JavaScript: var vs let and which one to use

Like many things in JavaScript, it has quirks to declare variables using the var keyword. var is a function-scoped keyword, so if we define a variable inside the if statement, the variable will be visible outside that scope. Creepy!

function wrong() {
  if (true) {
    var x = 5;
  }
  return x;
}

wrong(); // prints 5

If we change it to let, we get a sane result.

function right() {
  if (true) {
    let x = 5;
  }
  return x;
}

right(); // Uncaught ReferenceError: x is not defined

So the recommendation is to use the keyword let for declaring variables.