1// 1. filter()
2function removeDuplicates(array) {
3  return array.filter((a, b) => array.indexOf(a) === b)
4};
5// 2. forEach()
6function removeDuplicates(array) {
7  let x = {};
8  array.forEach(function(i) {
9    if(!x[i]) {
10      x[i] = true
11    }
12  })
13  return Object.keys(x)
14};
15// 3. Set
16function removeDuplicates(array) {
17  array.splice(0, array.length, ...(new Set(array)))
18};
19// 4. map()
20function removeDuplicates(array) {
21  let a = []
22  array.map(x => 
23    if(!a.includes(x) {
24      a.push(x)
25    })
26  return a
27};
28/THIS SITE/ "https://dev.to/mshin1995/back-to-basics-removing-duplicates-from-an-array-55he#comments"

1// JavaScript - finds if there is duplicate in an array. 
2// Returns True or False.
3
4const isThereADuplicate = function(arrayOfNumbers) {
5    // Create an empty associative array or hash. 
6    // This is preferred,
7    let counts = {};
8    // // but this also works. Comment in below and comment out above if you want to try.
9    // let counts = [];
10
11    for(var i = 0; i <= arrayOfNumbers.length; i++) {
12        // As the arrayOfNumbers is being iterated through,
13        // the counts hash is being populated.
14        // Each value in the array becomes a key in the hash. 
15        // The value assignment of 1, is there to complete the hash structure.
16        // Once the key exists, meaning there is a duplicate, return true.
17        // If there are no duplicates, the if block completes and returns false.
18        if(counts[arrayOfNumbers[i]] === undefined) {
19            counts[arrayOfNumbers[i]] = 1;
20        } else {
21            return true;
22        }
23    }
24    return false;
25}

1[2,7,65,9,4,2]