如何在JavaScript中处理字母数字和null？

var variable = "abc123";
if (typeof variable === "string") {
  // 处理字母
  var uppercase = variable.toUpperCase();
  var lowercase = variable.toLowerCase();
  console.log("Uppercase: " + uppercase);
  console.log("Lowercase: " + lowercase);
} else if (typeof variable === "number") {
  // 处理数字
  var parsedInt = parseInt(variable);
  var parsedFloat = parseFloat(variable);
  console.log("Parsed Int: " + parsedInt);
  console.log("Parsed Float: " + parsedFloat);
} else if (variable === null) {
  // 处理null
  console.log("Variable is null");
} else {
  console.log("Variable is not a string, number, or null");
}