A variable that has not assigned a value. And hold the value undefined. It is just like null.
A variable has defined with null value. It is like as undefined. It's store nothing.
There is only one number type. There is no specific type for integers. Number can written with or without decimal point. It is able to represent floating-point numbers, the number type has three symbolic values: +Infinity, -Infinity, and NaN (not-a-number).
It is logical a entity. A variable can have two values
New in ECMAScript6. A Symbol is a unique and immutable identifier.
The typeof Operator