JavaScript - Can't Assign Value To Specific String Value (array)

<script type="text/javascript">
Arr=['a','b','c'];
Arr2=Arr;
Arr[1]='_';
alert('Array 1: '+Arr);
alert('Array 2: '+Arr2);
// Both arrays become 'a','_','c'
</script>

<script type="text/javascript">
function Fruits() {


var selectfruit = newArray( //assigning values to fruit selected from dropdown box
			newArray("Banana", 1, 1),
			newArray("Apple", 1.2, 0.5),
			newArray("Mango", 1.1, 0.9),
			newArray("Orange", 0.1, 9.99)); 
var howmanyfruits = Number(document.getElementById("howmanyfruits").value); // how many fruits are you buying
var totalfruitsowned = Number(document.getElementById("totalfruitowned").value); // How many fruits do you already have

/* cash and coupons needed to buy fruits.
   cash is cpst for 1 fruit.
   coupons is cost for 1 fruit
   cash_all is cost for all fruits you're buying
   coupons_all is cost for all fruits you're buying
   
   each fruits requires cash AND coupons to be bought. Cash and coupons are tied to the first values in Array. 
   Eg. If you choose Apple that value would be 1.2  
   
   The 'fruitsmaxtobuy' variable is not tied to the first value, but the second one in array.
   If you choose Apple that value would be 0.5.
*/
   
var cash = Math.round(((totalfruitsowned * 0.51 * selectfruit) + 700)*10)/10;
var coupons = Math.round(((totalfruitsowned * 0.51 * selectfruit) + 850)*10)/10;
var cash_all = Math.round((howmanyfruits * cash)*10)/10; 
var coupons_all = Math.round((howmanyfruits * coupons)*10)/10; 

var fruitsmaxtobuy = Math.round((totalfruitsowned * 0.12 * selectfruit)*10)/10;

/*
Display Error if nothing is entered or if you forget to enter total fruits 
*/

if (((howmanyfruits=="" || howmanyfruits==null) && (totalfruitsowned=="" || totalfruitsowned==null)) || ((howmanyfruits==Number(document.getElementById("howmanyfruits").value)) && (totalfruitsowned=="" || totalfruitsowned==null)))
{document.getElementById("cash").innerHTML = "Error";
document.getElementById("coupons").innerHTML = "Error";
document.getElementById("cash_all").innerHTML = "Error";
document.getElementById("coupons_all").innerHTML = "Error";
document.getElementById("fruitsmaxtobuy").innerHTML ="Error"}
else {
document.getElementById("cash").innerHTML = cash;
document.getElementById("coupons").innerHTML = coupons;	
document.getElementById("cash_all").innerHTML = cash_all;
document.getElementById("coupons_all").innerHTML = coupons_all;
document.getElementById("fruitsmaxtobuy").innerHTML =fruitsmaxtobuy}
}
</script>

<form action="" id="fruitcost">
<table align="center" width="37.5%" cellpadding="0" cellspacing="0">
	<tbody>
    	<tr>
        	<th colspan="2" align="center">Fruit cost calcultor</th>
        </tr>    
   		<tr>
			<td>Select Fruit:</td>
            <td align="center"><select id="selectfruit">
      		 		<option>Banana</option>
        	    	<option selected>Apple</option>
        			<option>Mango</option>
        			<option>Orange</option>
   				</select>  
            </td>   
        </tr>
        <tr>
			<td>Total Fruits Owned:</td>
            <td align="center"><input id="totalfruitsowned" type="text" /></td>
		</tr>
        <tr>
			<td>How many fruits are you buying:</td>
            <td align="center"><input id="howmanyfruits" type="text" /></td>
		</tr> 
        <tr>
        	<td>Money Needed to buy 1 fruit:</td><td><font id="cash"></font></td>
        </tr>
        <tr>
        	<td>Coupons Needed to buy 1 fruit:</td><td><font id="coupons"></font></td>
        </tr>                              
        <tr>
        	<td>Money Needed:</td><td><font id="cash_all"></font></td>
        </tr>
        <tr>
        	<td>Coupons Needed:</td><td><font id="coupons_all"></font></td>
        </tr>
                <tr>
        	<td>Nr. of fruits you can buy:</td><td><font id="fruitsmaxtobuy"></font></td>
        </tr>
        <tr>
        	<td align="center" colspan="2"><input type="button" value="Submit" onclick="Fruits()" /></td>
        </tr>    
    </tbody>
</table>
    </form>

<et>ReplaceOnlyrpg</et>

<et>ReplaceOnly</et>