Compute the frequency of each item first.

{

    apples: 1,

    oranges: 4,

    bananas: 2

}

Then create an array from this frequency object which will also remove the duplicates.

[apples, oranges, bananas]

Now sort this array in descending order using the frequency map we created earlier.

function compareFrequency(a, b) {

    return frequency[b] - frequency[a];

}



array.sort(compareFrequency);

Here's the entire source (using the newly introduced Array functions in ECMA 5) and combining the de-duplication and frequency map generation steps,

function sortByFrequency(array) {

    var frequency = {};



    array.forEach(function(value) { frequency[value] = 0; });



    var uniques = array.filter(function(value) {

        return ++frequency[value] == 1;

    });



    return uniques.sort(function(a, b) {

        return frequency[b] - frequency[a];

    });

}

Same as above using the regular array iteration.

function sortByFrequencyAndRemoveDuplicates(array) {

    var frequency = {}, value;



    // compute frequencies of each value

    for(var i = 0; i < array.length; i++) {

        value = array[i];

        if(value in frequency) {

            frequency[value]++;

        }

        else {

            frequency[value] = 1;

        }

    }



    // make array from the frequency object to de-duplicate

    var uniques = [];

    for(value in frequency) {

        uniques.push(value);

    }



    // sort the uniques array in descending order by frequency

    function compareFrequency(a, b) {

        return frequency[b] - frequency[a];

    }



    return uniques.sort(compareFrequency);

}