如何基于带有提交按钮的选定选项调用条件函数

由于Result尚未定义的事实，您正在获得undefined。以下是您可能需要进行的更改。

由于您正在使用===进行评估，因此需要将order变量设置为int

var order = parseInt(select.options[select.selectedIndex].value);

下一步，移动x.innerHTML = Result到底部。

下面是最终结果(稍微整理一下):

function myFunction() {
    var x = document.getElementById("result");
    var listOne = Array();
    var listTwo = Array();
    var number = Math.floor(Math.random() * listOne.length);
    var letter = Math.floor(Math.random() * listTwo.length);
    var select = document.getElementById("mySelect");
    // casting it as an int. 
    // an alternative would be var order = +select.options[select.selectedIndex].value
    var order = parseInt(select.options[select.selectedIndex].value);
    var Result;
    listOne[0] = "1";
    listOne[1] = "2";
    listOne[2] = "3";
    listTwo[0] = "A";
    listTwo[1] = "B";
    listTwo[2] = "C";
    if (order === 1) {
        Result = listOne[number] + listTwo[letter] + listOne[number];
    }
    if (order === 2) {
        Result = listTwo[letter] + listOne[number] + listTwo[letter];
    }
    if (order === 3) {
        Result = listOne[number] + listTwo[letter] + listTwo[letter];
    }
    if (order === 4) {
        Result = listTwo[letter] + listOne[number] + listOne[number];
    }
    if (order === 5) {
        Result = listOne[number] + listOne[number] + listTwo[letter];
    }
    if (order === 6) {
        Result = listTwo[letter] + listTwo[letter] + listOne[number];
    }
    // add html AFTER Result has been set
    x.innerHTML = Result
}

DEMO

给你的提交按钮一个ID #submit

<input id="submit" type="submit" value="Generate" />

这就是你所需要的:

function myFunction(){
    var $result = document.getElementById("result");
    var $select = document.getElementById("mySelect"); 
    var order = $select.options[$select.selectedIndex].value;
    var listOne = ["1","2","3"]; // now it has length
    var listTwo = ["A","B","C"];
    var randNum  = Math.floor ( Math.random() * listOne.length ); 
    var randChar = Math.floor ( Math.random() * listTwo.length );
    var res = "";
    if (order === "1"){
      res = listOne[randNum] + listTwo[randChar] + listOne[randNum];
    }
    if (order === "2"){
      res = listTwo[randChar] + listOne[randNum] + listTwo[randChar];
    }
    if (order === "3"){
      res = listOne[randNum] + listTwo[randChar] + listTwo[randChar];
    }
    if (order === "4"){
      res = listTwo[randChar] + listOne[randNum] + listOne[randNum];
    }
    if (order === "5"){
      res = listOne[randNum] + listOne[randNum] + listTwo[randChar];
    }
    if (order === "6"){
      res = listTwo[randChar] + listTwo[randChar] + listOne[randNum];
    }  
    $result.innerHTML = res;
}
document.getElementById("submit").onclick = myFunction;

您可以将选项元素的值设置为字符串，如'nln'，以表示序列'数字字母数字'。

函数可以是这样的:

function getRandomString(s) {
  var a;
  var t = '';
  var data = {n: [0,1,2,3], l: ['a','b','c','d']};
  s = s.split('');
  for (var i=0, iLen=s.length; i<iLen; i++) {
    a = data[s[i]];
    t += a[Math.random()*a.length | 0];
  }
  return t;
} 

上面的值可以在"随机"字符串中重复。要获取唯一值，请使用splice从数组中获取值，而不是使用普通的属性访问:

    t += a.splice(Math.random()*a.length | 0, 1);

HTML可以是:

<form onsubmit="document.getElementById('result').innerHTML=getRandomString(this.mySelect.value);return false;">
  <select id=mySelect>
    <option value="nln" selected>one</option>
    <option value="lnl">two</option>
    <option value="nll">three</option>
    <option value="lnn">four</option>
    <option value="nnl">five</option>
    <option value="lln">six</option>
  </select>
  <input type="submit"value="Generate">
</form>
<div><span id="result"></span></div>

试试这个:

工作演示:JSFiddle

<div id="result"><i>Your result will appear here</i></div>
<form name="myform">
<select id="mySelect">
 <option value="1">one</option>
 <option value="2">two</option>
 <option value="3">three</option>
 <option value="4">four</option>
 <option value="5">five</option>
 <option value="6">six</option>
</select>
<input id="calc" type="button" value="Generate" />
</form>
function myFunction() {
    var x=document.getElementById("result");
    var listOne = [1, 2, 3];
    var listTwo = ["A", "B", "C"];
    var select = document.getElementById("mySelect");
    var order = +(select.options[select.selectedIndex].value);
    var number = Math.floor ( Math.random() * listOne.length ); 
    var letter = Math.floor ( Math.random() * listTwo.length );
    var result;
    if (order == 1) {
        result = listOne[number] + listTwo[letter] + listOne[number];
    }
    if (order === 2) {
        result = listTwo[letter] + listOne[number] + listTwo[letter];
    }
    if (order === 3) {
        result = listOne[number] + listTwo[letter] + listTwo[letter];
    }
    if (order === 4) {
        result = listTwo[letter] + listOne[number] + listOne[number];
    }
    if (order === 5) {
        result = listOne[number] + listOne[number] + listTwo[letter];
    }
    if (order === 6) {
        result = listTwo[letter] + listTwo[letter] + listOne[number];
    }
    x.innerHTML = result;
}
//Call the function;
document.getElementById("calc").onclick = myFunction;