Remove duplicates in an array using forEach

Question

Good Day, I am trying to remove duplicate element from an array using the forEach loop. But at this point in time i get some errors. Below are my codes

function removeDup(arr) {
    let result = arr.forEach((item, index) => { if (index > 1) item.shift() });
    return result;
}

I am not even sure if this code would work for removing duplicates because when i run it in a browser console i get this error

if (index > 1) item.shift(); ^

TypeError: item.push is not a function

First of all, how do i fix this error and secondly would this code work for removing duplicates?

item is element of array which won't have push method. Define result with empty array and then do result.push in foreach. However it looks that just checking index>1 won't eliminate duplicates. — binariedMe, Feb 19 '19 at 02:13
@LucienDubois I made a mistake. meant to write `item.shift`. edited the question — diagold, Feb 19 '19 at 02:15
No dude, item won't have shift method either. note that item is not array so it won't have array methods. — binariedMe, Feb 19 '19 at 02:16

binariedMe · Accepted Answer · 2019-02-19T02:24:46.077

5

You can try :

function removeDup(arr) {
  let result = []
  arr.forEach((item, index) => { if (arr.indexOf(item) == index) result.push(item) });
  return result;
}

Explanaton:

first initialize result with empty array. Then iterate through passed array, check if the index is its first occurance of item. push to result array. return result array.

Alternative solution :

function removeDup(arr) {
  let result = []
  arr.forEach((item, index) => { if (result.indexOf(item) === -1) result.push(item) });
  return result;
}

Explanation :

you can avoid checking for index as first occurance by checking if that element has already been pushed into result array or not.

edited Feb 19 '19 at 02:24

answered Feb 19 '19 at 02:15

binariedMe

4,309
1
18
34

obviously it won't work consistently with item being an object and not a primitive type. You might need to implement an equal function to find indexof – binariedMe Feb 19 '19 at 02:18
Nice solution. Works well. I can see you checked the `index` of each item with `forEach` loop and compared the `indexOf` that item to the `index`. Can you please explain your code? – diagold Feb 19 '19 at 02:19
@diagold editing to add explanation. – binariedMe Feb 19 '19 at 02:21
Alright . Thanks – diagold Feb 19 '19 at 02:24
I would suggest to try and go with alternative solution or if using es6, go with Tony's solution with Set. – binariedMe Feb 19 '19 at 02:25

arunmmanoharan · Answer 2 · 2019-02-19T02:29:23.000

2

function findDuplicates(data) {

  let result = [];

  data.forEach(function(element, index) {
    //Checks data.indexOf(1) which is 0 === 0, then push it to array
    //Now again when it checks the value of 1 which corresponds to a value of 1 but in forloop has an index of 5, so exits and duplicate values are not pushed
    if (data.indexOf(element) === index) {
      result.push(element)
    }
  });

  return result;
}

console.log(findDuplicates([1, 2, 3, 4, 5, 1, 2]))

edited Feb 19 '19 at 02:29

answered Feb 19 '19 at 02:17

arunmmanoharan

2,535
2
29
60

Can you please explain your code? – diagold Feb 19 '19 at 02:20
So you are looping over the array, and you are comparing whether the index of the current element in the array equals the index, push it into the new Array. – arunmmanoharan Feb 19 '19 at 02:22
Thats actually the part i dont really understand. The `indexOf` the element in the `array` equals what `index`? – diagold Feb 19 '19 at 02:24
I added some notes. If still confusing, throw the code in the browser console and use a debugger to debug. – arunmmanoharan Feb 19 '19 at 02:30

score 2 · Answer 3 · answered Feb 19 '19 at 02:17

Why not use a Set? e.g.

var arr = ['a', 'b', 'a']                                                                               
var unique = Array.from(new Set(arr))                                                                   
console.log(unique) // ['a', 'b']

score 1 · Answer 4 · edited Jun 20 '20 at 09:12

You can use filter it will return a new array set as your checked condition

The indexOf() method searches the array for the specified item, and returns its position.

The search will start at the specified position, or at the beginning if no start position is specified, and end the search at the end of the array.

Returns -1 if the item is not found.

If the item is present more than once, the indexOf method returns the position of the first occurence.

function removeDup(arr) {
    let result = arr.filter(function (item, pos) {return arr.indexOf(item) == pos});  
    // at first loop -> item = 1 -> so indexOf = 0 & pos = 0 -> so return data
    // at second loop -> item = 4 -> so indexOf = 1 & pos = 1 -> so return data
    // at third loop -> item = 5 -> so indexOf = 2 & pos = 2 -> so return data
    // at fourth loop -> item = 4 -> so indexOf = 1 & pos = 3 -> no return
    // at fifth loop -> item = 1 -> so indexOf = 0 & pos = 4 -> no return
    return result;
}

var d = removeDup([1,4,5,4,1]);
console.log(d);

Okay nice one but can you please explain your code? – diagold Feb 19 '19 at 02:28 — diagold, Feb 19 '19 at 02:28

Remove duplicates in an array using forEach

4 Answers4