Variables hold reusable data in a program and associate it with a name.

Variables are stored in memory.

The var keyword is used in pre-ES6 versions of JS.

let is the preferred way to declare a variable when it can be reassigned, and const is the preferred way to declare a variable with a constant value.

Variables that have not been initialized store the primitive data type undefined.

Mathematical assignment operators make it easy to calculate a new value and assign it to the same variable.

The + operator is used to concatenate strings including string values held in variables

In ES6, template literals use backticks ` and ${} to interpolate values into a string.

The typeof keyword returns the data type (as a string) of a value.