How to Count the Number of Occurrences of Each Item in an Array

Counting the occurrences / frequency of array elements

const arr = [2, 2, 5, 2, 2, 2, 4, 5, 5, 9];

function foo (array) {
  let a = [],
    b = [],
    arr = [...array], // clone array so we don't change the original when using .sort()
    prev;

  arr.sort();
  for (let element of arr) {
    if (element !== prev) {
      a.push(element);
      b.push(1);
    }
    else ++b[b.length - 1];
    prev = element;
  }

  return [a, b];
}

const result = foo(arr);
console.log('[' + result[0] + ']','[' + result[1] + ']')
console.log(arr)

How do I count the occurrences of a list item?

If you only want a single item's count, use the count method:

>>> [1, 2, 3, 4, 1, 4, 1].count(1)
3

**Important: this is very slow if you are counting multiple different items**

Each count call goes over the entire list of n elements. Calling count in a loop n times means n * n total checks, which can be catastrophic for performance.

If you want to count multiple items, use Counter, which only does n total checks.

How to count the number of occurrences of each item in an array?

no need to use jQuery for this task — this example will build an object with the amount of occurencies of every different element in the array in O(n)

var occurrences = { };
for (var i = 0, j = arr.length; i < j; i++) {
   occurrences[arr[i]] = (occurrences[arr[i]] || 0) + 1;
}

console.log(occurrences);        // {ab: 3, pq: 1, mn: 2}
console.log(occurrences['mn']);  // 2

Example fiddle

You could also use Array.reduce to obtain the same result and avoid a for-loop

var occurrences = arr.reduce(function(obj, item) {
  obj[item] = (obj[item] || 0) + 1;
  return obj;
}, {});

console.log(occurrences);        // {ab: 3, pq: 1, mn: 2}
console.log(occurrences['mn']);  // 2

Example fiddle

count the number of occurrence of each element in an array (in php)

As mentioned in the comments array_count_values() is the optimal solution in php. But you must write it out aka show your understanding on how to search an array its rather simple as well.

$a=['a','b','a','c','a','d'];
$output=[]; 

for($i = 0; $i < count($a); $i++){
    if(!isset($output[$a[$i]])){
      $output[$a[$i]] = 1;
    }

    else{
        $output[$a[$i]] = $output[$a[$i]] + 1;
    }
}

var_dump($output);

//output
array(4) {
  ["a"] => int(3)
  ["b"] => int(1)
  ["c"] => int(1)
  ["d"] => int(1)
}

Count the number of occurrences of each item in an array and regex

You can do it like this:

var data =
[
    {
        "Id": "10",
        "FileName": "TechnicalBook_2021-08-26T12:36:48Z",
        "Book": "ABC P1",
        "Location": "USA",
        "LastModified": "2021-08-26T12:36:48Z"
    },
    {
        "Id": "11",
        "FileName": "SocialBook_2021-08-26T12:36:48Z",
        "Book": "XYZ P1",
        "Location": "USA",
        "LastModified": "2021-08-26T15:36:48Z"
    },
    {
        "Id": "12",
        "FileName": "TechnicalBook_2021-08-26T15:36:48Z",
        "Book": "ABC P2",
        "Location": "USA",
        "LastModified": "2021-08-26T12:36:48Z"
    },
    {
        "Id": "13",
        "FileName": "SocialBook_2021-08-26T15:36:48Z",
        "Book": "XYZ P2",
        "Location": "USA",
        "LastModified": "2021-08-26T15:36:48Z"
    },
    {
        "Id": "14",
        "FileName": "SocialBook_2021-08-26T17:36:48Z",
        "Book": "XYZ P3",
        "Location": "USA",
        "LastModified": "2021-08-26T17:36:48Z"
    }
]

var prefixesAll = data.map(a => a.FileName.split('_')[0]);

var prefixesUnique = [...new Set(prefixesAll)];

var prefixCounts = prefixesUnique.map(a => ({
  "FileName": a,
  "Count": data.filter(b => b.FileName.startsWith(a)).length
}));

console.log(prefixesAll);
console.log(prefixesUnique);
console.log(prefixCounts);

How to count certain elements in array?

Very simple:

var count = 0;
for(var i = 0; i < array.length; ++i){
    if(array[i] == 2)
        count++;
}

Count each array item occurrence and return result as an object

Array.prototype.reduce() seems to be pretty close to what you're looking for:

const src = ['red', 'green', 'green', 'blue', 'purple', 'red', 'red', 'black'],

      result = src.reduce((acc,color) => (acc[color]=(acc[color]||0)+1, acc), {})
      
console.log(result)

.as-console-wrapper{min-height:100%;}

Counting occurrences of particular property value in array of objects

A simple ES6 solution is using filter to get the elements with matching id and, then, get the length of the filtered array:

const array = [  {id: 12, name: 'toto'},  {id: 12, name: 'toto'},  {id: 42, name: 'tutu'},  {id: 12, name: 'toto'},];
const id = 12;const count = array.filter((obj) => obj.id === id).length;
console.log(count);