Using lodash to check whether an array has duplicate values

Question

What do you all think would be the best (best can be interpreted as most readable or most performant, your choice) way to write a function using the lodash utilities in orde

Answer 1

No need to use lodash, use following code instead:

function getDuplicates(array, key) {
  return array.filter(e1=>{
    if(array.filter(e2=>{
      return e1[key] === e2[key];
    }).length > 1) {
      return e1;
    }
  })
}

Answer 2

As of ES6 you can simply use Set so this becomes:

let hasDuplicates = arr => new Set(arr).size != arr.length

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

Which somewhat negates the use of lodash in this particular case.

Answer 3

Well, there's always. lodash's _.uniq() function. That function actually returns a new array that only contains unique values, so checking to see if the length of the array has changed would get you your 'true' or 'false' value to return yourself, I believe.

Answer 4

You can try this code:

function hasDuplicates(a) {
  return _.uniq(a).length !== a.length; 
}

var a = [1,2,1,3,4,5];
var b = [1,2,3,4,5,6];

document.write(hasDuplicates(a), ',',hasDuplicates(b));

<script src="http://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.1.0/lodash.min.js"></script>

Answer 5

You could check that there is _.some element in the array which does not return its own location when looked up in the array. In other words, there is at least one element which has a match earlier in the array.

function hasDuplicates(array) {
    return _.some(array, function(elt, index) { 
        return array.indexOf(elt) !== index; 
    });
}

Perhaps this is faster than the _.uniq solution, since it will identify the first duplicated element right away without having to compute the entire unique-ified array.

Or, depending on your coding style and desire for readability, and if you want to use ES6 arrow functions for brevity:

var earlierMatch  = (elt, index, array) => array.indexOf(elt) !== index;
var hasDuplicates = array => _.some(array, earlierMatch);

Answer 6

I don't know lodash but I submit:

_.any(_.countBy(['foo', 'foo', 'bar']), function(x){ return x > 1; });

The problem with all the solutions proposed so far is that the entire input array needs processing to get an answer, even if the answer is obvious from the first 2 elements of the array.