Check if each item in an array is identical in JavaScript

Question

I need to test whether each item in an array is identical to each other. For example:

var list = ["l","r","b"]

Should evaluate as false, because each item is not identical. On the other hand this:

var list = ["b", "b", "b"]

Should evaluate as true because they are all identical. What would be the most efficient (in speed/resources) way of achieving this?

Does this answer your question? [Check if all values of array are equal](https://stackoverflow.com/questions/14832603/check-if-all-values-of-array-are-equal) — Heretic Monkey, Feb 07 '22 at 20:01

Dogbert · Accepted Answer · 2016-03-22T08:16:45.237

17

function identical(array) {
    for(var i = 0; i < array.length - 1; i++) {
        if(array[i] !== array[i+1]) {
            return false;
        }
    }
    return true;
}

edited Mar 22 '16 at 08:16

answered Mar 10 '12 at 13:53

Dogbert

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I opted for this solution, thanks. Pimvdb had a very elegant solution with array.every but that would of required adding a big chunk of code (for non-ES5 compliant browser support) to an already large document for just one instance of .every so I opted for this instead. – Nick Mar 10 '12 at 17:13
1

Your solution made the most sense to me. However, I'd change the "!=" to "!==" because your solution returns true in the following array [1,1,1,1,1,1,"1"] where the last character is a string. – Joffrey Baratheon Mar 21 '16 at 22:34
I'd like to see something similar but for objects. – MadHatter Oct 09 '17 at 05:15
Nice solution in traditional way. ES5 introduced Array.every(), which could be used for one linear solution as: https://stackoverflow.com/questions/9646943/check-if-each-item-in-an-array-is-identical-in-javascript/54826946#54826946 – Chang Feb 22 '19 at 12:37

pimvdb · Answer 2 · 2012-03-10T13:58:59.573

16

In ES5, you could do:

arr.every(function(v, i, a) {
   // first item: nothing to compare with (and, single element arrays should return true)
   // otherwise:  compare current value to previous value
   return i === 0 || v === a[i - 1];
});

.every does short-circuit as well.

edited Mar 10 '12 at 13:58

answered Mar 10 '12 at 13:52

pimvdb

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Nice, didn't know this existed and going to start using this myself. Just adding a pointer to the docs for array.every https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/every – Shane Mar 10 '12 at 14:10
Thanks, that's a very elegant solution. I went for Dogberts solution in the end to avoid adding the array.every code to support older browsers - but in principle this would have worked nicely. – Nick Mar 10 '12 at 17:15
Here no need to compare each predecessor with the current value i.e. v===a[i-1]. Anyways a you can find one linear solution at: https://stackoverflow.com/a/54826946/8958729 – Chang Feb 22 '19 at 12:33

Jen · Answer 3 · 2020-04-09T23:44:16.110

7

You could always do a new Set, and check the length.

var set1 = [...new Set(list)].length === 1;

edited Apr 09 '20 at 23:44

answered May 07 '18 at 12:45

Jen

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score 5 · Answer 4 · edited Nov 20 '20 at 02:28

The one line answer is:

arr.every((val, ind, arr) => val === arr[0]);

You can look into Array.every for more details.

Note:

Array.every is available ES5 onwards.
This method returns true for any condition put on an empty array.
Syntax: arr.every(callback[, thisArg]) or array.every(function(currentValue, index, arr), thisValue)
It does not change the original array
The execution of every() is short-circuited. As soon as every() finds an array element that doesn't match the predicate, it immediately returns false and doesn't iterate over the remaining elements

score 2 · Answer 5 · answered Jan 12 '21 at 14:52

2

arr.every(i=>i==arr[0]) //will return true if all items in arr are identical

answered Jan 12 '21 at 14:52

Muhammad Raheel

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score 2 · Answer 6 · answered Mar 10 '12 at 13:51

2

function matchList(list) {
  var listItem = list[0];

  for (index in list) {
    if(list[index] != listItem {
       return false;
    }
  }

  return true;
}

answered Mar 10 '12 at 13:51

Jivings

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it will fail at the last element of the array. – Gonzalo Larralde Mar 10 '12 at 13:52

score 1 · Answer 7 · edited Feb 07 '22 at 19:58

function identical(array) {
  // a variable holding standard value
  //against this standard value we are examining the array
  var standard = array[1];
  for (var i = 0; i < array.length; i++) {
    if (array[i] !== standard) {
      return false;
    }
  }
  return true;
}

identical([1, 1, 1, 1, 1]); //return true
identical(['a', 'a', 'a']); //return true
identical(['a', 'a', 'b'])

function identical(array) {
  // a variable holding standard value
  //against this standard value we are examining the array
  var standard = array[1];
  for (var i = 0; i < array.length; i++) {
    if (array[i] !== standard) {
      return false;
    }
  }
  return true;
}

identical([1, 1, 1, 1, 1]); //return true
identical(['a', 'a', 'a']); //return true
identical(['a', 'a', 'b'])

score 1 · Answer 8 · answered Mar 10 '12 at 13:52

1

var list = ["b", "b", "b"];
var checkItem = list[0];
var isSame = true;
for (var i = 0; i < list.length; i++) {
  if (list[i] != checkItem) {
    isSame = false;
    break;
  }
}
return isSame;

answered Mar 10 '12 at 13:52

Waynn Lue

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Just return false when you first encounter it. – Jivings Mar 10 '12 at 13:54
Yeah, I wasn't sure if the OP wanted it for use later, or returned in a function. – Waynn Lue Mar 10 '12 at 13:56
You can use every() for one linear solution. Ref: https://stackoverflow.com/a/54826946/8958729 – Chang Feb 22 '19 at 12:29

score 0 · Answer 9 · edited May 23 '17 at 12:00

0

My suggestion would be to remove duplicates (check out Easiest way to find duplicate values in a JavaScript array), and then check to see if the length == 1. That would mean that all items were the same.

edited May 23 '17 at 12:00

Community

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answered Mar 10 '12 at 13:52

Kory Sharp

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That would be O(n^2) (or O(n log n) depending on the sorting algorithm). Too slow. – Dogbert Mar 10 '12 at 13:55
I was actually suggesting the second answer (should have specified), that remove duplicates without sorting. – Kory Sharp Mar 10 '12 at 15:09

score 0 · Answer 10 · answered Mar 10 '12 at 13:52

0

function allEqual(list)
{
    if(list.length == 0 || list.length == 1)
    {
      return true;
    }

    for (index in list) {
    if(list[index] != list[index+1] {
       return false;
    }
  }

  return true;

}

answered Mar 10 '12 at 13:52

Erix

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Like my first answer: it will fail at the last element of the array. – Jivings Mar 10 '12 at 13:59

Check if each item in an array is identical in JavaScript

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