Create a new array from an existing array that holds the count for how many times an item exist using javascript/jquery

Question

I have a list of items in an array and i want to create a new array that is an array of objects with the key being the value from the existing array and the value being the number of times that item was in the existing array. Hope that makes since, if not hopefully this example will help. No third party libraries like lodash or underscore please.

Any help would be greatly appreciated, thank you.

var arr = [a,a,a,b,c,c,d,e,f,f,f,f];
new array =[{a:3},{b:1},{c:2},{d:1},{e:1},{f:4}];

My attempt so far:

            countryCodes.sort();
            var result = count(countryCodes);
            console.log(result[0]+": "+result[1]);
            function count(arr) {
                var a = [], b = [], prev;
                arr.sort();
                for ( var i = 0; i < arr.length; i++ ) {
                    if ( arr[i] !== prev ) {
                        a.push(arr[i]);
                        b.push(1);
                    } else {
                        b[b.length-1]++;
                    }
                    prev = arr[i];
                }
                return [a, b];
            }

Answer 1

You're close.

 var a = "aa", b = "bb", c = "cc", d = "dd", e = "ee", f = "ff"; var arr = [a,a,a,b,c,c,d,e,f,f,f,f]; function getCount(arr) { var result = {}; for (var val of arr) result[val] = result[val] + 1 || 1; return result; } var totals = getCount(arr); document.write(JSON.stringify(totals)); 

Answer 2

Open console when press run button.

 var arr = ['a', 'a', 'a', 'b', 'c', 'c', 'd', 'e', 'f', 'f', 'f', 'f']; function getObjectFromArray(a) { var result = [], alreadyAdded = []; a.forEach(function(item) { var key = item, obj = {}; if (alreadyAdded.indexOf(item) != -1) return; alreadyAdded.push(item) obj[key] = getCount(a, item) result.push(obj) }) return result; } function getCount(a, el) { var count = 0; a.forEach(function(element) { if (element == el) count++ }) return count; } console.log(getObjectFromArray(arr)) 

Answer 3

Here is another working way.

   function count(arr) {
    var prev = arr[0];
    var current;
    var newArr = [];
    var count = 0;
    var obj = {};
    for (var i = 0; i < arr.length; i++) {
        current = arr[i];
        if (i == arr.length - 1) {
            if (prev == current) { 
                count++;
                obj = {};
                obj[prev] = count;
                newArr.push(obj);
            }
            else {
                obj = {};
                obj[prev] = count;
                newArr.push(obj);
                count = 1;
                obj = {};
                obj[current] = count;
                newArr.push(obj);
            }
        }
        else if (prev == current) {
            count++;
        }
        else {
            obj = {};
            obj[prev] = count;
            newArr.push(obj);
            count = 1;
            prev = current;
        }
    }
    return newArr;
}
        console.log(count(['a', 'a', 'a', 'b', 'c', 'c', 'd', 'e', 'f', 'f', 'f', 'f']));

Answer 4

This can be a very simple task with Array.prototype.reduce()

 var arr = ["a","a","a","b","c","c","d","e","f","f","f","f"], res = arr.reduce((p,c) => { var i = p.findIndex(e => !!e[c]); !!~i ? p[i][c]++ : p.push({[c]:1}); return p},[]); document.write("<pre>" + JSON.stringify(res) + "</pre>"); 

OK the below one should be faster like o(2n) and it's nothing more longer than two lines of object assignments.

 var arr = ["a","a","a","b","c","c","d","e","f","f","f","f"], mid = arr.reduce((p,c) => {!!p[c] ? p[c]++ : p[c]=1; return p},{}), res = Object.keys(mid).map(e => ({[e]:mid[e]})); document.write("<pre>" + JSON.stringify(res) + "</pre>");