Alert user whether each array value is number or not

Question

My goal is to create an array, and when a button is clicked, it goes through each individual item of the array and alerts whether it's a number or not.

<body>
    <form id = "form">
        <p id = "instruction">Enter all values in the textbox, separated by a space.</p>
        <textarea id = "textarea" rows = "8" columns = "60"></textarea>
            <input id = "submit" type = "submit" value = "Sort Alphabetically" onclick = "stringSplitAlpha();">
            <p id = "p"></p>
        </form>
<script>
function stringSplitAlpha() {
    var str = document.getElementById("textarea").value;
    var arr1 = str.split(" ");
    for (var i = 0; i < arr1.length; i++) {
        window.alert(isNaN(arr1[i]));
        if (isNaN(arr1[i]) == 'true') {
            window.alert(arr1[i] + "is not a number.");
        } else {
            window.alert("arr1[i] + "is a number.");
        }
    }
}
</script>
</body>

When I run the code, everything works fine at the beginning. I put in some values in the textbox for the array, and when I click the button, window.alert(isNaN(arr1[i])); shows "true" on a letter and "false" on a number. This is expected.

However, when it says "true" for a letter, the next alert shows "... is a number." This comes from

if (isNaN(arr1[i]) == 'true') {
            window.alert(arr1[i] + "is not a number.");
        } else {
            window.alert("arr1[i] + "is a number.");
        }

, but is totally unexpected. window.alert(isNaN(arr1[i])); came up as "true" so shouldn't it say "... is not a number"? I was stumped here. I thought maybe there was some sort of syntax error, but there doesn't seem to be any. I tried both double and triple equal signs, to no avail.

Just to clarify, if I put in "1 34 hello" in the textbox, this is what would show up in the alerts, in order:

"false," "1 is a number," "false," "34 is a number," "true," "hello is a number". Only the last alert of "hello is a number" is incorrect.

Thanks in advance.

Jon · Accepted Answer · 2020-07-28 13:43:02Z

1

The isNaN function returns a boolean value, not a string. Instead of comparing it to 'true', try it without the comparison. The other option would be to remove the quotation marks from around true and your original example should work. I further condensed it below since the best practice way you would see this in code is to omit the comparison for a boolean value.

if (isNaN(arr1[i])) {
  window.alert(arr1[i] + " is not a number.");
} else {
  window.alert(arr1[i] + " is a number.");
}

Further Documentation: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN

edited Jul 28, 2020 at 13:43

answered Jul 28, 2020 at 13:36

Jon

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2 Comments

Sirswagger21 Over a year ago

Yes! It worked! Will accept it when I can. One question - how does the if statement work without any sort of "true" or "false" statement, just the isNaN(arr1[i]) itself?

Jon Over a year ago

I'm happy it worked for you! It works because the if statement is designed to evaluate true vs false automatically. A true evaluation will always go into the "if" block and a false evaluation will always go into the "else" block. A boolean value will always be true or false which is what is returned from isNaN.

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