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How to find duplicate values in a JavaScript array?

Web Development Javascript
#1

I need to check an array in Javascript to find any duplicate values. so what is the easiest to do that other than looping through the array using the usual Javascript looping methods…etc.

best regards.

2 Likes
#2

You can add this function, or tweak it and add it to Javascript’s Array prototype:

Array.prototype.unique = function () {
    var r = new Array();
    o:for(var i = 0, n = this.length; i < n; i++)
    {
    	for(var x = 0, y = r.length; x < y; x++)
    	{
    		if(r[x]==this[i])
    		{
                alert('this is a DUPE!');
    			continue o;
    		}
    	}
    	r[r.length] = this[i];
    }
    return r;
}

var arr = [1,2,2,3,3,4,5,6,2,3,7,8,5,9];
var unique = arr.unique();
alert(unique);
#3

You could sort the array and then run through it and then see if the next (or previous) index is the same as the current. Assuming your sort algorithm is good, this should be less than O(n2):

var arr = [9, 9, 111, 2, 3, 4, 4, 5, 7];
var sorted_arr = arr.sort(); // You can define the comparing function here. 
                             // JS by default uses a crappy string compare.
var results = [];
for (var i = 0; i < arr.length - 1; i++) {
    if (sorted_arr[i + 1] == sorted_arr[i]) {
        results.push(sorted_arr[i]);
    }
}

alert(results);