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>

http://www.somacon.com/p117.php

Check or Uncheck all Checkboxes

javascript, function

JavaScript