Use one JavaScript function with two forms
<p>I have a JavaScript function for checking form input which sends an AJAX request to a PHP script. Here is the HTML:</p>

<pre><code>&lt;form method="post" id="textform35" action="insert_text.php"&gt;
    &lt;textarea id="text" cols="80" rows="3" placeholder="forecast text"&gt;&lt;/textarea&gt;
    &lt;input type=hidden id="text_old" value=""&gt;
    &lt;input type=hidden id="sin_type" value="short"&gt;
    &lt;input type=hidden id="den" value="0"&gt;
    &lt;input type=hidden id="grad" value="България-в"&gt;
    &lt;input type=hidden id="span" value="1"&gt;
    &lt;input type="button" name="Submit" value="Submit" onclick="validate_sendForm()"&gt;
&lt;/form&gt;

&lt;form method="post" id="textform70" action="insert_text.php"&gt;
    &lt;textarea id="text" cols="80" rows="12" placeholder=" &gt;&gt; "&gt;&lt;/textarea&gt;
    &lt;input type=hidden id="text_old" value=""&gt;
    &lt;input type=hidden id="sin_type" value="short"&gt;
    &lt;input type=hidden id="den" value="0"&gt;
    &lt;input type=hidden id="grad" value="България-в"&gt;
    &lt;input type=hidden id="span" value="1"&gt;
    &lt;input type="button" name="Submit" value="Запази" onclick="validate_sendForm()"&gt;
&lt;/form&gt;
</code></pre>

<p>As you can see, there are two forms with identical elements but with different <code>id</code>s.</p>

<p>The function <code>validate_sendForm()</code> is as below:</p>

<pre><code>function validate_sendForm() {
    var text=document.getElementById("text").value;
    var y=document.getElementById("text_old").value;

    if (text==y) {
        if (y=='') {
            alert("some text ");
            return false;
        } else {
            alert("another text ");
            return false;
        }
    } else {
        var city_id   = document.getElementById("grad").value;
        var fore_code = document.getElementById("sin_type").value;
        var date_for  = document.getElementById("den").value;
        var date_span = document.getElementById("span").value;

        var xhttp = new XMLHttpRequest();
        var params="grad="+city_id+"&amp;den="+date_for+"&amp;span="+date_span+"&amp;sin_file="+fore_code+"&amp;tekst="+text;
        xhttp.open("POST", "insert_text.php", true);
        xhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
        xhttp.setRequestHeader("Content-length", params.length);
        xhttp.setRequestHeader("Connection", "close");

        xhttp.onreadystatechange = function() {
            if (xhttp.readyState == 4 &amp;&amp; xhttp.status == 200) {
                alert(xhttp.responseText);
            }
        }

        xhttp.send(params);
    }
};
</code></pre>

<p>How can I use the function with both forms? Now it takes the values of the first form and never looks at the second one. I suppose I have to use "this" but have no idea where or how. Please, don't suggest that I use jQuery because I can not.</p>