delete in javascript

Key word delete.

1. Delete global object.

x = 42;         // creates the property x on the global object
var y = 43;     // creates the property y on the global object, and marks it as non-configurable
myobj = {
  h: 4,
  k: 5
};

// x is a property of the global object and can be deleted
delete x;       // returns true

// y is not configurable, so it cannot be deleted
delete y;       // returns false 

// delete doesn't affect certain predefined properties
delete Math.PI; // returns false 

// user-defined properties can be deleted
delete myobj.h; // returns true 

// myobj is a property of the global object, not a variable,
// so it can be deleted
delete myobj;   // returns true

function f() {
  var z = 44;

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

2. Function

function Foo(){}
Foo.prototype.bar = 42;
var foo = new Foo();

// returns true, but with no effect,
// since bar is an inherited property
delete foo.bar;           

// logs 42, property still inherited
console.log(foo.bar);

// deletes property on prototype
delete Foo.prototype.bar; 

// logs "undefined", property no longer inherited
console.log(foo.bar);

3. Array

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
}