/**
 * 跨浏览器，向控制台写入消息
 * @param message
 */
function log(message) {
  if (typeof console == "object") {
    console.log(message);
  } else if (typeof opera == "object") {
    opera.postError(message);
  } else if (typeof java == "object" && typeof java.lang == "object") {
    java.lang.System.out.println(message);
  }
}

function sum(num1, num2) {
  log("Entering sum(), arguments are " + num1 + "," + num2);
  log("Before calculation");
  var result = num1 + num2;
  log("After calculation");
  log("Exiting sum()");
  return result;
}

function log(message) {
  var console = document.getElementById("debuginfo");
  if (console == null) {
    console = document.createElement("div");
    console.id = "debuginfo";
    console.style.background = "#dedede";
    console.style.border = "1px solid silver";
    console.style.padding = "5px";
    console.style.width = "400px";
    console.style.position = "absolute";
    console.style.right = "0px";
    console.style.top = "0px";
    document.body.appendChild(console);
  }
  console.innerHTML += "<p>" + message + "</p>";
}

function divide(num1, num2){
  return num1/num2;
}

function divide(num1, num2){
  if(typeof num1 !="number" || typeof num2 !="number"){
    throw new Error("divide(): 两个参数都必须是数值")
  }
  return num1/num2;
}

<script type="text/javascript">
  function assert(condition, message) {
    if (!condition) {
      throw new Error(message);
    }
  }
  function divide(num1, num2) {
    assert(typeof num1 == "number" && typeof num2 == "number", "divide():Both arguments must be numbers.");
    return num1 / num2;
  }
  divide(1,"tt");
</script>