Form validation

This works, however after it says you must create a title the form continues to submit. How do i halt it until something is in the field?

Thanks

function isEmpty(elem, helperMsg){
if(elem.value.length == 0){
alert(helperMsg);
elem.focus();
return true;
}
return false;
}

<input type=”submit” class=”submit”onclick=”isEmpty(document.getElementById(‘req1’), ‘You must create a title in the title field’)”>

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