Edit in JSFiddle

$(document).ready(function() {
    $("#myForm").on("input",function(e) {
       e.preventDefault();
        var invalid = $(":invalid");
        console.log("there are "+invalid.length+" invalid items");
    });
});

<form id="myForm">
    <input type="text" name="foo" required>
    <input type="submit">
</form>