Validate form with checkboxes using Javascript

Here's a jQuery solution, I'll add a plain JS one in a few moments.

$('form[name="form1"]').on('submit', function(e) {
    if(!$('#filters input:checkbox:checked').length) {
        e.preventDefault();
        alert('Please select at least one filter.');
    }
});

This codes does not require the onsubmit inline event.

Since you are not familiar with jQuery I'll explain it more thoroughly:

• $('form[name="form1"]') creates a jQuery object containing all elements matching the selector. It would be faster if you gave your form id="form1" and used $('#form1')
• .on() binds an event handler. The first argument passed to the callback function is a wrapped event object which we'll need to prevent the form from being submitted if necessary.
• $('#filters input:checkbox:checked') selects all checked checkboxes that are children of #filters. :checkbox and :checked are pseudo-selectors which will only match checkboxes that are currently checked)
• .length is the number of elements in the jQuery object - if nothing is checked it's zero
• e.preventDefault(); prevents the default action of the event from being executed, i.e. the form will not be submitted.

Usually you would wrap the whole code with $(document).ready(function() { ... }); to make it execute as soon as the DOM is ready - if you put the <script> tag containing the code after the </form> tag of your form, it's not necessary though.

If you really want a plain JS solution, try this:

function Validate() {
    var container = document.getElementById('filters');
    var checked = 0;
    for(var i = 0; i < container.childNodes.length; i++) {
        var elem = container.childNodes[i];
        if(elem.tagName == 'INPUT' && elem.type == 'checkbox' && elem.checked) {
            checked++;
        }
    };
    if(checked) {
        return true;
    }
    alert('Please select at least one filter.');
    return false;
}