Variable Declaration

In summary, 

• Variables can be thought of as named holders or containers. 
• var keyword is used to create and declare a variable. 
• let variables are declared in a similar manner to var declared variables. But we use the let keyword rather than var; this has scope constraints. The let declared variables cannot be redeclared. 
• The const declaration creates a constant whose scope is often either global or local to the declared block during which we declare it. The value of a constant is read-only; you cannot change it through reassignment, and you can’t redeclare it.
• A data type is a data description. To communicate correctly with values, programming languages need to have distinctive data types. ● There are six primitive (fundamental) data types:
• String a series of characters.
• Number is a numerical value.
• Two other uncommon number types
• Theoretically NaN is a type of number, NaN means Not a Number. NaN is the product of attempting to do difficult arithmetic with other data types. 
• Infinity is also a type of number, either the answer of dividing by 0 or calculating too large a number. 
• Boolean is considered a value that is either true or false. 
• An undefined variable has no value. 
• null is a value that represents nothing. 
• A Symbol could be a special and unchanging data type.
  

References: