Determining whether an array contains duplicate values

Question

I would like to scan through a JS array and determine if all the elements are unique, or whether the array contains duplicates.

example :

my_array1 = [1, 2, 3] 
my_array2 = [1, 1, 1]

I want get result like this :

my_array1 must be return true, because this array element is unique
and array2 must be return false, because this array element is not unique

How can I go about writing this method?

Answer 1

Sort your array first of all, and then go for a simple comparison loop.

function checkIfArrayIsUnique(arr)  {
   var myArray = arr.sort();

    for (var i = 0; i < myArray.length; i++) {
        if (myArray.indexOf(myArray[i]) !== myArray.lastIndexOf(myArray[i])) { 
            return false; 
        } 
    } 

    return true;
}

Answer 2

if you want to check for uniqueness you can also do this.As stated on the comment i do not assert this is as the only best option.There are some great answers down below.

var arr = [2,3,4,6,7,8,9];
var uniq = []; // we will use this to store the unique numbers found
               // in the process for doing the comparison

var result = arr.slice(0).every(function(item, index, array){
  if(uniq.indexOf(item) > -1){
    // short circuit the loop
    array.length=0; //(B)
    return false;
  }else{
    uniq.push(item);
    return true;
  }
});

result --> true

arr.slice(0) creates a temporary copy of the array, on which the actual processing is done.This is because when the uniqueness criteria is met i clear the array (B) to short circuit the loop.This will make sure the processing stops as soon as the criteria is met.

And will be more nicer if we expose this as a method on a Array instance. so we can do something like this [1,2,3,5,7].isUnique();

Add the following snippet and you are ready to go

Array.prototype.isUnique = function() {
    var uniq = [];
    var result = this.slice(0).every(function(item, index, arr) {
        if (uniq.indexOf(item) > -1) {
            arr.length = 0;
            return false;
        } else {
            uniq.push(item);
            return true;
        }
    });
    return result;
};

arr.isUnique() --> true

DEMO