Exchanging last letter of a word using the 'replace() method

Question

I’m doing an exercise that asks me to turn the word Roberto in Roberta using the method replace(), and by my logic this solution should work:

var roberto = 'Roberto';

roberto.replace( roberto.charAt(6), 'a' );
// Raberto

Using the charAt() and passing the letter position by parameter works smoothly with all other letters of the word, but when I step to position 6, that would be just the letter I need (the last in the case) it takes the second letter instead of the last. Would anyone know to tell me why this occurs?

OBS.: The solution HAS than using the method replace().

Answer 1

This is happening because the letter the is repeated twice. One at position 2 and the other at position 6. This way replace the first occurrence only, not the second or third.

You will not succeed using pure texts, as shown in your example. You may have this same problem several times and it is not interesting to go rogue to get the last occurrence only.

Alternatively, using the replace, you can use regular expression for this, looking for any character at the end of the string.

var roberto = 'Roberto';

roberto = roberto.replace(/.$/, "a");

console.log(roberto);

Answer 2

If you have to use this function the only way is to use a Regex to determine a pattern that does what you want.

You can’t pass the position and you can’t just pass which character will be changed because it will do so in the first one you find.

Then you need to find a regular expression pattern that finds the last character to swap, in case the $ does it for you.

var roberto = 'Roberto';
console.log(roberto.replace(/o$/, 'a'));

I put in the Github for future reference.

Has documentation to study everything you can do with Regex. It’s very powerful and every pattern you want can be found, even if it’s a little complex to create something like this.

I prefer to do this in a more performative way without using the replace(), precisely for this reason and for being less readable, but there is the solution according to the requirement.