Variables declared by var keyword are scoped to the immediate function body 

(hence the function scope) while let variables are scoped to the immediate 

enclosing block denoted by { } (hence the block scope).

// Example

function run() {

  var foo = "Foo";

  let bar = "Bar";

​

  console.log(foo, bar); // Foo Bar

​

  {

    var moo = "Mooo"

    let baz = "Bazz";

    console.log(moo, baz); // Mooo Bazz

  }

​

  console.log(moo); // Mooo

  console.log(baz); // ReferenceError

}