CODEWITHSUNDEEP
javascriptnan
darsie
darsie

Reputation: 147

NaN comparison returns true?

The last days I read how NaN always compares false even with itself and how to compare stuff when NaN may occur, and now I made a JS that compares two NaN true. WTF? Or did I compare 'NaN' strings?

http://www.bksys.at/bernhard/JS-NaN-compare-true.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>radioactivity calculator</title>
</head>

<body>

<form name="form1">
a: <input type="text" name="a"><br>
b: <input type="text" name="b"><br>
x: <input type="text" name="x"><br>

</form>

<script type="text/javascript">

    document.form1.a.value=Math.sqrt(-1);
    document.form1.b.value=(1/0)/(1/0);
    document.form1.x.value=(document.form1.a.value==document.form1.b.value);

</script>

</body>
</html>

Upvotes: 1

Views: 776

Answers (3)

James Donnelly
James Donnelly

Reputation: 128791

You are indeed comparing the string "NaN" against another string "NaN", which equates to true. The value held in text input elements is always pulled as a String type.

A simple way to resolve this is to prefix your values with the Unary Plus (+) operator to convert them to integer values (you can drop those brackets, too):

document.form1.x.value = +document.form1.a.value == +document.form1.b.value;

Example

document.form1.a.value = Math.sqrt(-1);
document.form1.b.value = (1/0) / (1/0);
document.form1.x.value = +document.form1.a.value == +document.form1.b.value;
<form name="form1">
  a: <input  type="text" name="a" size="20" value="a"><br>
  b: <input  type="text" name="b" size="20" value="b"><br>
  x: <input  type="text" name="x" size="20" value="x"><br>
</form>

Note: As RobG pointed out in his comment below it's important to note here that converting the string value "NaN" to an integer with the Unary Plus operator converts it directly to NaN because the string cannot be replicated as a numeric value. The same would happen if both of your input elements contained the value "Foo" - or even contained two completely different non-numeric string values. Whilst this solution does work, it may yield undesired results if you are to extend this code to handle non-numeric values as well.

Upvotes: 3

Curtis Mattoon
Curtis Mattoon

Reputation: 4742

NaN is a special value in JavaScript. It doesn't even equal itself (also a quick way to test):

var a = parseInt('seven');
if (a == a) {
    alert("a == a"); 
} else {
    alert("a != a"); // This will happen
}
if (a == 'NaN') {
    // Won't happen
} else {
    alert("NaN is not equal to the string 'NaN'"); // Here
}

http://jsfiddle.net/u951v90o/

Upvotes: -1

Norman Breau
Norman Breau

Reputation: 2417

This is a JavaScript gotcha ;)

The proper way to compare NaN is to use the isNaN method.

var a = 'a' + 5; //NaN

if (isNaN(a)) {
    //do something
}

Upvotes: 0

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