function datechange(element) {
// Expect input as d/m/y
    function isValidDate(s) {
      var bits = s.split('/');
      var d = new Date(bits[2], bits[1] - 1, bits[0]);
      return d && (d.getMonth() + 1) == bits[1];
    }
 [document.getElementById("checkinner").value].forEach(function(s) {
 if(isValidDate(s)){
     console.log(s + ' : ' + isValidDate(s))
 }else{
 console.log('not valid')
 }
     
});
}

                        <span class="input-group-addon"><i class="fa fa-calendar"> Check in</span>
                        <input id="checkinner" type="date" onchange="datechange(this)" class="form-control" data-inputmask="'alias': 'dd/mm/yyyy'" placeholder="Enter checkin date of birth">

    function datechange(element) {
        var date = Date.parse(element.value.toString());
        if (isNaN(date))
            alert('This is not a date object');
        else
            alert('This is a date object');
    }