javascript 显示全局变量与隐式全局变量的区别

var a = 'a'; // 显式声明的全局变量
b = 'b'; // 隐式声明的全局变量
 
console.log(a); // a
console.log(b); // b
console.log(window.a); // a
console.log(window.b); // b

// 显式声明的全局变量不能被删除
delete a; // 返回 false 
 
// 隐式声明的全局变量可以被删除
delete b; // 返回 true 
 
// 删除情况
console.log(typeof a); // string
console.log(typeof b); // undefined

Object.getOwnPropertyDescriptor(window, a); // {value: "a", writable: true, enumerable: true, configurable: false}
Object.getOwnPropertyDescriptor(window, b); // {value: "b", writable: true, enumerable: true, configurable: true}

b = 'b';
var descriptor = Object.getOwnPropertyDescriptor(window, b);
descriptor.configurable = false;
Object.defineProperty(window, b, descriptor);
delete b; // 返回 false 

// 定义三个全局变量
var global_var = 1;
global_novar = 2; // 反面教材
(function () {
  global_fromfunc = 3; // 反面教材
}());
 
// 试图删除
delete global_var; // false
delete global_novar; // true
delete global_fromfunc; // true
 
// 测试该删除
typeof global_var; // "number"
typeof global_novar; // "undefined"
typeof global_fromfunc; // "undefined"

var global = (function () {
  return this;
}());