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[TOC]
# ES6學習手札

> 至于為什么開始學習ES6，主要是因為小程序，因為小程序目前很好的支持ES6

## 一、常量const
~~~
  const num = 520;
  console.log(PI);
  num = 3; // 報錯: Uncaught TypeError: Assignment to constant variable.
  const num = 3.1; // 報錯: Uncaught SyntaxError: Identifier 'num' has already been declared
~~~
**一旦聲明以后就不能更改了，區別于var，只在聲明的塊級中有效。**

## 二、作用于let
~~~
function test(){
    let x=1;
    let y=2;
    {
        let x=10;
        let y=20;
    }
    console.log(x,y)
  }
	test();//輸出結果，是x=1，y=2
~~~
                    
                
        
                
            
        
                
    
                
    
                
        
                
    
                

                







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