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                            ## 一、let
let用來聲明變量。它的用法類似于`var`，但是所聲明的變量，只在`let`命令所在的代碼塊內有效；
~~~javascript
{
  let a = 10;
  var b = 1;
}
a // ReferenceError: a is not defined.
b // 1
~~~
## 二、const
`const`聲明一個只讀的常量。一旦聲明，常量的值就不能改變；
~~~javascript
const PI = 3.1415;
PI // 3.1415
PI = 3;
// TypeError: Assignment to constant variable.
~~~

                    
                
        
                
            
        
                
    
                
    
                
        
                
    
                

                







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