delete

delete expression 

delete object.property
delete object['property']

var Employee = {
  age: 28,
  name: 'abc',
  designation: 'developer'
}

console.log(delete Employee.name   // returns true
console.log(delete Employee.age    // returns true

// When trying to delete a property that does 
// not exist, true is returned 
console.log(delete Employee.salary // returns true

var Employee = {};
Object.defineProperty(Employee, 'name', {configurable: false}

console.log(delete Employee.name  // returns false

var nameOther = 'XYZ';

// We can access this global property using:
Object.getOwnPropertyDescriptor(window, 'nameOther'  

// output: Object {value: "XYZ", 
//                  writable: true, 
//                  enumerable: true,
//                  configurable: false}

// Since "nameOther" is added using with the
// var keyword, it is marked as "non-configurable"

delete nameOther;   // return false

function Employee() { 
  delete salary;
  var salary;
}

Employee(

"use strict";

function Employee() {
  delete salary;  // SyntaxError
  var salary;        
}

// Similarly, any direct access to a function
// with delete will raise a SyntaxError

function DemoFunction() {
  //some code
}

delete DemoFunction; // SyntaxError

// creates the property adminName on the global scope
adminName = 'xyz';            

// creates the property empCount on the global scope
// Since we are using var, this is marked as non-configurable. The same is true of let and const.
var empCount = 43;

EmployeeDetails = {
  name: 'xyz',
  age: 5,
  designation: 'Developer'
};

// adminName is a property of the global scope.
// It can be deleted since it is created without var.
// Therefore, it is configurable.
delete adminName;       // returns true

// On the contrary, empCount is not configurable, 
// since var was used.
delete empCount;       // returns false 

// delete can be used to remove properties from objects  
delete EmployeeDetails.name; // returns true 

// Even when the property does not exists, it returns "true"
delete EmployeeDetails.salary; // returns true 

// delete does not affect built-in static properties
delete Math.PI; // returns false 

// EmployeeDetails is a property of the global scope.
// Since it defined without "var", it is marked configurable
delete EmployeeDetails;   // returns true

function f() {
  var z = 44;

  // delete doesn't affect local variable names
  delete z;     // returns false
}

function Foo() {
  this.bar = 10;
}

Foo.prototype.bar = 42;

var foo = new Foo(

// Returns true, since the own property
// has been deleted on the foo object
delete foo.bar;           

// foo.bar is still available, since it 
// is available in the prototype chain.
console.log(foo.bar

// We delete the property on the prototype
delete Foo.prototype.bar; 

// logs "undefined" since the property
// is no longer inherited
console.log(foo.bar           

var trees = ['redwood', 'bay', 'cedar', 'oak', 'maple'];
delete trees[3];
if (3 in trees) {
    // this does not get executed
}

var trees = ['redwood', 'bay', 'cedar', 'oak', 'maple'];
trees[3] = undefined;
if (3 in trees) {
    // this gets executed
}