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JavaScript isNaN() Function Topic


MichalB
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I would like to suggest to change the example in the JavaScript isNaN() Function topic (https://www.w3schools.com/jsref/jsref_isnan.asp) to the one below.

I have added parseFloat  of every test example.  It shows that sometimes isNaN returns false, but parseFloat returns NaN.  Interesting might be also treatment of the date.

 

 

<!DOCTYPE html>
<html>
<body>

<p>The isNaN() function returns true if the value is NaN (Not-a-Number), and false if not.</p>

<p>Click the button to check whether a number is an illegal number.</p>

<button onclick="myFunction()">Try it</button>

<p id="demo"></p>

<script>
function myFunction() {
  var res = "";
  res = res + isNaN(123) + ": 123 -> " + parseFloat(123) + "<br>";
  res = res + isNaN(-1.23) + ": -1.23 -> " + parseFloat(-1.23) + "<br>";
  res = res + isNaN(5-2) + ": 5-2 -> " + parseFloat(5-2) + "<br>";
  res = res + isNaN(0) + ": 0 -> " + parseFloat(0) + "<br>";
  res = res + isNaN('123') + ": '123' -> " + parseFloat('123') + "<br>";
  res = res + isNaN('Hello') + ": 'Hello' -> " + parseFloat('Hello') + "<br>";
  res = res + isNaN('2005/12/12') + ": '2005/12/12' -> " + parseFloat('2005/12/12') + "<br>";
  res = res + isNaN('') + ": '' -> " + parseFloat('') + "<br>";
  res = res + isNaN(true) + ": true -> " + parseFloat(true) + "<br>";
  res = res + isNaN(undefined) + ": undefined -> " + parseFloat(undefined) + "<br>";
  res = res + isNaN(null) + ": null -> " + parseFloat(null) + "<br>";
  res = res + isNaN('NaN') + ": 'NaN' -> " + parseFloat('NaN') + "<br>";
  res = res + isNaN(NaN) + ": NaN -> " + parseFloat(NaN) + "<br>";
  res = res + isNaN(0 / 0) + ": 0 / 0 -> " + parseFloat(0 / 0) + "<br>";

  document.getElementById("demo").innerHTML = res;
}
</script>

</body>
</html>

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parseFloat does something different than what isNaN checks for.  isNaN converts to a Number, and then checks if that is NaN.  Converting to a Number is not the same as using parseFloat:

var res = "";
  res = res + isNaN(123) + ": 123 -> " + Number(123) + "\n";
  res = res + isNaN(-1.23) + ": -1.23 -> " + Number(-1.23) + "\n";
  res = res + isNaN(5-2) + ": 5-2 -> " + Number(5-2) + "\n";
  res = res + isNaN(0) + ": 0 -> " + Number(0) + "\n";
  res = res + isNaN('123') + ": '123' -> " + Number('123') + "\n";
  res = res + isNaN('Hello') + ": 'Hello' -> " + Number('Hello') + "\n";
  res = res + isNaN('2005/12/12') + ": '2005/12/12' -> " + Number('2005/12/12') + "\n";
  res = res + isNaN('') + ": '' -> " + Number('') + "\n";
  res = res + isNaN(true) + ": true -> " + Number(true) + "\n";
  res = res + isNaN(undefined) + ": undefined -> " + Number(undefined) + "\n";
  res = res + isNaN(null) + ": null -> " + Number(null) + "\n";
  res = res + isNaN('NaN') + ": 'NaN' -> " + Number('NaN') + "\n";
  res = res + isNaN(NaN) + ": NaN -> " + Number(NaN) + "\n";
  res = res + isNaN(0 / 0) + ": 0 / 0 -> " + Number(0 / 0) + "\n";

console.log(res)

That shows the results that are expected.  I agree that isNaN should be associated with parseFloat and parseInt though, because that's usually when you need to test.  I typically convert first and then test to see if that result is NaN instead of using isNaN on the value to be converted.

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