Declaration

Known as

let = Lets set a variable that can be mutated/altered later

let age = 23;

age = 24;

const = set constant variable that should not be mutated/altered later

const birthYear = 1980;

birthYear = 1981;

var = should not be used in modern (2016+) Java script.

var; was the precursor to let and const , found only in pre 2016 Js code.

A declaration of let declares a set of Variables that can be altered.

A declaration of const declares a set of Variables that should not be altered.

var is pre 2016 JS update and was used before the introduction of let and const