How do I change the last letter of a sting based on a user prompt

// Returns the number and pluralized form, like "5 cats" or "1 dog", given
// a noun and count. However, there are a few exceptions. In this exercise,
// add "es" in the following cases:
// 1. the given noun ends with 'o'. For example, '5 potatoes'.
// 2. the noun ends in "f" or "fe" change the "f" to a "v" and add "-es."
// 3. the noun ends in "y", change the "y" to a "i" and add "-es."
// HINT: to replace the last character, you can use `replace()` with Regex.
// For example, if you would like to replace the last 'e' with 'o' in a string `str`,
// you can use `str.replace(/e$/, 'o')`.
if ((count > 1) || (count < 0) || (count == 0) && slice(-1) == o$) {

  result = count + " " + noun.replace(/o$/, "oe") + "s";
}


if ((count > 1) || (count < 0) || (count == 0) && slice(-1) == f$) {

  result = count + " " + noun.replace(/f$/, "v") + "es";
}


if ((count > 1) || (count < 0) || (count == 0) && slice(-1) == y$) {

  result = count + " " + noun.replace(/y$/, "i") + "es";
}


else(result = count + " " + noun)



// DO NOT CHANGE THIS.
console.log(result);

0 degrees
Failed

0 thieves
Failed

1 thief
Passed

1 family
Passed

2 families
Passed

1 party
Passed

1 apple
Passed

2 apples
Failed

10 balloons
Failed

1 balloon
Passed

3 knife
Failed

2 potatoes
Passed

10 tomatoes
Passed

2 parties
Passed

-40 degrees
Failed