字符串Concat两个变量，并填充了另一个变量，并检查第四变量值是否在第四变量数组和显示中可用

这是用于组合文本和获取值的函数。字符串Concat两个变量并填充了另一个变量，并检查第四变量值是否在第四变量数组中可用，并且显示

function myFunction() {
  var price = {
    audi: 200,
    bmw: 300,
    volvo: 500
  };
  var x = document.getElementById("mySelect").value;
  var y = document.getElementById("mySelect2").value;
  var p = {
    LondonColombo: 100,
    ColomboLondon: 250
  };
  var z = x + y
  var txt = "";
  if (z in p) {
    txt = price[z]
  }
  document.getElementById("demo").value = z;
  document.getElementById("cost").value = txt;
}

<!DOCTYPE html>
<html>
<body>
  <select id="mySelect" onchange="myFunction()">
    <option value="audi">audi</option>
    <option value="bmw"> bmw </option>
    <option value="volvo">volvo </option>
    <option value="London">London </option>
  </select>
  <select id="mySelect2" onchange="myFunction()">
    <option value="audi">audi</option>
    <option value="bmw"> bmw </option>
    <option value="volvo">volvo </option>
    <option value="Colombo">Colombo </option>
    <input id="demo">
    <input id="cost">

您看到的不确定是因为在txt = price[z]中，价格未定义。而不是使用p[z]

function myFunction() {
  var price = {
    audi: 200,
    bmw: 300,
    volvo: 500
  };
  var x = document.getElementById("mySelect").value;
  var y = document.getElementById("mySelect2").value;
  var p = {
    LondonColombo: 100,
    ColomboLondon: 250
  };
  var z = x + y
  var txt = "";
  if (z in p) {
    txt = p[z]
  }
  document.getElementById("demo").value = z;
  document.getElementById("cost").value = txt;
}

<select id="mySelect" onchange="myFunction()">
  <option value="audi">audi</option>
  <option value="bmw"> bmw </option>
  <option value="volvo">volvo </option>
  <option value="London">London </option>
</select>
<select id="mySelect2" onchange="myFunction()">
  <option value="audi">audi</option>
  <option value="bmw"> bmw </option>
  <option value="volvo">volvo </option>
  <option value="Colombo">Colombo </option>
</select>
<input id="demo">
<input id="cost">

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