is not a function");
    }
  }
);
                            

Proxy

Lets add MethodSink to the end of the prototype chain.

> var obj = {
      foo: 1 ,
      __proto__: MethodSink,
      __noSuchMethod__: function(name, args) {
        return name;
      }
    }
> obj.foo
1
> obj.bar()
"bar"
> obj.toString
function toString() {
    [native code]
}
                            

Proxy

We can now proxy __noSuchMethod__.

> obj.bar
(function (...args) {
      return receiver.__noSuchMethod__(name, args);
    })
> var foo = obj.bar
js> foo()
"bar"
                            

Arrow Functions

Shorter function expressions.

> var square = x => x * x;
> square(5)
25
                            

Arrow Functions

Take a look at this peice of code.

function Person(){
  this.age = 0;

  setInterval(function growUp(){
    this.age++;
  }, 1000);
}

var p = new Person();
                            

Arrow Functions

Arrow functions lexically bind the this value.

function Person(){
  this.age = 0;

  setInterval(() => {
    this.age++;
  }, 1000);
}

var p = new Person();