确定表单上是否存在字段 [英] determining if a field exists on a form

问题描述

我有一个表单字段（一系列复选框），它是从数据库动态创建的，因此表单上可能不存在该字段（如果数据库中没有匹配的值）。我有一些代码需要根据字段是否存在来执行，如果确实存在，则拉入所选的值。我似乎无法得到javascript来承认这个字段存在。这是我尝试过的：

I have a form field (a series of checkboxes) that's being created dynamically from a database, so it's possible that the field will not exist on the form (if there are no matching values in the database). I have some code that needs to execute based on whether the field exists, and pull in the values that are selected if it does exist. I can't seem to get javascript to acknowledge that this field exists, though. Here's what I've tried:

function displayAction(){
    var f = document.adminForm;
    var a = f.action;

    if(f.prefix.value!="-") {
        a = a + '&task=callExclusionDisplay&prefix=' + f.prefix.value;
    }
    else {
        var exclusions = document.getElementById("exclusions");
        if (exclusions != null){
            alert("exclusions set");
            a = a + '&task=callExclusionCreate&prefix=' + f.prefix.value + '&exclusions=' + exclusions.join();
        }
    }
    alert('after if, action is ' + a);
}

代码永远不会通过if语句检查exclusion是否为null，即使我在查看页面时有许多名为exclusions的复选框（id也设置为排除）。问题是！= null，因为它是一组复选框，而不是一个表单元素？我怎样才能让它发挥作用？如果我跳过测试的null，如果数据库没有返回任何匹配的值，则代码会抛出有关排除的错误。

The code never passes the if statement checking to see if exclusions is not null, even though when I look at the page there are a number of checkboxes named exclusions (with the id also set to exclusions). Is the issue with !=null because it's a group of checkboxes, rather than a single form element? How can I get this to work? If I skip the test for null, the code throws errors about exclusions not being defined if the database doesn't return any matching values.

推荐答案

您正在使用 document.getElementById ，但表单元素具有名称。
尝试 f.elements.namedItem（exclusions）而不是 exclusions！= null

You're using document.getElementById, but form elements have a name. Try f.elements.namedItem("exclusions") instead of exclusions != null

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