The following rules are to be followed while naming variables in JavaScript:
* You should not use any of the JavaScript reserved keyword as variable name. For example, break or boolean variable names are not valid.
* JavaScript variable names should not start with a numeral (0-9). They must begin with a letter or the underscore character. For example, 123name is an invalid variable name but _123name or name123 is a valid one.
* JavaScript variable names are case sensitive. For example, Test and test are two different variables.
4 Ways to declare a JavaScript variable: Using var, let, const, or nothing. Always declare JavaScript variables with var, let, or const.
* The var keyword is used in all JavaScript code from 1995 to 2015. If you want your code to run in older browser, you must use var.
* The let and const keywords were added to JavaScript in 2015. The variable type Let shares lots of similarities with var but unlike var, it has scope constraints.
* Const is another variable type assigned to data whose value cannot and will not change throughout the script.