indexOf method in JavaScript

Question

So I have the line

var a = document.cardform.cardnumber.value.toString;

Then later:

else if (a.indexOf(' ')!=0 ||a.indexOf('+')!=0 || a.indexOf('-')!=0){

This checks that the user hasn't entered any unwanted +'s, -'s, or ' 's. The line seems to raise an error that looks like this:

Uncaught TypeError: undefined is not a function.

I'm confused as to what it's telling me. Is it saying undefined isn't a function, or that 'a' is an incompatible data type? How do I fix this problem?

It's telling you that the function String.prototype.toString doesn't have an indexOf property. — user400654
– user400654, Commented Mar 10, 2015 at 22:19
toString is a function, but in any case, value is already a string. — elclanrs
– elclanrs, Commented Mar 10, 2015 at 22:19
Also, you probably want to check whether indexOf returns -1. It will return 0 if that character is the first character in the string (assuming you actually call toString first). — Austin Mullins
– Austin Mullins, Commented Mar 10, 2015 at 22:20
The built-in (on modern browsers) trim method on strings may be of more use. var a = document.cardForm.cardnumber.value.trim(); — user400654
– user400654, Commented Mar 10, 2015 at 22:21

Adjit · Accepted Answer · 2015-03-10 22:23:51Z

2

You need to actually call the toString() function.

var a = document.cardform.cardnumber.value.toString();

Now, a is a string and has the method .indexOf()

answered Mar 10, 2015 at 22:23

Adjit

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2 Comments

recursive recursion Over a year ago

Thank you! I feel stupid now, spent 30 minutes and all it was was a set of parentheses...

Adjit Over a year ago

No problem, but as others noted, when getting the value of an input field you don't need to call the toString() function.

dfsq · Accepted Answer · 2015-03-10 22:30:48Z

1

If you really want to use toString method, then you need to call it, otherwise a is a function which doesn't have any indexOf method.

But you don't really need to do this, because document.cardform.cardnumber.value is already a string. Remember that form elements values are always strings.

Finally you can make a check much simpler with a basic regular expression:

else if (/[-+\s]/.test(a)) {

answered Mar 10, 2015 at 22:30

dfsq

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