function SetAllCheckBoxes(FormName, FieldName, CheckValue)
{
if(!document.forms[FormName])
return;
var objCheckBoxes = document.forms[FormName].elements[FieldName];
if(!objCheckBoxes)
return;
var countCheckBoxes = objCheckBoxes.length;
if(!countCheckBoxes)
objCheckBoxes.checked = CheckValue;
else
// set the check value for all check boxes
for(var i = 0; i < countCheckBoxes; i++)
objCheckBoxes[i].checked = CheckValue;
}
<form method="GET" action="page17.php" name="myForm" onsubmit="return false;">
<label for="myCheckbox1">
<input type="checkbox" name="myCheckbox" value="1" id="myCheckbox1">
I like Britney Spears </label>
<br>
<label for="myCheckbox2"><input type="checkbox" name="myCheckbox" value="2" id="myCheckbox2">
I like Hillary Duff </label>
<br>
<label for="myCheckbox3"><input type="checkbox" name="myCheckbox" value="3" id="myCheckbox3">
I like Mandy Moore </label>
<br><input type="button" onclick="SetAllCheckBoxes('myForm', 'myCheckbox', true);" value="I like them all!">
&nbsp;&nbsp;
<input type="button" onclick="SetAllCheckBoxes('myForm', 'myCheckbox', false);" value="I don't like any of them!">
</form>