Why is this javascript variable outputting, "undefined," combined with the proper output?

Question

my son started learning javascript, and I have been trying to learn some as well, to help keep his interest up and answer his questions. I have a little C++ experience, and I was toying with the language and tried writing the below javascript, only to see that it keeps outputting "undefined" with whatever the correct answer is. I'd really appreciate anyone shedding some light on the matter for me. Thanks.

var word = prompt("Enter some text");
var reversedWord = reverseString(word);
alert(reversedWord);

function reverseString(text)
{
    var reversedText = "";
    var length = text.length;

    for(var i = length; i >= 0; i--)
    {
    reversedText += text[i];
    }
    return reversedText;
}

This codepen.io/Maniraj_Murugan/pen/abvjjXo should help you .. — Maniraj Murugan
– Maniraj Murugan, Commented May 13, 2020 at 16:47

Gopinath · Accepted Answer · 2020-05-13 18:03:36Z

1

Javascript uses the keyword 'undefined' to mean 'null'.

The problem observed in the above code is commonly referred as the 'Array index out of bound' error.

This is happening because of an attempt being made to access an element in an array from a position that does not exist.

In an array of n elements, the positions are numbered from 0 to n-1
I.E., from 0 to (length - 1).

Therefore, while writing a for-loop on an array, it must be ensured that the reference is made to positions from 0 to (length - 1) only.

The problem in the above code can be resolved by updating the for-loop to loop between (length - 1) and 0 only.

Here is the updated code that does not show 'undefined':

function reverseString(text) {
    var reversedText = "";
    var length = text.length;
    for(var i = (length -1); i >= 0; i--) {
        reversedText += text[i];
    }
    return reversedText;
}

answered May 13, 2020 at 18:03

Gopinath

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1 Comment

Jaws88 Over a year ago

Thanks. Dumb mistake on my part. Appreciate you explaining it!

Johnny Zabala · Accepted Answer · 2020-05-13 16:42:57Z

1

The property length correspond to the amount of elements in the string. For example 'you'.length === 3. But the position of the letters is zero based (0, 1 and 2).

In your example when you say i = length -> i === 3, and text[3] === undefined.

Set the variable length to: var length = text.length - 1; and it will work.

answered May 13, 2020 at 16:42

Johnny Zabala

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Comments

Josiah Powell · Accepted Answer · 2020-05-13 16:47:07Z

1

the array goes from 0 to length -1 so text[length] is undefined try:

for(var i = length -1; i >= 0; i--)
{
reversedText += text[i];
}

answered May 13, 2020 at 16:47

Josiah Powell

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Comments

Quentin · Accepted Answer · 2020-05-13 16:42:52Z

0

You start at the index equal to the length of the string.

So given:

"Foo"
 012

It has a length of three, so you start at index 3, which is undefined.

You need to start at var i = length - 1

answered May 13, 2020 at 16:42

Quentin

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