变量提升的考试题

看了几道变量提升的小题想分享给大家，都很简单，我这里就不解释原因了。

• 我们先说一下预编译的过程

  1. 创建AO对象
  2. 找形参和变量声明，放入AO中，值为undefined
  3. 形参实参相统一
  4. 找函数声明，给AO中相应的的对象赋函数体
• 下面我们来看几个题：

1.

console.log(bar());//11 打印的是函数的返回值
function bar(){
     foo = 100;
     function foo(){}
     var foo = 11;
     return foo;
}
1. AO {

}
2. AO {
    foo: undefined //变量声明
}
3. AO {
    foo: undefined //无形参
}
4. AO {
    foo: function foo(){}
}

2.

function bar(){
     return foo;
     foo = 100;
     function foo(){}
     var foo = 11;
}
console.log(bar());//fun
1. AO {

}
2. AO {
    foo: undefined //变量声明
}
3. AO {
    foo: undefined //无形参
}
4. AO {
    foo: function foo(){}
}

3.

function test(){
     console.log(b);//unde
     if(a){
         var b = 100;
     }
     c = 234;
     console.log(c);//234
}
var a;
test();
a = 10;
console.log(c);//234
console.log(global);//unde
global = 100;
console.log(global);//100
function fn(){
 console.log(global);//unde
 global = 200;
 console.log(global);//200
 var global = 300;
 console.log(global) //300
}
fn();
var global;
console.log(test);//fun

4.

 function test(test){
     console.log(test);//fun
     var test = 234;
     console.log(test);//234
     function test(){}
}
test(1);
var test = 123;
console.log(test);//123

5.

function test(a,b){
     console.log(a);//fun
     console.log(b);//unde
     var b = 234;
     console.log(b);//234
     a = 123;
     console.log(a);//123
     function a(){}
     var a;
     b = 28;
     var b = function(){}
     console.log(a);//123
     console.log(b);//fn
}
test(1);

6.

function test(a,b){
     console.log(a);//1
     c = 0;
     var c;
     a = 3;
     b = 2;
     console.log(b);//2
     function b(){}
     function d(){}
     console.log(b);//2
}
test(1);

7.

function fn(a){
     console.log(a);//fun
     var a = 123;
     console.log(a);//123
     function a(){}
     console.log(a);//123
     var b = function(){}
     console.log(b);//fun
     function d(){};
}
fn(1);

8.

console.log(a);//fun
a();//fun
var a = 3;
function a(){
    console.log(a);
}
console.log(a);//3
a = 6;
a();//error a 不是一个函数 a = 6

9.

a(3);
var a = 1;
function a(a) {
    console.log(a); //3
    console.log(b); //fun
    b = 1;
    c = 2;
    console.log(b); //1
    var b = 5;
    console.log(d); //fun
    function b(){};
    var a = 6;
    console.log(c); // 2
    if(false){
        var d = 1;
        c = 5;
    }
    console.log(c); //2
    function d(){};
    console.log(d); //fun
    arguments[0] = 7;
    console.log(a); //7
}
console.log(a); // 1
console.log(c); // 2

10.

function Foo(){
     getName = function(){
        alert(1);
     };
     return this;
}
Foo.getName = function(){alert(2);};
Foo.prototype.getName = function(){alert(3);};
var getName = function(){alert(4);};
function getName(){alert(5);};

Foo.getName();//2
getName();//4
Foo().getName();//1
new Foo.getName();//2
new Foo().getName();//3
new new Foo().getName();//3

其实上面一道题有关于变量提升但是不是重点因为还有原型、继承、运算符优先级等知识点。

• 闲扯

  • parseInt

  console.log(parseInt('a', 16)); //10
  console.log(parseInt(function(){}, 16)); //15 'f'
  console.log(parseInt(parseInt, 16)); //15 'f'
  console.log(parseInt(0.000002)); //0
  console.log(parseInt(0.0000002)); //2 '2e-7'  大于等于7位才会用科学计数法表示
  console.log(parseInt(false, 16)); //250 'fa'
  console.log(parseInt("113", 2)); //3 '11'

  • 立即执行函数的写法

  (function a() {
      console.log(1);
  })();
  
  (function a() {
      console.log(1);
  }());
  
  +function a() {
      console.log(1);
  }();
  
  -function a() {
      console.log(1);
  }();
  
  !function a() {
      console.log(1);
  }();
  
  false || function a() {
      console.log(1);
  }();
  
  true && function a() {
      console.log(1);
  }();
  
  var a = function a() {
      console.log(1);
  }();

上面的代码都相当于是立即执行函数，其实主要是你把它变成一个表达式它就能立即执行。也就是**只有表达式才能被执行符号执行**。

+ 两个数组相加
```
let a = [1, 2],
    b = [5, 3];
console.log(a + b); //1,25,3  相当于调用了toString()
```
+ ==相等比较
我们都知道```+0、-0、false、null、undefined、""、NaN```都是假值。
我们来看几个例子：
```
console.log(a + b); //1,25,3
console.log(false == ""); //true
console.log(false == []); //true
console.log(false == 0); //true
console.log(false == null); //false
console.log(false == undefined); //false
console.log(false == NaN); //false
console.log(false == {}); //false
```
```
console.log("" == []); //true
console.log("" == 0); //true
console.log("" == null); //false
console.log("" == undefined); //false
console.log("" == NaN); //false
console.log("" == {}); //false
```
```
console.log(0 == ""); //true
console.log(0 == []); //true
console.log(0 == null); //false
console.log(0 == undefined); //false
console.log(0 == NaN); //false
console.log(0 == {}); //false
```
还有几个比较难以理解的例子：
```
console.log([] == ![]); //true   因为![] = false， false == []是成立的。
2 == [1] //true
"" == [null] //true
0 == "/n" //true 因为'/n'被转换为0
```
+ [] + {}的问题
```
console.log([] + {}); // "[object Object]"  [] = "", "" + {} 调用{}的toString()
console.log({} + []); // 0 
只不过是转换的前后顺序不同
```