Catching White Spaces in Javascript Code

Question

In the following program when only the White Spaces are entered, it shows ex2 exception value less than 5, instead of showing ex4 exception This is not a valid number, I couldn't understand the logic behind it.

<html>
<head>
    <title></title>
    <script type="text/javascript">
        function promptCheck() {

            var val=prompt("Enter a Number between 5 and 10","");

            try {
             if(val=="") {
                 throw "ex1";
             }

             else if(val<5) {
                 throw "ex2";
             }

             else if(val>10) {
                 throw "ex3";
             }

             else if(isNaN(val)) {
                 throw "ex4";
             }
            }

            catch(err) {
                if(err=="ex1") {
                    alert("You have not entered any value");
                }
                 if(err=="ex2") {
                    alert("Value less than 5");
                }
                 if(err=="ex3") {
                    alert("Value greater than 10");
                }
                 if(err=="ex4") {
                    alert("This is not a valid number");
                }
            }

        }
    </script>
</head>

<body>
    <input type="button" value="Bring Mouse on Me!" onmouseover="promptCheck()" />
</body>
</html>

score 2 · Answer 1 · edited May 23 '17 at 11:48

2

In a numeric context, white-space convert to a zero. And zero is clearly lower than 5.

alert('  ' * 1);    // Shows 0

To solve the problem, you can use parseFloat, which will print NaN for spaces.
Another option is to use RegExps, to ensure that the input consists of integers:

var val = prompt("Enter a Number between 5 and 10", "");
val = /\d+/.exec(val); // Numbers if valid, null otherwise

// OR, instead of the previous line:
if (/\D/.test(val) { // If the input contains a non-digit character, error.
    throw "ex4";
} else ...

For string-to-number conversion, see this comparison of number-conversion methods. You can see what happens for a given input.

edited May 23 '17 at 11:48

Community

1
1

answered Feb 11 '12 at 14:37

Rob W

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It should be added that reordering the `else`s would solve the problem. – moteutsch Feb 11 '12 at 14:39
Ok... fair enough. Now what logic is applied to catch White Spaces? – sandbox Feb 11 '12 at 14:41
@moteutsch That will not solve the problem, because `isNaN` will check whether white-space, that is, zero, is `NaN`. That's false. – Rob W Feb 11 '12 at 14:41
Yes, but that's not exactly what I meant. I meant that you didn't actually give a solution, you only mentioned part of the problem. – moteutsch Feb 11 '12 at 14:47
@moteutsch Not a solution? Have you missed that block of code at the middle of my answer? Note: I have shown two possible solutions, on a separate line (up and down from `// OR, instead ... `. If you want to reject any non-number characters, use method two. – Rob W Feb 11 '12 at 14:49
I posted my original comment before you posted the second code sample. – moteutsch Feb 11 '12 at 14:54

score 2 · Accepted Answer · answered Feb 11 '12 at 14:42

2

That happens because string with only whitespaces is treated as empty string and it it is converted to 0.

So

"    "*1 // => 0

What you need to do is to parse value beforehand:

var value = parseInt(val, 10); // would be NaN in case of empty string

answered Feb 11 '12 at 14:42

Ilya Tsuryev

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Catching White Spaces in Javascript Code

2 Answers2