How to check if two numbers are equal in javascript

Question

Let's say we have an array: let arr = [10,10,5,11,5]

How could I check for all the numbers that are equal? ( basically duplicates )

What i thought of is a forEach and compare every number but that's not very optimal. A filter ? Maybe. Anyone has any good ideea that would be optimal?

What do you mean by "all the numbers that are equal"? You want the unique numbers in the array? You want to know if they're *all* equal? Or...? — Dave Newton, Mar 05 '19 at 22:19
What is your desired output? An array without any duplicate elements? — 4castle, Mar 05 '19 at 22:19
I'm not sure who's downvoting all the answers, but it's non-sensical: the problem statement isn't sufficient--if anything the question should be downvoted. — Dave Newton, Mar 05 '19 at 22:20
@ChrisD A `filter` also iterates and runs a comparison. You need to be more specific with your problem statement. — Dave Newton, Mar 05 '19 at 22:21

score 0 · Answer 1 · answered Mar 05 '19 at 22:21

You can use reduce to build down the values you need

let arr = [10, 10, 5, 11, 5];

let duplicates = getDuplicates(arr);

console.log(duplicates);

function getDuplicates(arr) {
  let duplicates = arr.reduce((acc, item) => {
    if(acc[item] !== undefined) {
      acc[item] = true;
    } else {
      acc[item] = false;
    }
    return acc;
  }, {});
  
  return Object.keys(duplicates).filter(item => duplicates[item]);
}

Adrian Brand · Answer 2 · 2019-03-05T22:30:44.380

You can use filter

let arr = [10,10,5,11,5];

const duplicates = array => array.filter((item, index) => array.some((i, idx) => i === item && idx < index));

console.log(duplicates(arr));

or reduce

let arr = [10,10,5,11,5];

const duplicates = array => array.reduce((results, item, index) => {
  if (!results.some(i => i === item) && array.some((i, idx) => i === item && index !== idx)) {
    results.push(item);
  }
  return results;
}, []);

console.log(duplicates(arr));

score -1 · Answer 3 · answered Mar 05 '19 at 22:17

You can use a Set to remove the duplicates and then compare the lengths of the two arrays.

let arr1 = [10,10,5,11,5]
let arr2 = [10,11,5]

function hasDuplicates(arr) {
  return arr.length != [...new Set(arr)].length
}

console.log(hasDuplicates(arr1))
console.log(hasDuplicates(arr2))

Arthur · Answer 4 · 2019-03-05T22:28:19.533

Really quick with every

let arr = [10,10,5,11,5]
//arr = [10,10,10,10]
if (arr.every( v => v === arr[0] )) {
  console.log("yes")
} else {
  console.log("no")
}

Or using Lodash

let arr = [10,10,5,11,5]
//arr = [10,10,10,10]
if (_.uniq(arr).length == 1) {
  console.log("yes")
} else {
  console.log("no")
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.11/lodash.min.js"></script>

How to check if two numbers are equal in javascript

4 Answers4