Effect of declared and undeclared variables

What is the major difference between JavaScript declared and undeclared variables, since the delete operator doesn't work on declared variables?

 var y = 43;     // declares a new variable
 x = 42;

 delete x;       // returns true  (x is a property of the global object and can be deleted)
 delete y;       // returns false (delete doesn't affect variable names)

Why does this happen? Variables declared globally are also the properties of the window object, so why can't it be deleted?

Solution 1:

Declared and undeclared global variables

The mechanism for storing and accessing them is the same, but JavaScript treats them differently in some cases based on the value of the configurable attribute (described below). In regular usage, they should behave the same.

Both exist in the global object

Below are some comparisons of declared and undeclared global variables.

var declared = 1;  // Explicit global variable (new variable)
undeclared   = 1;  // Implicit global variable (property of default global object)

window.hasOwnProperty('declared')    // true
window.hasOwnProperty('undeclared')  // true

window.propertyIsEnumerable('declared')    // true
window.propertyIsEnumerable('undeclared')  // true

window.declared     // 1
window.undeclared   // 1

window.declared   = 2;
window.undeclared = 2;

declared     // 2
undeclared   // 2

delete declared     // false
delete undeclared   // true
delete undeclared   // true (same result if delete it again)

delete window.declared     // false
delete window.undeclared   // true (same result if delete it yet again)
delete window.undeclared   // true (still true)

Both declared and undeclared global variables are properties of the window object (the default global object). Neither one is inherited from a different object through the prototype chain. They both exist directly in the window object (since window.hasOwnProperty returns true for both).

The configurable attribute

For declared global variables, the configurable attribute is false. For undeclared global variables, it's true. The value of the configurable attribute can be retrieved using the getOwnPropertyDescriptor method, as shown below.

var declared = 1;
undeclared = 1;

(Object.getOwnPropertyDescriptor(window, 'declared')).configurable     // false
(Object.getOwnPropertyDescriptor(window, 'undeclared')).configurable   // true

If the configurable attribute of a property is true, the attributes of the property can be changed using the defineProperty method, and the property can be deleted using the delete operator. Otherwise, the attributes cannot be changed, and the property cannot be deleted in this manner.

In non-strict mode, the delete operator returns true if the property is configurable, and returns false if it's non-configurable.

Summary

Declared global variable

Is a property of the default global object (window)
The property attributes cannot be changed.
Cannot be deleted using the delete operator

Undeclared global variable

Is a property of the default global object (window)
The property attributes can be changed.
Can be deleted using the delete operator

Solution 2:

The main difference is when you're declaring variables inside a function. If you use var when you're declaring a variable inside a function, then that variable becomes a local variable. However, if you don't use var, then the variable becomes a global variable no matter where you declare it (inside or outside a function).

Solution 3:

When any variable is created via Variable Declaration in JavaScript , these properties are created with "DontDelete" attribute , which basically means that variable you created cannot be Deleted using "delete" expression. All the functions, arguments , function parameters by default are created with this DontDelete attribute. You can think of DontDelete as a flag.

var y = 43;
delete y;         //returns false because it is has a DontDelete attribute

Whereas Undeclared assignment doesn't set any attributes like DontDelete . So when we apply delete operator on this undeclared variable , it returns true.

x = 42;
delete x;        //returns true because it doesn't have a DontDelete attribute

The difference between property assignment and variable declaration — latter one sets DontDelete, whereas former one doesn’t. That's why undeclared assignment creates a deletable property.

Link on how exactly delete operator works