Javascript - 为什么我的功能出现NAN错误

Question

<script type="text/javascript">
  var sheep = prompt("How many sheeps do we have in the park?");
  var rangers = prompt("How many rangers do we have in the park?");

  function feedPerRanger (sheep, rangers) {
     alert("Each Park Ranger should load " + sheep*2/rangers + 
           " lb(s) of feed into his/her BART today.");
  }

  feedPerRanger ();
</script>

Hello

I'm still a beginner so this is probably a rookie mistake. The script is suppose to alert the user as to how much feed each park ranger is suppose to load on to his/her BART. It's suppose to be 2 lbs per every sheep divided by the number of rangers. I keep getting the following NAN error. Can anyone explain why?

"Each Park Ranger should load NaN lb(s) of feed into his/her BART today."

Answer 1

Change this:

function feedPerRanger (sheep, rangers) {
    alert("Each Park Ranger should load " + sheep*2/rangers + " lb(s) of feed into his/her BART today.");
}

with this:

function feedPerRanger () {
    alert("Each Park Ranger should load " + sheep*2/rangers + " lb(s) of feed into his/her BART today.");
}

Now it works; but (in my opinion) it is not the better way to do what you want.

You may use your function properly, passing two generic parameter as following:

function feedPerRanger(sheep, rangers) {
    alert("Each Park Ranger should load " + sheep*2/rangers + " lb(s) of feed into his/her BART today.");
}

var sheep = prompt("How many sheeps do we have in the park?");
var rangers = prompt("How many rangers do we have in the park?");

feedPerRanger(sheep, rangers);

Answer 2

Edit - as Ian has suggested change the names of the parameters.

You should pass the sheep and rangers variables to the function. And you should also check if they contain numbers. Consider this:

var sheep = prompt("How many sheeps do we have in the park?");
// sheep = "some random string";
// alert ( sheep / 2 );

I can press "h" or write whatever I want. Look at prompt from MDN

result (of prompt) is a string containing the text entered by the user, or the value null.

So add a little check if the variable is not a number with isNaN like this:

isNaN(sheep)

Also I can press Cancel in the prompt , then sheep or rangers will be null . If you change the function like this, there will be no chance to have another NaN :

function feedPerRanger(sheep, rangers) {
    if ((sheep && !isNaN(sheep)) && (rangers && !isNaN(rangers))) {
        alert("Each Park Ranger should load " + sheep * 2 / rangers +
            " lb(s) of feed into his/her BART today.");
    } else {
        alert("please use numbers");
    }
}

var sheepCount = prompt("How many sheeps do we have in the park?");
var rangersCount = prompt("How many rangers do we have in the park?");

feedPerRanger(sheepCount, rangersCount);

Fiddle

Answer 3

Your function definition accepts two parameters.

function feedPerRanger (sheep, rangers)

You give none in your call:

feedPerRanger ();

(Additionally, there's no space between the name of the function and the parentheses)

When you create your function definition, you're giving names to local variables. That is, sheep and ranger are the names given to the parameters passed so that they can be used in the function. So, when you create the function definition, sheep and ranger are not the variables you created above.

There are two ways to fix this.

1. Remove your parameters from the function definition:

    function feedPerRanger(){...} 

2. Pass in your variables as parameters to the function:

    function feedPerRanger(numSheep, numRangers){ alert("Each Park Ranger should load " + numSheep*2/numRangers + " lb(s) of feed into his/her BART today."); } feedPerRanger(sheep, rangers)