js delete

var a = "123";

delete a; //false

console.log(a); //123

b = "123";

delete b; //true;

console.log(b)//Uncaught ReferenceError: b is not defined

var c = Object.getOwnPropertyDescriptor(window,"a");

console.log(c.configurable) //false

var d = Object.getOwnPropertyDescriptor(window,"b");

console.log(d.configurable) //true