call javascript function on asp dropdown list change
<p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
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<pre class="snippet-code-js lang-js prettyprint-override"><code>        &lt;script type="text/javascript"&gt;
            function ddl(id) {
      //          var id = document.getElementById('&lt;%=DDLquestionType.ClientID%&gt;').value;
                if (id == "Open") {
                    AmericanAnswer.style.display = 'none';
                    document.getElementById('Answer1_Btn').style.visibility = false;
                    
                }
                alert(id);
            }
        &lt;/script&gt;</code></pre>
<pre class="snippet-code-html lang-html prettyprint-override"><code>                &lt;asp:DropDownList ID="DDLquestionType" CssClass="ddlQuestionType box"  runat="server" AutoPostBack="True" onchange="ddl(this)" &gt;
                    &lt;asp:ListItem Text="American" Value="American"&gt;&lt;/asp:ListItem&gt;
                    &lt;asp:ListItem Text="Open" Value="Open"&gt;&lt;/asp:ListItem&gt;
                    &lt;asp:ListItem Text="Yes/No" Value="YesNo"&gt;&lt;/asp:ListItem&gt;
                    &lt;asp:ListItem Text="Numerical" Value="Numerical"&gt;&lt;/asp:ListItem&gt;
                &lt;/asp:DropDownList&gt;</code></pre>
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</p>

<p>I have this java script function:</p>

<p>Which I try to call on drop down list selection change</p>

<p>It doesn't work. 
I also tried
    onchange="javascript:ddl()" and function without parameters, 
    ononchange="javascript:ddl(this);", 
    ononchange="javascript:ddl(this.value);", 
and many others.
I'm new to java script, any links with explanations also will be highly appreciated</p>