JS basics

1. async vs defer

<script src="1.js" defer></script>
<script src="2.js" defer></script>

defer - preserve the execution order.

Naming: 

jquery obj ? $name : name 

variables in lowerCamelCase

6 data types (5 primary types):
1. number
2. string
3. boolean
4. null
5. undefined

6. object

binary + : cast to string if one of operands is string
binary -,/,%,* : cast to number 
unary + : cast to number

Operators priority: 

inc/dec => unary operators => binary operators => assignment 
left associative. 
If both operators have the same priority from left to right

assignment returns value;

=== strict comprasion (compare types and then values)
var a = 0 == false // true
var b = 0 === false // false

undefined == null // true

undefined cast to number = > Nan ; +undefined == Nan
null cast to number = > 0 ; +null == 0 

When using > < >= <= on null | undefined - they cast to numbers.
When using == != on null | undefined : no cast. They are unique. 

so null >= 0 => false because 

Standars says that null or undefined == only to null or undefined.

 0,  "", null и undefined, NaN is false when used in if 
 
z = 1 || 0 // 1 ( || returns first true)
z = 1 && 2 // 1 ( && returns last if all true or first false)
z = 0 && 1  // 0  

 
Cast to number:

empty string => to zero
string : trim white spaces (\t, \n) => try to number => if fail => Nan;

Cast to bool:

undefined, null => false
any number (except zero) => true
zero => false
any string (except empty) => true
empty string => false 
object => true  


Functions

if/else, switch, for, while, do..while - do not affect on scope of variables.

hoisting

function without return ? undefined ;

// Function Declaration
function sum(a, b) {
  return a + b;
}

// Function Expression
var sum = function(a, b) {
  return a + b;
}

Function declared as Function declaration will be created before execution of the code.

var age = prompt('Сколько вам лет?');

var sayHi = (age >= 18) ?
  function() { alert('Прошу Вас!');  } :
  function() { alert('До 18 нельзя'); };

sayHi()

^ works with dunction expresion, but dont with declaration

Every single function call has own execution context;


function f () {
..
}

var g = f;
f = null;
g() // run time error

Solution: Named Function Expression

var f = function factorial(n) {
  return n ? n*factorial(n-1) : 1;
};

var g = f; (g now has factorial function not f)
f = null;

alert( g(5) ); ok