How to count duplicate value in an array in javascript

Question

Currently, I got an array like that:

var uniqueCount = Array();

After a few steps, my array looks like that:

uniqueCount =          


        

Answer 1

var counts = {};
your_array.forEach(function(x) { counts[x] = (counts[x] || 0)+1; });

Answer 2

Single line based on reduce array function

const uniqueCount =  ["a", "b", "c", "d", "d", "e", "a", "b", "c", "f", "g", "h", "h", "h", "e", "a"];
const distribution = uniqueCount.reduce((acum,cur) => Object.assign(acum,{[cur]: (acum[cur] || 0)+1}),{});
console.log(JSON.stringify(distribution,null,2));

Answer 3

You can have an object that contains counts. Walk over the list and increment the count for each element:

var counts = {};

uniqueCount.forEach(function(element) {
  counts[element] = (counts[element] || 0) + 1;
});

for (var element in counts) {
  console.log(element + ' = ' + counts[element]);
} 

Answer 4

You can solve it without using any for/while loops ou forEach.

function myCounter(inputWords) {        
    return inputWords.reduce( (countWords, word) => {
        countWords[word] = ++countWords[word] || 1;
        return countWords;
    }, {});
}

Hope it helps you!

Answer 5

I stumbled across this (very old) question. Interestingly the most obvious and elegant solution (imho) is missing: Array.prototype.reduce(...). All major browsers support this feature since about 2011 (IE) or even earlier (all others):

var arr = ['a','b','c','d','d','e','a','b','c','f','g','h','h','h','e','a'];
var map = arr.reduce(function(prev, cur) {
  prev[cur] = (prev[cur] || 0) + 1;
  return prev;
}, {});

// map is an associative array mapping the elements to their frequency:
document.write(JSON.stringify(map));
// prints {"a": 3, "b": 2, "c": 2, "d": 2, "e": 2, "f": 1, "g": 1, "h": 3}

Answer 6

Quickest way:

Сomputational complexity is O(n).

function howMuchIsRepeated_es5(arr) {
	const count = {};
	for (let i = 0; i < arr.length; i++) {
		const val = arr[i];
		if (val in count) {
			count[val] = count[val] + 1;
		} else {
			count[val] = 1;
		}
	}

	for (let key in count) {
		console.log("Value " + key + " is repeated " + count[key] + " times");
	}
}

howMuchIsRepeated_es5(['a','b','c','d','d','e','a','b','c','f','g','h','h','h','e','a']);

The shortest code:

Use ES6.

function howMuchIsRepeated_es6(arr) {
	// count is [ [valX, count], [valY, count], [valZ, count]... ];
	const count = [...new Set(arr)].map(val => [val, arr.join("").split(val).length - 1]);

	for (let i = 0; i < count.length; i++) {
		console.log(`Value ${count[i][0]} is repeated ${count[i][1]} times`);
	}
}

howMuchIsRepeated_es6(['a','b','c','d','d','e','a','b','c','f','g','h','h','h','e','a']);