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# Const VS Let VS Var

## Background

* var happened first , let and const occurs in es6
* They are not types, but are keywords, since javaScript is weak and dynamic(check errors after running the program) typed

## Const

* the value of variable cannot be changed
* cannot re-declared
* cannot be undefined

## Let

* Cannot re-declared the variable  
* Example:

```javascript
let test = 2;
let test = 3; // result :failed
```

* block-scoped
* Example:

```javascript
 function(){
   if(true){
     let i = 1;
    }
    console.log(i);// error
  }
```

## Var

* Function-Scoped
* Can be re-declared
* Hoisting
* Example:

```javascript
var test = 2;
var test = 3; // result : test will be 3

console.log(x);
// undefined
var x = 10;
```