JavaScript: Check/Uncheck box depending on dropdown menu selection
Question
I am trying to get a check box to tick its self if a drop down list item is selected. For example, if 'Entry 0' is selected in the drop down, i want the check box '0 is selected' to be checked. Also, if an entry other than 'Entry 0' is selected, I want the check box '0 is selected' to be unchecked. The code i have uses 2 drop down menu items, and 2 corresponding check boxes. Currently only check box 1 works, and does not deselect when another drop down item is selected.
I have searched the net and google for hours and cannot seem to find a solution. I am new to JavaScript, but need this for an online form. (once I understand how to get this solution working, I should be able to put it into my online form.)
<body>
<script language="JavaScript">
function onChange() {
if (document.formName3.selectName3 = 'Entry 0')
//{ alert("Option changed")
{ document.formName3.c_0.checked = true;
document.formName3.c_1.checked = false;
}
else if (document.formName3.selectName3 = 'Entry 1')
//{ alert("Option changed")
{document.formName3.c_1.checked = true;
document.formName3.c_0.checked = false;
}}
{ alert("Begin")
document.formName3.c_1.checked = true;
}
</script>
<form
name="formName3" id="formName3"
onSubmit="return false;"
>
<p>
<select
name="selectName3"
onChange="onChange()"
>
<option
value="Option 0" selected="selected"
>
Entry 0
<option
value="Option 1"
>
Entry 1
</select>
<p>
<input name="c_0" type="checkbox" id="c_0" />
<label for="c_0">0 Is selected</label>
</p>
<p>
<input name="c_1" type="checkbox" id="c_1" />
<label for="c_1">1 Is selected</label>
</p>
</form>
</body>
</html>
Can you let me know how to get it working please? Once I understand how to do this, i can apply it as a feature to my online form. Thanks!!!
4 answers
solution1
1 2011-10-26 15:47:17
There are a couple of problems here.
The following conditional will be true every time it is executed because you are using one equals sign instead of two. (= instead of ==). You need to always use two equals signs when comparing values.
if (document.formName3.selectName3 = 'Entry 0')
Second, the way you are trying to find the value of the currently selected option is incorrect. Instead of:
document.formName3.selectName3
You can use the following:
document.formName3.selectName3.options[document.formName3.selectName3.selectedIndex].value
There are many other ways, but this is acceptable.
To put these two suggestions together, the code now becomes:
function onChange() {
var selectValue = document.formName3.selectName3.options[document.formName3.selectName3.selectedIndex].value;
if (selectValue == 'Entry 0') {
document.formName3.c_0.checked = true;
document.formName3.c_1.checked = false;
} else if (selectValue == 'Entry 1') {
// ... ;
}
}
Here is some more info:
solution2
0 2011-10-26 15:45:51
Start by turning all of your single "=" symbols into "===" in your 'if' conditions.
if ( foo === bar ){
<<do something>>
}
solution3
0 2011-10-26 15:46:26
if (document.formName3.selectName3 = 'Entry 0')
This is an assignment and not an equality check. Use == or === .
In the above code, the first if always evaluates to true .
document.formName3.selectName3
Returns a DOM element, not the value.
To get the selected value from a select box, do:
var index = document.formName3.selectName3.selectedIndex;
var selectedValue = document.formName3.selectName3.options[ index ].value;
If you have a lot of options and checkboxes, the code will become quite extensive and a pain to maintain. You might want to consider using a library like jQuery or Prototype.
solution4
0 2011-10-26 15:53:08
This code does what you want:
<html>
<body>
<script type="text/javascript">
function onChange() {
var selectBox = document.formName3.selectName3;
var val = selectBox.value;
i = 0;
while(document.formName3["c_" + (i++)])
document.formName3["c_" + (i++)].checked = false;
document.formName3["c_" + val].checked = true;
}
</script>
<form name="formName3" id="formName3" onSubmit="return false;">
<p>
<select name="selectName3" onChange="onChange()">
<option value="0" selected="selected">Entry 0</option>
<option value="1">Entry 1</option>
</select>
</p>
<p>
<input name="c_0" type="checkbox" id="c_0" />
<label for="c_0">0 Is selected</label>
</p>
<p>
<input name="c_1" type="checkbox" id="c_1" />
<label for="c_1">1 Is selected</label>
</p>
</form>
</body>
</html>
Some pointers to help you learn:
The current value of the select box is obtained with selectBox.value . This value is equal to the currently selected option' s value contents.
The function first clears all checkboxes, by taking advantage of a couple of javascript features. Notice the while loop. What that does is:
A. If an element named document.formName3["c_" + (i++)] , which is directly equivalent to document.formName3.c_0 exists, then set its value to false.
B. Increments i by one.
C. Repeats, until i is equal to 2, at which point it can't find a checkbox like that, and stops.
After all checkboxes have been cleared, the appropriate checkbox is checked.
Another final note: In javascript, you can access properties of objects by using the brackets syntax. What that means, is that object1.property1 is directly equivalent to object1["property1"] .
Cheers, and I hope this helps.
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