JS作用域的练习

Js作用域练习demo1

var v = "hello";
(function(){
    alert(v);
    var v = "world";
})();
alert(v);

//以上代码执行如下
var v = "hello";
(function(){
   VO = {
          arguments:{ },
          v:undefined
   }
   alert(v);//undefined
   v = "world";
})();
alert(v);//hello

Js作用域练习demo2

(function(){
    alert(a+b+c);//NaN
    var a = "1";
    var f = function(){};
    var b = "2";
    var c = "3";
})();

//以上代码执行如下
(function(){
    VO = {
       arguments:{},
       a:undefined,
       f:undefined,
       b:undefined,
       c:undefined
    }
    alert(a+b+c);//NaN
    a = "1";
   f = function(){};
   b = "2";
   c = "3";
})();

JS作用域练习demo3

//函数声明是整体提升
//函数表达式 只提升声明的变量  不会带着函数体一起提升
(function(){
    f2(); //2
    f1(); //报错f1 is not a function
    var f1 = function(){
         alert(2)
    };
    function f2(){
         alert(1);
    }
})();

//以上代码执行如下
(function(){
    VO = {
       arguments:{},
       f2:function () {
          alert(1);
       },
       f1:undefined,
    }
    f2(); //1
    f1(); //报错f1 is not a function
    f1 = function(){
       alert(2)
    };
})();

JS作用域练习demo4

var a = 1;
function outer(){
     var b = 2;
     function inner(){
        var c = 4;
        alert(a);//1
     }
     inner();
}
outer();

JS作用域练习demo5

var a = 1;
function check(){
    a = 100;
    alert(a);//100
}
check();
alert(a);//100

JS作用域练习demo6

var v = "hello";
if(true){
    alert(v); //hello
    var v = "world";
}
alert(v); //world

//以上代码执行如下
VO = {
   v:undefined,
}
v = "hello";
if(true){
    alert(v);//hello
    v = "world";
}
alert(v);//world

JS作用域练习demo7

function rain(){
    var x = 1;
    function man(){
        x = 100;
    }
    man();
    alert( x );
}
rain();

//以上代码执行如下
function rain(){
    VO = {
        arguments:{},
        x:undefined,
    }
    x = 1;
    function man(){
       VO = {
          arguments:{}
        }
        x = 100;
    }
    man();
    alert( x );//100
}
rain();

JS作用域练习demo8

var x = 1;
function rain(){
   alert( x );//undefined
   var x = 'man';
   alert( x );//man
}
rain();

//以上代码执行如下
var x = 1;
function rain(){
    VO = {
       arguments:{},
       x:undefined
    }
    alert( x );//undefined
    x = 'man';
    alert( x );//man
}
rain();

JS作用域练习demo9

function rain(){
   x = 100;
}
rain();
alert( x );//100

JS作用域练习demo10

var t='a';
function test2(){
    alert(t);//undefined
    var t = 'b';
    alert(t);//b
}
test2();

JS作用域练习demo11

var t='a';
function test2(){
   alert(t);//a
   t = 'b';
   alert(t);//b
}
alert(t);//a
test2();
alert(t);//b

JS作用域练习demo12

var a="Hello";
function test(){
   alert(a); //Hello
   a="World";
   alert(a); //World
}
test();

JS作用域练习demo13

var a =1;
function test(){
   alert(a);//udefined
   a=4;
   alert(a)//4
   var a=2;
   alert(a);//2
}
test();
alert(a);//1

最后

这些题目在大多数面试中可能都会遇到，知其原理才能看清每个题目的套路在哪里。