Sort a Javascript Array by frequency and then filter repeats

// returns most frequent to least frequent

Array.prototype.byCount= function(){
    var itm, a= [], L= this.length, o= {};
    for(var i= 0; i<L; i++){
        itm= this[i];
        if(!itm) continue;
        if(o[itm]== undefined) o[itm]= 1;
        else ++o[itm];
    }
    for(var p in o) a[a.length]= p;
    return a.sort(function(a, b){
        return o[b]-o[a];
    });
}

//test

var A= ["apples","oranges","oranges","oranges","bananas","bananas","oranges"];
A.byCount()

/* returned value: (Array) oranges,bananas,apples */

I was actually working on this at the same time - the solution I came up with is pretty much identical to Anurag's.

However I thought it might be worth sharing as I had a slightly different way of calculating the frequency of occurrences, using the ternary operator and checking if the value has been counted yet in a slightly different way.

function sortByFrequencyAndFilter(myArray)
{
    var newArray = [];
    var freq = {};

    //Count Frequency of Occurances
    var i=myArray.length-1;
    for (var i;i>-1;i--)
    {
        var value = myArray[i];
        freq[value]==null?freq[value]=1:freq[value]++;
    }

    //Create Array of Filtered Values
    for (var value in freq)
    {
        newArray.push(value);
    }

    //Define Sort Function and Return Sorted Results
    function compareFreq(a,b)
    {
        return freq[b]-freq[a];
    }

    return newArray.sort(compareFreq);
}

Basic strategy:

Create an object to use as a hash table to track the frequency of each item in the array to be sorted.

Create a new array containing the item, frequency pairs.

Sort this array on frequency in descending order.

Extract the items from that array.

Code:

function descendingUniqueSort(toBeSorted) {
    var hash = new Object();
    toBeSorted.forEach(function (element, index, array) { 
                           if (hash[element] == undefined) {
                               hash[element] = 1;
                           }
                           else {
                               hash[element] +=1;
                           }});
    var itemCounts = new Array();
    for (var key in hash) {
       var itemCount = new Object();
       itemCount.key = key;
       itemCount.count = hash[key];
       itemCounts.push(itemCount);
    }
    itemCounts.sort(function(a,b) { if(a.count<b.count) return 1; 
        else if (a.count>b.count) return -1; else return 0;});

    return itemCounts.map(function(itemCount) { return itemCount.key; });
 }

var arr = ["apples", "oranges", "oranges", "oranges", "bananas", "bananas", "oranges"].sort();
var freq = {};
for (var s in arr) freq[s] = freq[s] ? freq[s] + 1 : 0;
arr.sort(function(a, b) { return freq[a] > freq[b] ? -1 : 1; });
for (var i = arr.length - 1; i > 0; i--) if (arr[i] == arr[i - 1]) arr.splice(i,1);
alert(arr.join(","));

for the first step to compute

{
    oranges: 4,
    bananas: 2,
    apples: 1
}

you can use countBy function of underscroe.js

var all=["apples", "oranges", "oranges", "oranges", "bananas", "bananas", "oranges"];
var frequency=_.countBy(all,function(each){return each});

so frequency object will contain frequency of all unique values, and you can get an unique list by simply calling _.uniq(all), and to sort that unique list by the _.sortBy method of underscore and using your frequency object you can use

_.sortBy(_.uniq(all),function(frequencyKey){return -frequency[frequencyKey]});

-ve sign is used here to sort the list in decending order by means of frequency value as per your requirement.

You can check the the documentation of http://underscorejs.org/ for further optimization by your own trick :)

Create a counter of the array's elements using reduce:

arr.reduce(
  (counter, key) => {counter[key] = 1 + counter[key] || 1; return counter}, 
  {}
);

Sort the counter object using sort on Object.entries and finally show only keys.

const arr = ["apples", "oranges", "oranges", "oranges",
  "bananas", "bananas", "oranges"
];

// create a counter object on array
let counter = arr.reduce(
  (counter, key) => {
    counter[key] = 1 + counter[key] || 1;
    return counter
  }, {});
console.log(counter);
// {"apples": 1, "oranges": 4, "bananas": 2}

// sort counter by values (compare position 1 entries)
// the result is an array
let sorted_counter = Object.entries(counter).sort((a, b) => b[1] - a[1]);
console.log(sorted_counter);
// [["oranges", 4], ["bananas", 2], ["apples", 1]]

// show only keys of the sorted array
console.log(sorted_counter.map(x => x[0]));
// ["oranges", "bananas", "apples"]

Let me put a minimal code to get unique values (and with frequencies) in ES6.

var arr = ["apples", "oranges", "oranges", "oranges", "bananas", "bananas", "oranges"];
console.log([...new Set(arr)])
console.log([...new Set(arr)].map(fruit=>({[fruit]:arr.filter(v=>(v===fruit)).length})))

It is also applied to array of objects to aggregate some properties.

var arr = [{"fruit":"apples"}, {"fruit":"oranges"}, {"fruit":"oranges"}, {"fruit":"oranges"}, {"fruit":"bananas"}, {"fruit":"bananas"}, {"fruit":"oranges"}];
console.log(arr.reduce((x,y)=>{if(x[y.fruit]) {x[y.fruit]++;return x;} else {var z={};z[y.fruit]=1;return Object.assign(x,z);}},{}))