# what is javascript? II
## function and let, var, const relationship
1. all of function has return value. if you not write return it will be like this
"Return NormalCompletion(undefined)."

2. parameters vs arguments

var foo = function( a, b, c ) {}; // a, b, and c are the parameters

foo( 1, 2, 3 ); // 1, 2, and 3 are the arguments

3. function rule

function A(){
    function B(){
        console.log("B")
    }
}

A() //nothing happen
B() //b is not defined

function A(){
    function B(){
        console.log("B")
    }
    B()
}
A() //b

it will not error

function A(a,b){
    console.log(a,b)
}

A(1,2,3) //1,2

one of method to avoid

function A(a=1,b=2,c=3){
    console.log(a,b,c)
}

A(1,2,3) //1,2

order of precedence
Variable assignment over function declaration

var double = 22;

function double(num) {
  return (num*2);
}

console.log(typeof double); // Output: number

Function declarations over variable declarations

var double;

function double(num) {
  return (num*2);
}

console.log(typeof double); // Output: function

function hoisting
https://www.digitalocean.com/community/tutorials/understanding-hoisting-in-javascript

3. anonymous function
this function is no necessary function name, kind of this using case about ...
https://www.geeksforgeeks.org/javascript-anonymous-functions/

var a = function(){ console.log("hi")}
a()    //hi

var b = function(x){ console.log(x)}
b("c") // c

4. arrow function
like a anonymous function, but it other way to display
like that ... https://www.w3schools.com/js/js_arrow_function.asp

var foo = (x,y,n)=>{};

5. scope chain
javascipt has 3 kind of type always use in script .. has different scope

d = 4 // this is global type

//limit in function
function A(){
var a;
}

//limit in block
{
let b = 1;
const c = function(){}
}

6. rule of them's declare

var a = 1
var a = 2
var a 

let b = 3
let b = 4 //error cant same name 
let e;    //error cant declare no assign value    

const a = 4
a=5      //error after assign cant change

7. about type of declare and execute

function A(){
console.log(a)
var a = 3 //print undefined
}

function B(){
console.log(b)
let b = 4 //print ReferenceError it mean  "temporal dead zone"
}

function C(){
console.log(c)
const c = function(){ 
console.log("c") 
} //print ReferenceError it mean  "temporal dead zone"
}

8. rule of global
because function A will find b, if cant find, will find outside
it mean will change outside value, like this example

let b = 1
function A(){
    b = 3
}

A()
console.log(b) //3

in this example, becuase run function A will run function B, it will find a
but in function cant find, so B go outside to find a, cause change function A's a value, after B function already find a, it will not continue find a more!

var a = "123"
function A(){
    var a = 4
    function B(){
        a = 3
        console.log(a)
    }
    B()
    console.log(a)
}

A()
console.log(a)

//3
//3
//123

in this case, console.log(a) a is assgin by var a, it same, let a it limit on black
so ans is undefind

if(false){
    var a=1
}
console.log(a)

if(false){
    let a=1
}
console.log(a) //a is var a!!!not a let a!!!

that will explanation

if(false){
    var a=1
}
console.log(a)

if(false){
    let b=1
}
console.log(b) //b is not defind

how about it? because i it var it not limit so console.log can get it, 5 is end of for loop i++

for(var i = 0; i<5; i++){

}

console.log(i) //i is 5

###### tags: `JavaScript`