some code's meaning of javascript?
<pre><code> &lt;script language="JavaScript" type="text/javascript"&gt;     
     var city=[
     ["city1","city2","city3","city4"],
    ["city5","city6","city7"],
     ["city8","city9","city10"],
     ];

     function getCity(){            
         var sltProvince=document.form1.province;

        var sltCity=document.form1.city;             

         var provinceCity=city[sltProvince.selectedIndex - 1];     

         sltCity.length=1;    

         for(var i=0;i&lt;provinceCity.length;i++){
             sltCity[i+1]=new Option(provinceCity[i],provinceCity[i]);
         }
     }
&lt;/script&gt;
 &lt;FORM METHOD=POST ACTION="" name="form1"&gt;
         &lt;SELECT NAME="province" onChange="getCity()"&gt;
             &lt;OPTION VALUE="0"&gt;select province &lt;/OPTION&gt;
             &lt;OPTION VALUE="province1"&gt;province 1 &lt;/OPTION&gt;
             &lt;OPTION VALUE="province2"&gt;province2&lt;/OPTION&gt;
             &lt;OPTION VALUE="province3"&gt;province3 &lt;/OPTION&gt;
         &lt;/SELECT&gt;
         &lt;SELECT NAME="city"&gt;
            &lt;OPTION VALUE="0"&gt;select the city&lt;/OPTION&gt;
        &lt;/SELECT&gt;
     &lt;/FORM&gt;
</code></pre>

<p>the above code is according to the province select its city. there are some lines i don't understand well. expect some one can explain it. thank you.</p>

<p>1, what's these lines do and meaning? </p>

<pre><code>var provinceCity=city[sltProvince.selectedIndex - 1];
</code></pre>

<p>and</p>

<pre><code>  sltCity.length=1;  
  sltCity[i+1]=new Option(provinceCity[i],provinceCity[i]);
</code></pre>
