JS中的var let const

• var属于function scope 而 let和const属于block scope。

• let和const属于ES6新特性。

• const定义的变量类似Java中的final关键字，其值不可修改。

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function(){
  var a = 123;	
  if(a == 123){
    let b = 456;
  }
  console.log(b); // Uncaught ReferenceError: b is not defined
  const c = 789;
  c = 0; // Uncaught TypeError: Assignment to constant variable.
}
console.log(a); // Uncaught ReferenceError: a is not defined

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