var keyword

var x;
x = 123;

// Or simply
var x = 123;

Repeat use of var does not cause an error.

var x;
var x;

let keyword

let y;
y = 123;

let z = 123;

Note that let can only be used once for a variable.

let y;
let y;
// Uncaught SyntaxError: Identifier 'y' has already been declared

const keyword

const y = 123;

const y;
// Uncaught SyntaxError: Missing initializer in const declaration