How to send multiple variables to a php file with XMLHttpRequest using post method?

Question

I want to send multiple variables to a php file within one XMLHttpReqyest , I can send 1 but I can't figure how to send more than 1.

for example if I have:

The HTML:

<form method="POST">
    <input type="text" id="firstVal"/>
    <input type="text" id="secondVal"/>
    <input type="submit" id='send'/>
</form>

The JS:

var val1 = documennt.getElementById('firstVal');
var val2 = documennt.getElementById('secondVal');
var sendData = documennt.getElementById('send');

function checkData(){
    var xhr = new XMLHttpRequest();
    xhr.open("POST" , 'file.php' , true);
    xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    xhr.onreadystatechange  = function(){   
        if(xhr.readyState == 4 && xhr.status == 200){
            console.log(xhr.responseText);
        }
    }
    xhr.send();
}

sendData.addEventListener('click' , checkData);

How to send these variables to the file.php ?