js通过var定义全局变量与在window对象上直接定义属性的区别说明

var name = "xiaomi";
console.log(window.name);//"xiaomi"

var num1=123;
window.num2=456;
delete num1;
delete num2;
console.log(num1);  //123
console.log(num2);  //num2 is not defined

var num1=123;
window.num2=456;
Object.getOwnPropertyDescriptor(window, "num1");
//Object {value: 123, writable: true, enumerable: true, configurable: false}
Object.getOwnPropertyDescriptor(window, 'num2');
//Object {value: 456, writable: true, enumerable: true, configurable: true}

console.log(num1); // undefined
var num1=123;
console.log(num2); // ReferenceError: a is not defined
window.num2=456;

function () {
    var num1 = 123;
    window.num2 = 456;
}
console.log(num1); //num1 is not defined
console.log(num2); //456

var a  = 2; //全局变量 
function func()
{
    var a = 1;  //局部变量    
     a = 2;     //全局变量 
}

  var test = 5;//全局变量 
  function a(){ 
    var a = 3;//局部变量 
    alert(a); 
  } 
  function b(){ 
    alert(test); 
  } 
  //a();//调用a方法，那么方法里面的内容才会执行 
  //b();//同上 

    test = 5;//全局变量 
    function a(){ 
      aa = 3;//也是全局变量 
      alert(test); 
    } 
    //a();  //输出5 
    //alert(aa);//这里也可以方法a()方法里面的变量，因为aa是全局变量 

<script> 
  window.test = 50; 
  alert(test);//输出50 
</script>