document.getElementById("square1").onclick = function disappear() {

        document.getElementById("square1").style.visibility = "hidden";

    }

        document.getElementById("square2").onclick = function disappear() {

        document.getElementById("square2").style.visibility = "hidden";

    }

        document.getElementById("square3").onclick = function disappear() {

        document.getElementById("square3").style.visibility = "hidden";

    }

        document.getElementById("square4").onclick = function disappear() {

        document.getElementById("square4").style.visibility = "hidden";

    }

        document.getElementById("square5").onclick = function disappear() {

        document.getElementById("square5").style.visibility = "hidden";

    }

        document.getElementById("square6").onclick = function disappear() {

        document.getElementById("square6").style.visibility = "hidden";

    }

for (var i = 1; i < 7; i++) {

       document.getElementById("square" + i).onclick = function disappear() {

            document.getElementById("square" + i).style.visibility = "hidden";

       }

              }