只有在物品存在的情况下才定义变量

Question

在本网站上的几位同事的帮助下，我能够获得以下脚本的变体，以根据他们在下拉菜单中进行的选择填充某些文本字段。基本上，他们从下拉列表中选择他们的位置，并用他们的地址（input_19）和城市，州和邮政编码（input_21）填充文本字段。当将其扩展到多个用途时，我意识到有时套件编号需要位于单独的字段中，因此我添加了两个额外的变量来分割地址（input_26）和套件编号（input_21）。在没有input_21的情况下，看起来代码不成功。只有在字段存在的情况下才有办法使用脚本部分？

// JavaScript Document 

// Pre populated array of data 
var myData1 = new Array(); 
myData1['Addison'] = '14295 Midway Road, Suite 100'; 
myData1['Arlington'] = '1241 W. Green Oaks Blvd., Suite 101'; 
myData1['Colleyville'] = '1210 Hall Johnson Road, Suite 150'; 
myData1['Fire Wheel'] = '650 Beebalm Lane, Suite 260'; 
myData1['Flower Mound'] = '3020 Corporate Court, Suite 400'; 
myData1['Fort Worth'] = '4421 Oak Park Lane, Suite 102'; 
myData1['Legacy'] = '7500 Dallas Pkwy., Suite 175'; 
myData1['Mansfield'] = '1405 N. Highway 287, Suite 101'; 
myData1['McDermott'] = '3915 McDermott Road, Suite 2000'; 
myData1['Preston Center'] = '8201 Preston Road, Suite 450'; 
var myData2 = new Array(); 
myData2['Addison'] = 'Addison, TX 75001'; 
myData2['Arlington'] = 'Arlington, TX 76013'; 
myData2['Colleyville'] = 'Colleyville, TX 76034'; 
myData2['Fire Wheel'] = 'Garland, TX 75040'; 
myData2['Flower Mound'] = 'Flower Mound, TX 75028'; 
myData2['Fort Worth'] = 'Fort Worth, TX 76109'; 
myData2['Legacy'] = 'Plano, TX 750241'; 
myData2['Mansfield'] = 'Mansfield, TX 76063'; 
myData2['McDermott'] = 'Plano, TX 75025'; 
myData2['Preston Center'] = 'Dallas, TX 75225'; 
var myData3 = new Array(); 
myData3['Addison'] = 'Suite 100'; 
myData3['Arlington'] = 'Suite 101'; 
myData3['Colleyville'] = 'Suite 150'; 
myData3['Fire Wheel'] = 'Suite 260'; 
myData3['Flower Mound'] = 'Suite 400'; 
myData3['Fort Worth'] = 'Suite 102'; 
myData3['Legacy'] = 'Suite 175'; 
myData3['Mansfield'] = 'Suite 101'; 
myData3['McDermott'] = 'Suite 2000'; 
myData3['Preston Center'] = 'Suite 450'; 
var myData4 = new Array(); 
myData4['Addison'] = '14295 Midway Road'; 
myData4['Arlington'] = '1241 W. Green Oaks Blvd.'; 
myData4['Colleyville'] = '1210 Hall Johnson Road'; 
myData4['Fire Wheel'] = '650 Beebalm Lane'; 
myData4['Flower Mound'] = '3020 Corporate Court'; 
myData4['Fort Worth'] = '4421 Oak Park Lane'; 
myData4['Legacy'] = '7500 Dallas Pkwy.'; 
myData4['Mansfield'] = '1405 N. Highway 287'; 
myData4['McDermott'] = '3915 McDermott Road'; 
myData4['Preston Center'] = '8201 Preston Road'; 




var element = document.querySelector('form.cart'); 
element.input_1.onchange = updateText; 
function updateText() { 
var obj_sel = element.input_1; 
element.input_19.value = myData1[obj_sel.value]; 
var obj_sel = element.input_1; 
element.input_21.value = myData2[obj_sel.value]; 
var obj_sel = element.input_1; 
element.input_26.value = myData3[obj_sel.value]; 
var obj_sel = element.input_1; 
element.input_31.value = myData4[obj_sel.value]; 
} 

谢谢！

Answer 1

如果对象不存在，您的脚本会抛出一个错误，因为您正试图将属性分配给undefined。由于你没有catch那个错误，脚本就会停止运行。在尝试设置其值之前，可以使用if语句来确认对象不是undefined。你也不需要你用它每次重复var obj_sel = element.input_1;：这应该解决您的问题

function updateText() { 
    var obj_sel_value = element.input_1.value; 

    if(element.input_19) 
    element.input_19.value = myData1[obj_sel_value]; 
    if(element.input_21) 
    element.input_21.value = myData2[obj_sel_value]; 
    if(element.input_26) 
    element.input_26.value = myData3[obj_sel_value]; 
    if(element.input_31) 
    element.input_31.value = myData4[obj_sel_value]; 
} 

，但也有其他事情可以做，使你的代码更好，保存自己在未来的时间。例如，不是每个myData对象都有一个变量，您可以使用一组对象。如果您发现自己的变量名称后缀为数字，这很好地表明您应该考虑用一个数组替换所有这些变量。

var myData = []; 
myData[0] = {}; 
myData[0]['Addison'] = '14295 Midway Road, Suite 100'; 
... 
myData[0]['Preston Center'] = '8201 Preston Road, Suite 450'; 
myData[1]['Addison'] = 'Addison, TX 75001'; 
... 
myData[1]['Preston Center'] = 'Dallas, TX 75225'; 
myData[2] = {}; 
... 
... 

还要注意，我代替你new Array()电话与{}。这是有道理的，因为你使用它像一个对象而不是数组。 Javascript数组是一个有序的集合，通过数字键进行索引。如果你有字符串键，你正在处理一个对象，而不是一个数组。一个对象是（键，值）对的无序集合。