Variables
Variables are usually declared in JavaScript with the var keyword.
var price;
JavaScript is an untyped language, you cannot declare a variable to be a string or number. Variables can hold any type and data types are converted automatically behind the scenes. See Dynamic Data Types
If you don’t use var the variable will be declared as global. You should avoid using global variables as they can result in unwanted side effects and are a frequent source of bugs! See Variable Scope.
After a variable is declared its value is undefined.
A value is assigned to a variable with the equals sign.
price = 500;
A variable can be assigned a value when it is declared.
var price = 500;
A variable can be emptied by setting its value to null.
price = null;
If you re-declare a variable, the variable will not lose its value.
var travelType = "Car";
var travelType; // travelType is still "Car"
If you assign a value to variable that has not been declared with var, the variable will automatically be declared as a global variable. See Variable Scope.
Variables names must start with a letter or underscore and cannot use any Reserved Words.
Use short names for variables which you use only in nearest code.
Multi-word names add precision from right to left, adjectives are always at the left side.
Use camel-case.