JavaScript - How To Initialize A Negative Value?

If a negative number is entered into the reading variable it should be displayed as a zero. This works with the first calculations such as average but right at they very end the the alert wont display the highest average it just displays the one which has a negative number typed in to it.
Code:

 var region=0
 var reading=0
 var average=0
 var count=0
 var readingSum=0
 var catergory=0
 var dry=0
 var normal=0
 var wet=0
 var countWet=0
 var countNormal=0
 var countDry=0
 var totalLoop=0
 var numLocations=0
 var maxAverage=0
 var maxRegion=0
 var maxReadings=0
 var totalReadings=0

     // ** Find out how many locations ** //
     
     numLocations=eval(prompt("How many locations are you taking readings for?"));

     // ** Loop for multiple entries ** //
     
      for(totalLoop=1; totalLoop<=numLocations; totalLoop=totalLoop+1)
      {
      
      // ** Reset total for loop ** //
      
       readingSum=0

     // ** Get Region ** //
     
     region = prompt("Enter region please");
     
     // ** Get Readings ** //
     
     totalReadings = eval(prompt("Enter readings please"));
    
     // ** Get readings and Prevent negative number ** //
     if(totalReadings<=0)
     {
     alert("Please enter valid reading")
     }
     else
     {
     alert("You entered " + totalReadings);
     }
     
     // ** Ask for indivdual readings ** //
     
      for(count=1; count<=totalReadings; count=count+1)
      {
       
       reading=eval(prompt("Enter reading please"));
       
       // ** Prevent negative number ** //
        if(reading<0)
     {
     reading=0
     alert("Please enter valid reading")
     }
        // ** Work out Total of readings ** //
       
      readingSum= readingSum + reading
      }
      
    // ** Work out average ** //  
    average= readingSum/totalReadings
 
    // ** Put average into a category and display everything ** //
     if(average<5)
     {
     catergory="dry"
     countDry=countDry+1
     }
     else if(average>=5 && average<20)
     {
    catergory="normal"
    countNormal= countNormal+1
     }
     else if(average>20)
     {
     catergory="wet"
     countWet=countWet+1
     }
     
     
    alert("Location is " + region + "\nNumber of weakly readings taken is " + totalReadings + "\nTotal of readings is " + readingSum + "\nAverage is " + average + "\nLocation is " +  catergory);
     
     // ** Find Highest average ** //
     
     if((average>maxAverage) || (totalReadings>maxReadings))
     {
     maxAverage=average
     maxRegion=region
     }
     }
     
     // ** Final Report ** //
     
     alert("Number of locations is " + numLocations + "\nDry is " + countDry + "\nNormal is " + countNormal + "\nWet is " + countWet + "\nLocation with highest average reading is " + maxRegion + "\nHighest average is " + maxAverage);

Do i need a while loop? i am only allowed to use while, for and if statements

I'm just starting out learning and have sort of given myself a couple of basic problems to solve. I pretty quickly found something that's apparently hard to Google.

I've made a page that shows a counter number, starting at 0, with buttons that will increase or decrease the number by 1 or 10.

So I'd like to figure out how to show that number in red when it is a negative number and in green when it is a positive number. I've tried a couple of things, including an if... else, but here's my latest crack:

Code:

var counter=0;

function countUp() {
	lastCounter=counter;
	counter++;
	display();
}
function countDown() {
	lastCounter=counter;
	counter--;
	display();
}
function tenUp() {
	lastCounter=counter;
	counter+=10;
	display();
}
function tenDown() {
	lastCounter=counter;
	counter-=10;
	display();
}
function lastNumber() {
	counter=lastCounter
	document.getElementById("number").innerHTML=lastCounter;
}

function display () {
	number=counter+'';
	switch(counter) {
		case counter== 0;
			document.getElementById("number").innerHTML=number.fontcolor(white);
			break;
		case counter> 0;
			document.getElementById("number").innerHTML=number.fontcolor(green);
			break;
		case counter< 0;
			document.getElementById("number").innerHTML=number.fontcolor(red);
			break;
	} //end switch
} //end display

I suspect that what I'm doing totally wrong has to do with how I'm describing the case or with calling one function inside another function, since it works up until I start messing with that (had the document.getElementByID stuff in the countUp, countDown, etc., functions before) but I don't know. Thanks in advance for any help you can offer.