delete operator with var and non var variables

Question

I tried delete a variable in javascript using delete operator, but found some issue. Can you guys please explain the below code, and why it is happening

>> var a = 5;

>> delete a
false

>>a
5

>> b=5;

>>delete b
true

>>b
ReferenceError b is not defined

why var a = 5 and b = 5 are different?

Answer 1

When a variable is created using a variable declaration (i.e. using var) then it is created with its deleteable flag set to false.

When a variable is created implicitly by assignment without being declared, its deleteable flag is set to true.

It is a peculiarity of the global execution context that variables are also made properties of the global object (this doesn't happen in function or eval code). So when you do:

var a;

Then a is a variable and also a property of the global (window in a browser) object and has its deleteable flag set to false. But:

a = 'foo';

creates a as a global variable without a declaration, so its deleteable flag is set to true.

The result is that you can delete global variables created implicitly, but not those created by declarations (including function declarations).