javascript count occurrences in array code example

Example 1: how to count occurences in an array with javascript

// This function counts instances of elements in an array
// the return object has the array elements as keys
// and number of occurrences as it's value
const arrToInstanceCountObj = arr => arr.reduce((obj, e) => {
  obj[e] = (obj[e] || 0) + 1;
  return obj;
}, {});

arrToInstanceCountObj(['h', 'e', 'l', 'l', 'o', 'w', 'o', 'r', 'l', 'd'])
/*
  {
    h: 1,
    e: 1,
    l: 3,
    o: 2,
    w: 1,
    r: 1,
    d: 1,
  }
*/

Example 2: javascript Count the occurrences of a value in an array

const countOccurrences = (arr, val) => arr.reduce((a, v) => (v === val ? a + 1 : a), 0);

// Examples
countOccurrences([2, 1, 3, 3, 2, 3], 2);                // 2
countOccurrences(['a', 'b', 'a', 'c', 'a', 'b'], 'a');  // 3

Example 3: javascript count occurrences in string

function countOccurences(string, word) {
   return string.split(word).length - 1;
}
var text="We went down to the stall, then down to the river."; 
var count=countOccurences(text,"down"); // 2

Example 4: javascript Count the frequency of a value in an array

const countOccurrences = arr => arr.reduce((prev, curr) => (prev[curr] = ++prev[curr] || 1, prev), {});

// Examples
countOccurrences([2, 1, 3, 3, 2, 3]);               // { '1': 1, '2': 2, '3': 3 }
countOccurrences(['a', 'b', 'a', 'c', 'a', 'b']);   // { 'a': 3, 'b': 2, 'c': 1 }

Example 5: count number of times an element is occuring in an array in javascript

Array.prototype.frequencies = function() {
    var l = this.length, result = {all:[]};
    while (l--){
       result[this[l]] = result[this[l]] ? ++result[this[l]] : 1;
    }
    // all pairs (label, frequencies) to an array of arrays(2)
    for (var l in result){
       if (result.hasOwnProperty(l) && l !== 'all'){
          result.all.push([ l,result[l] ]);
       }
    }
    return result;
};

var freqs = [5, 5, 5, 2, 2, 2, 2, 2, 9, 4].frequencies();
alert(freqs[2]); //=> 5
// or
var freqs = '1,1,2,one,one,2,2,22,three,four,five,three,three,five'
             .split(',')
             .frequencies();
alert(freqs.three); //=> 3