Reversing string

JavaScript
1

I can not figure out why I do get “undefined” output in console log
Down below you can see my code that is trying to reverse a string. But unfortunately, I do not know why I do get “undefined” in the console log instead of every letter. I guess the logic should be correct but I am not 100% confident about it.

  **This is my code so far**


function reverseString(str) {
var string = str;
var newString = "";
for (let i = 0; i < string.length; i++) {
  newString += string[-1 + (-i)];
}
str = newString;
return str;
}

reverseString("hello");

  **Your browser information:**

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.114 Safari/537.36

Challenge: Reverse a String

Link to the challenge:

2

So do this in paper:

If i is 0, what is -1 + -0
If i is 1, what is -1 + -1

And so on

You can loop in any direction you know, you don’t have to start from zero and use ++.

3

It is most likely the logic :slight_smile:
For example: accessing the last character of a string looks like this

const str = "carrots"
const lastCharOfStr = str[str.length - 1]
console.log(lastCharOfStr)  //--> s

You tried to do it bit differently.

4

Thank you for you answer. I am used to calling last character of a string with index “-1” from Python and also the later ones “-2” (second to last character and so on), … . This does not work in javascript?

6

unfortunately JS is not so flexible in that department. index -1 is considered non existant. If you try [1,2,3,4].indexOf(5) it will return -1. Like other people pointed out, you can use a more old fashined approach and count string chars from back to front(starting from last char index and decrement the number as the loop goes)

7

By your way, the line in for loop should be
newString += string[string.length - 1 - i]
changing the index to the last index and decreasing for each character of the string

8

Yeah, doesn’t work like Python.

Arrays are basically just JS objects. There are a few special properties unique to arrays but on the whole you can think of

myArray = [10,20,30]

as

myArray = { "0": 10, "1": 20, "2": 30, length: 3 }

Bracket notation is how you access object properties. There isn’t a key “-1”, so myArray[-1] is undefined. It doesn’t do anything clever here, it’s just a basic object lookup.

Just as an FYI, at, which is not available yet but is likely to be in the language next year (and afaics has buy-in from every browser so should roll out everywhere very quickly one it’s approved, + is extremely easy to polyfill for browsers that don’t support it) lets you do what you’re looking for – myArray.at(-1) will do exactly what you’re expecting.

9

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