《你不知道的JavaScript（上卷）》笔记：提升

1. 2

   a = 2;
   var a;
   console.log(a); // 2

2. undefined

   console.log(2); // undefined
   var a = 2;

3. 函数被提升

   foo();
   function foo() {
       console.log( a ); //undefined
       var a = 2;
   }

   相当于：

   function foo() {
       console.log( a ); //undefined
       var a = 2;
   }
   foo();

4. TypeError

   foo(); // 不是ReferenceError, 而是TypeError!
   var foo = function bar() {
       // ...
   }

5. 函数优先

   foo(); //1
   var foo;
   function foo() {
       console.log(1);
   }
   foo = function() {
       console.log(2);
   }

6. 覆盖

   foo(); //3
   function foo() {
       console.log(1);
   }
   var foo = function() {
       console.log(2);
   }
   function foo() {
       console.log(3);
   }

7. if else

   foo(); // "b"
   var a = true;
   if(a) {
       function foo() {
           console.log("a");
       }
   }else{
       function foo() {
           console.log("b");
       }
   }