Several methods to deduplicate an array.

deduplicate-array.js

let arr = [1, 5, 'a', 3, 'f', '3', 5, 3, 'b', 'e', 'f'];
let ans = deduplicate(arr);

console.log(ans);
// Expected output: [1, 5, "a", 3, "f", "3", "b", "e"]

function deduplicate(arr) {
  let isExist = (arr, x) => {
    for(let i = 0; i < arr.length; i++) {
      if (arr[i] === x) return true;
    }
    return false;
  }

  let ans = [];
  arr.forEach(element => {
    if(!isExist(ans, element)) ans.push(element);
  });
  return ans;
}

deduplicate-using-filter-indexof.js

let arr = [1, 5, 'a', 3, 'f', '3', 5, 3, 'b', 'e', 'f'];
let ans = deduplicate(arr);

console.log(ans);
// Expected output: [1, 5, "a", 3, "f", "3", "b", "e"]

function deduplicate(arr) {
  return arr.filter((value, index, arr) => arr.indexOf(value) === index);
}

deduplicate-using-includes.js

let arr = [1, 5, 'a', 3, 'f', '3', 5, 3, 'b', 'e', 'f'];
let ans = deduplicate(arr);

console.log(ans);
// Expected output: [1, 5, "a", 3, "f", "3", "b", "e"]

function deduplicate(arr) {
  let isExist = (arr, x) => arr.includes(x);
  let ans = [];

  arr.forEach(element => {
    if(!isExist(ans, element)) ans.push(element);
  });
  
  return ans;
}

deduplicate-using-indexof.js

let arr = [1, 5, 'a', 3, 'f', '3', 5, 3, 'b', 'e', 'f'];
let ans = deduplicate(arr);

console.log(ans);
// Expected output: [1, 5, "a", 3, "f", "3", "b", "e"]

function deduplicate(arr) {
  let isExist = (arr, x) => arr.indexOf(x) > -1;
  let ans = [];

  arr.forEach(element => {
    if(!isExist(ans, element)) ans.push(element);
  });
  
  return ans;
}

deduplicate-using-set-array.from.js

let arr = [1, 5, 'a', 3, 'f', '3', 5, 3, 'b', 'e', 'f'];
let ans = deduplicate(arr);

console.log(ans);
// Expected output: [1, 5, "a", 3, "f", "3", "b", "e"]

function deduplicate(arr) {
  let set = new Set(arr);
  return Array.from(set);
}

deduplicate-using-set-spread.js

let arr = [1, 5, 'a', 3, 'f', '3', 5, 3, 'b', 'e', 'f'];
let ans = deduplicate(arr);

console.log(ans);
// Expected output: [1, 5, "a", 3, "f", "3", "b", "e"]

function deduplicate(arr) {
  let set = new Set(arr);
  return [...set];
}