var age = prompt("please enter your age");
      if (age >= 18) {
        alert("you are allowed to enter");
      } else {
        alert("you are not allowed to enter");
      }
      if (age > 50) {
        alert("welcome to our website!");
      }
    //   in this case, if age is greater than 50, both alerts will be shown because both conditions are checked separately
    //   and both if structures are independent of each other and true conditions will be executed
 <script>
      var person = prompt("who's there?");
      if (person === "Admin") {
        var pass = prompt("password?");
        if (pass === "TheMaster") {
          alert("Welcome!");
        } else {
          alert("Wrong password");
        }
      }
      else {
        alert("I don't know you");
      }
    //   last example with (?:, ternary if) operator
        person === "Admin"
          ? (prompt("password?")) === "TheMaster"
            ? alert("Welcome!")
            : alert("Wrong password")
          : null;
        person !== "Admin" ? alert("I don't know you") : null;
      // example with switch case statement
      switch (person) {
        case "Admin":
          var pass = prompt("password?");
          switch (pass) {
            case "TheMaster":
              alert("Welcome!");
              break;
            default:
              alert("Wrong password");
              break;
          }
          break;
        default:
          alert("I don't know you");
          break;
      }
    </script>