In JavaScript, a variable is a container for storing data. There are three ways to declare a variable in JavaScript:

• Using the var keyword (e.g. var x = 5;)
• Using the let keyword (e.g. let y = 6;)
• Using the const keyword (e.g. const z = 7;)

Variables declared with var were used in all JavaScript code from 1995 to 2015, but the let and const keywords were added to JavaScript in 2015. It is considered good programming practice to always declare variables before use.

Variables can store any type of JavaScript value, such as strings, numbers, and booleans. Once a variable is declared, it can be initialized with a value using the assignment operator = (e.g. let message; message = Hello;) . Variables can be reassigned a new value at any point after they are declared.

It is important to follow certain rules when naming variables in JavaScript. Variable names must start with a letter, underscore _, or dollar sign $, and after the first letter, they can use numbers as well as letters, underscores, or dollar signs. JavaScripts reserved keywords cannot be used as variable names.

In summary, a variable in JavaScript is a named storage location that can store any type of JavaScript value. Variables can be declared using the var, let, or const keyword, and must be named according to certain rules.