这个问题在这里已经有了答案：

为什么此javascript代码在控制台上打印“未定义”(（1个答案）
两天前关门了。
我用javascript制作了一个简单的计算器。我在下面提供这些代码。但问题是当我运行这些代码时。我看到一个未定义的输出。我不知道为什么会出现这段文字。我想把它去掉。

function addition(x, y) {
  var sum = x + y;
  document.write("Addition of two number is : " + sum);
}

function substract(x, y) {
  var sub = x - y;
  document.write("Subtraction of two number is : " + sub);
}

function multiply(x, y) {
  var multiply = x * y;
  document.write("Multipication of two number is : " + multiply);
}

function division(x, y) {
  var division = x / y;
  document.write("Division of two number is : " + division);
}

var x = parseInt(prompt("Enter the first number : "));
var y = parseInt(prompt("Enter the second number : "));

var operator = prompt("Enter the operator : ");

if (operator == "+") {
  document.write(addition(x, y));
} else if (operator == "-") {
  document.write(substract(x, y));
} else if (operator == "*") {
  document.write(multiply(x, y));
} else if (operator == "/") {
  document.write(division(x, y));
} else {
  document.write("Invalid Operator. Please choose operator between +,-,* or /. <br> Thanks for using our calculator. ");
}