Javascript: Count duplicates in an array

array_dupplicate_counter.js

/** 
Problem:

You have a javascript array that likely has some duplicate values and you would like a count of those values.
Solution:

Try this schnippet out.
*/


function compressArray(original) {
 
	var compressed = [];
	// make a copy of the input array
	var copy = original.slice(0);
 
	// first loop goes over every element
	for (var i = 0; i < original.length; i++) {
 
		var myCount = 0;	
		// loop over every element in the copy and see if it's the same
		for (var w = 0; w < copy.length; w++) {
			if (original[i] == copy[w]) {
				// increase amount of times duplicate is found
				myCount++;
				// sets item to undefined
				delete copy[w];
			}
		}
 
		if (myCount > 0) {
			var a = new Object();
			a.value = original[i];
			a.count = myCount;
			compressed.push(a);
		}
	}
 
	return compressed;
};

// It should go something like this:

var testArray = new Array("dog", "dog", "cat", "buffalo", "wolf", "cat", "tiger", "cat");
var newArray = compressArray(testArray);
 
/*
console: [
	Object { value="dog", count=2}, 
	Object { value="cat", count=3}, 
	Object { value="buffalo", count=1}, 
	Object { value="wolf", count=1}, 
	Object { value="tiger", count=1}
]
*/

function countDup(arr = []) {
  const set = new Set(arr);
  const uniqueArr = Array.from(set);
  if (uniqueArr.length === arr.length) return;

  const map = new Map();
  uniqueArr.forEach((uniqueVal) => {
    let n = 0;
    arr.forEach((val) => {
      if (val === uniqueVal) {
        n++;
      }
    });
    map.set(uniqueVal, n);
  });
  console.log('newMap ', map);
  for (const [key, val] of map) {
    console.log(key, val);
  }
}

array = ['Apples', 'Oranges', 'Apples', 'Strawberries', 'Apples'];

For a one liner to get unique values and their counts and put them in a dictionary :

reducedArray = array.reduce((acc, datum) => { 
                              let count = acc[datum];
                              acc[datum] = count === undefined ? 1 : ++count;
                              return acc;
                           }, {});

acc = accumulator that indexes according to datum
and returns a key value pair with key as datum and count as the number of times the datum appears in the array.

o/p: (try console.log(reducedArray))
Apples: 3
Oranges: 1
Strawberries: 1

array = ['abc', '123', 'abc', 'xyz', 'abc'];

For a one liner using reduce to count unique distinct values and put them in a dictionary:

reducedArray= array.reduce((acc, datum) => {
      let count= acc[datum];
      acc[datum] = count === undefined ? 1 : ++count;
      return acc;
    }, {});

acc: indexes the dictionary using datum and increments the count if it finds a duplicate

console: 
abc: 3
123: 1
xyz: 1

@davidnagli Thanks Buddy.. it really helped me with just one line code change.
appreciated

Thanks for this @ralphcrisostomo! I implemented one change, though. I needed to use data repeatedly in different ways. The "delete copy[w]" on line 27 line really threw my data off. I changed it for "i++" and was successful in keeping my data intact. Thanks again!!

Had to search high and low for this solution, every tutorial and answer before was only outputting in the console. You are a legend! Thanks you!

@williamlsh those first three lines can be reduced to one via ES6 spread syntax!

let uniqueArr = [...new Set(arr)];

Thank you!