JavaScript Calculator alert dont divide by 0
<p><strong>i have to do alert about dont divide by 0, how do it</strong> </p>

<pre><code>&lt;!doctype html&gt; 
&lt;html lang="pl"&gt; 
&lt;head&gt; 
&lt;link rel="Stylesheet" type="text/css" href="style1.css" /&gt;
&lt;meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /&gt;
&lt;title&gt;ZAD 5 - Kalkulator&lt;/title&gt;
&lt;meta name="owner" content="Adam Mika" /&gt;
&lt;meta name="description" content="dskjadhjkhgkjdgkjgdkasgds" /&gt;
&lt;/head&gt;
&lt;body&gt;
&lt;div id="lewa"&gt;
MENU:&lt;ul&gt;
&lt;li&gt;&lt;a href="zad1.html"&gt;Zadanie 1&lt;/a&gt;&lt;/li&gt;
&lt;li&gt;&lt;a href="zad2.html"&gt;Zadanie 2&lt;/a&gt;&lt;/li&gt;
&lt;li&gt;&lt;a href="zad3.html"&gt;Zadanie 3&lt;/a&gt;&lt;/li&gt;
&lt;li&gt;&lt;a href="zad4.html"&gt;Zadanie 4&lt;/a&gt;&lt;/li&gt;
&lt;li&gt;&lt;a href="zad5.html"&gt;Zadanie 5&lt;/a&gt;&lt;/li&gt;
&lt;/ul&gt;
&lt;/div&gt;
&lt;div id="prawa"&gt;
ZADANIE 5 - Kalkulator
&lt;br&gt;&lt;/br&gt;
        &lt;form name="kalk"&gt;
            &lt;table align="center"&gt;
                &lt;tr&gt;
                    &lt;td &gt;
                        &lt;input name="txtEkran" type="text" value=0 readonly /&gt;
                    &lt;/td&gt;
                &lt;/tr&gt;
                &lt;tr&gt;
                    &lt;td&gt;
                        &lt;input type="button" value=7 onclick="cyfraClick(7)" /&gt;
                        &lt;input type="button" value=8 onclick="cyfraClick(8)" /&gt;
                        &lt;input type="button" value=9 onclick="cyfraClick(9)" /&gt;
                        &lt;input type="button" value="/" onclick="operatClick('/')" /&gt;
                    &lt;/td&gt;
                &lt;/tr&gt;
                &lt;tr&gt;
                    &lt;td&gt;
                        &lt;input type="button" value=4 onclick="cyfraClick(4)" /&gt;
                        &lt;input type="button" value=5 onclick="cyfraClick(5)" /&gt;
                        &lt;input type="button" value=6 onclick="cyfraClick(6)" /&gt;
                        &lt;input type="button" value="*" onclick="operatClick('*')" /&gt;
                    &lt;/td&gt;
                &lt;/tr&gt;
                &lt;tr&gt;
                    &lt;td&gt;
                        &lt;input type="button" value=1 onclick="cyfraClick(1)" /&gt;
                        &lt;input type="button" value=2 onclick="cyfraClick(2)" /&gt;
                        &lt;input type="button" value=3 onclick="cyfraClick(3)" /&gt;
                        &lt;input type="button" value="-" onclick="operatClick('-')" /&gt;
                    &lt;/td&gt;
                &lt;/tr&gt;
                &lt;tr&gt;
                    &lt;td&gt;
                        &lt;input type="button" value=0 onclick="cyfraClick(0)" /&gt;
                        &lt;input type="button" value="=" onclick="rownaClick()" /&gt;
                        &lt;input type="button" value="+" onclick="operatClick('+')" /&gt;
                        &lt;input type="button" value="CE" onclick="kasuj('CE')" /&gt;
                    &lt;/td&gt;
                &lt;/tr&gt;
            &lt;/table&gt;
        &lt;/form&gt;
        &lt;/div&gt;

        &lt;script src="zad5.js"&gt;
        &lt;/script&gt;
      &lt;/body&gt;
   &lt;/html&gt;
</code></pre>

<p>JS CODE</p>

<pre><code>var formKalk = window.document.kalk;
            var operat;
            var cyfraPierwsza = true;
            var walor1 = 0, walor2 = 0;

            function cyfraClick(cyfra){
                if (cyfraPierwsza || formKalk.txtEkran.value == 0){
                    formKalk.txtEkran.value  = cyfra;
                    cyfraPierwsza = false;
                }else{
                    formKalk.txtEkran.value += cyfra;
                }
            }

            function operatClick(opr){
                var txtEkran = formKalk.txtEkran.value;
                walor1 = parseFloat(txtEkran);
                operat = opr;
                cyfraPierwsza = true;
            }

            function rownaClick(){
                var rezultat;
                var txtEkran = formKalk.txtEkran.value;
                walor2 = parseFloat(txtEkran);
                if(operat == "/") {
                rezultat = walor1 / walor2;
                }else if(operat == "*") {
                    rezultat = walor1 * walor2;
                }else if(operat == "-"){
                    rezultat = walor1 - walor2;
                }
                else {
                    rezultat = walor1 + walor2;
                }

                if(rezultat == Infinity) rezultat = 0;




                formKalk.txtEkran.value = rezultat;
                walor1 = rezultat;
                cyfraPierwsza = true;
            }


            function kasuj(c){
                formKalk.txtEkran.value = 0;
                cyfraPierwsza = true;

                if (c == "C"){
                    walor1 = 0;
                    operat = "";
                }
            }
</code></pre>

<hr>

<p>What do you think is causing it?</p>