i have an egual function that oarse the result of the string concatenation, i've done the + "1" cuz doig the concatenation and parsing it i can write multi-digit number.
uno.addEventListener("click", Uno);

function Uno () {

Append(1);

if (x == "" && y == "") {

x = "1";

}

if (x !== "" && checker !== undefined && y == "") {

y = "1";

}

else if (x !== "" && y == "" && checker == undefined) {

x = x + "1";

}

else if (x !== "" && y !== "" && checker !== undefined) {

y = y +"1";

}

}
uguale.addEventListener("click", Uguale);
function Uguale () {
if (checker == "+") {
x = parseInt(x);
y = parseInt(y);
console.log((x + y));
}
if (checker == "-") {
console.log(parseInt(x - y));
}
if (checker == "*") {
console.log(parseInt(x * y));
}
if (checker == "/") {
console.log(parseInt(x / y))
}
}