Var: Var is the oldest keyword for variable declaration in JavaScript. It is function scoped which means that it is visible inside the function it is declared in.
Example:
var x = 10;
Let: Let is the new keyword for variable declaration in JavaScript. It is block scoped which means that it is visible inside the block it is declared in.
Example:
let y = 20;
Const: Const is also a keyword for variable declaration in JavaScript. It is also block scoped and it cannot be reassigned or redeclared.
Example:
const z = 30;