Create new array with only the duplicate items in Javascript using for loops

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I need to write a function to find all the elements that appear more than once in the array and then return those elements in an array. Example: Input: [2,7,4,10,12,27,4,7,7,12, 10] Output: [4,7,10,12]

This is what i have so far:

let arr= [4,4,6,8,8,9,10,10]

var method1 = function(a) {
  var counts = [];
    for(var i = 0; i <= a.length; i++) {
        for(var j = i; j <= a.length; j++) {
            if(i != j && a[i] == a[j]) {
                counts.push(a[i]);
                }
              }
            }

    return counts;
}
console.log(method1(arr));

It works when the array only have two of a number, but not if there are more. How can I push to counts only one of each number independently of how many duplicates there are?

thanks for any help!


You could take a hash table for counting the values and if you have a count of two, take this element.

var array = [2, 7, 4, 10, 12, 27, 4, 7, 7, 12, 10],
    hash = Object.create(null),
    result = array.filter(v => (hash[v] = (hash[v] || 0) + 1) === 2);

console.log(result);

javascript return repeated items from an array only once Code , “javascript return repeated items from an array only once” Code Answer loop each to find duplicate numbers in array JS � var findDuplicate javascript given an array of integers, return only the values that have duplicates in the This is likely because you're using an outdated version of create-react-app. For finding duplicate values in JavaScript array, you’ll make use of the traditional for loops and Array reduce method. Using For Loop Let’s see how you can find duplicates in an array using for loop. The logic is you’ll separate the array into two array, duplicate array and unique array. You’ll iterate over the values in the array and push the items into unique list if it already doesn’t exist. It it already exists you have found a duplicate and hence push it into duplicate array. …


You use Array.prototype.filter() and compare indexOf() and lastIndexOf() of element to check if occurs more than once

const method1 = (arr) => [...new Set(arr.filter(elm => arr.indexOf(elm) !== arr.lastIndexOf(elm)))]
console.log(method1([2,7,4,10,12,27,4,7,7,12, 10]))

Handling Array Duplicates Can Be Tricky - DEV, Tagged with javascript, webdev, discuss. Now, what if we have an array of object duplicates? Considering what we've just read, it's not possible to use something simple like Define the resulting arrays ( unique and duplicates ); Loop through the source array; Try to locate the current item within the� JavaScript for loops iterate over each item in an array. JavaScript arrays are zero based, which means the first item is referenced with an index of 0. Referencing items in arrays is done with a numeric index, starting at zero and ending with the array length minus 1. The syntax to access an array member


Use a marker that would signal that item was already seen. It can be done with a dictionary

let arr= [4,4,6,8,8,9,10,10];

function solution(arr) {
  const marker = {};
  const duplicates = []
  for (const item of arr) {
    if (marker[item]) {
      duplicates.push(item);
    } else {
      marker[item] = true;
    }
  }
  return duplicates;
}

const s = solution(arr);
console.log(s);

How to Remove Duplicate Values from a JavaScript Array, How to Remove Duplicate Values from a JavaScript Array You can use the indexOf() method in conjugation with the push() remove the Alternatively, you can use the newly introduced ES6 for-of loop instead of for loop to perform How to display all items or values in an array using loop in jQuery � How to create a string� You can create array simply as – var arrayName = []. In this tutorial we are going to explain how you can create array and access it’s element. For loop is used to access the array elements in javascript. We will explain the for loop with example and demo so that we can understand how it works.


How to Remove Array Duplicates in ES6, Here are 3 ways to filter out duplicates from an array and return only the unique values The filter() method creates a new array of elements that pass the conditional we provide. In other words, if the Let's step in and walk through what happens as we loop through the array. 4 Ways to Combine Strings in JavaScript� Finally, If you want to get a distinct value in Javascript array with performance improvement then use the array = […new Set(array)] method. It does not make process slow. If you use filter() + indexOf() method then performance might get slow in the large amount of data. See also. Javascript array map() Javascript array foreach()


Check duplicate values in javascript array in 5 ways, Iterate over the array using a for loop. With ES6, we have a javascript Set object which stores only unique elements. A Set create a Set with array elements. let fruits = new Array(2); console.log(fruits.length); // 2 console.log(fruits[0]); // undefined Array constructor with multiple parameters. If more than one argument is passed to the constructor, a new Array with the given elements is created.


How to get all unique values (remove duplicates) in a JavaScript , notations � Lower and Upper Bound Theory � Analysis of Loops � Solving Recurrences Given an array which contains some values, the task is to remove the duplicate Method 2: Using array filter() method: The arr.filter() function is used to create a new array from existing array consisting of only those elements from the� if you want the array without the duplicates: const filtered = a.filter((val, i) => a.indexOf(val) === i); NB: these only work for arrays of primitive values – William Myers May 25 '17 at 13:44 If you use findIndex() instead of indexOf(), it will stop when it finds a match instead of continuing with the rest of the array. – bobpal Mar 1 at


PHP array_unique() Function, The array_unique() function removes duplicate values from an array. If two or more array Note: The returned array will keep the first array item's key type. ) and keys method, enables you to create a temporary array of size N to produce the indexes, and then a new array that can be assigned to your variable: var foo = [ Array(N).keys() ]; Fill/Map. You can first create the size of the array you need, fill it with undefined and then create a new array using map, which sets each element to the index.