How do you declare a variable in JavaScript, and what are the rules for naming one?

You declare a variable with let name = value;, where the name follows a few simple rules and the value can be reused later.

A variable is a named box for storing a value so you can refer to it again later in the program. You create one with the let keyword, optionally giving it a starting value:

let answer = 42;
let value = 1 + answer;   // value is now 43

Rules for the name (simplified):

• Allowed characters: letters, digits, the dollar sign $, and the underscore _.
• It must start with a letter (not a digit).
• Names are case-sensitive — myVar and myvar are two different variables.
• You can't use reserved keywords like let, function, or if as a name.

Beyond the rules, programmers follow naming conventions so code reads consistently: camelCase for variables and functions (firstName, getUserData) and UPPER_CASE for constants that never change (MAX_SIZE). These aren't enforced by the language, but following them makes code far easier to read.