Var & Let & Const

let and const Introduced in ES6 (ES6 is also known as ECMAScript 6 and ECMAScript 2015)

let - declares block level variables:

// myLetVariable is *not* visible out here

for (let myLetVariable = 0; myLetVariable < 5; myLetVariable++) {
  // myLetVariable is only visible in here
}

// myLetVariable is *not* visible out here

const - never changes:

const Pi = 3.14; // variable Pi is set
Pi = 1; // will throw an error because you cannot change a constant variable.

var - available from the function:

// myVarVariable *is* visible out here

for (var myVarVariable = 0; myVarVariable < 5; myVarVariable++) {
  // myVarVariable is visible to the whole function
}

// myVarVariable *is* visible out here

developer.mozilla.org