Filter an Array of strings using Filtered String and Remove from Original Array

I want to remove some elements from the array containing the word Evil (filterString).

let guests = ["Partner", "Evil Nice Relative 1", "Nice Relative 2", "Evil One", "another evil", "another one", "another evil is here", "strange Evil is here", "someone Nicer", "Ugly Evil Bad"];

const filteredArray = [];
const filterString = "Evil";

function checkEvil() {
    guests.filter((element, index) => {
        if (element.toLowerCase().indexOf(filterString.toLowerCase()) !== -1) {
            console.log(index);
            guests.splice(index,1);
        } else {
            filteredArray.push(element);
        }
    });
    console.log(guests);
}

Here is what I get for the original array (guests):

    ['Partner', 'Nice Relative 2', 'another evil', 'another one', 'strange Evil is here', 'someone Nicer']

Just want the guests array updated once the desired string (Evil) is filtered.

Since you want to mutate the original array then you can do as:

let guests = [
  "Partner",
  "Evil Nice Relative 1",
  "Nice Relative 2",
  "Evil One",
  "another evil",
  "another one",
  "another evil is here",
  "strange Evil is here",
  "someone Nicer",
  "Ugly Evil Bad",
];

const filterString = "Evil";

function checkEvil() {
  for (let i = guests.length - 1; i >= 0; i--) {
    const element = guests[i];
    if (element.toLowerCase().indexOf(filterString.toLowerCase()) !== -1) {
      guests.splice(i, 1);
    }
  }
  console.log(guests);
}

checkEvil();

1) You can easily achieve the result using filter and match as:

const arr = [
  "Partner",
  "Nice Relative 2",
  "another evil",
  "another one",
  "strange Evil is here",
  "someone Nicer",
];
const result = arr.filter((s) => !s.match(/evil/i));
console.log(result);

2) You can also do this using forEach and match as:

let guests = [
  "Partner",
  "Evil Nice Relative 1",
  "Nice Relative 2",
  "Evil One",
  "another evil",
  "another one",
  "another evil is here",
  "strange Evil is here",
  "someone Nicer",
  "Ugly Evil Bad",
];

const filteredArray = [];
const filterString = "Evil";

function checkEvil() {
  guests.forEach(element => {
    if (!element.match(/evil/i)) filteredArray.push(element);
  });
}
checkEvil();
console.log(filteredArray);