(Angular.js) How to count how many item there are in an array by filter?

Question

In angular JS, I have a variable that keeps track of people and their age

$scope.people = [
    {
        name: "Lucy",
        age: "18"
    }, {
        name: "Michael",
        age: "24"
    }, {
        name: "Lisa",
        age: "46"
    }
];

The user can add more people via a simple form:

<input ng-model="newPerson.name">
<input ng-model="newPerson.age">
<button ng-click="addNewPerson()">Add</button>

At the bottom of the page, I want a simple pie graph showing the population by age group eg.(>18, 18-25, 26-35, 36-45, 45<). To do that, I need to be able to filter the $scope.people by age, get the number of each group.

I know I could use normal javascript to loop through the whole array, get a count of each age group. And whenever a new person is added, just increment the count in that particular group, but I was wondering if there is a more efficient and angular-esque way to do this?

nbarbosa · Accepted Answer · 2016-04-26 03:39:27Z

Here's one way to implement this with Array.Prototype.filter and Array.Prototype.reduce. Note the the filter is applied on $scope.ageBrackets which contains the pre-processed data prepared by the forEach loop. This is efficient for small range values such as the age of a person. This is one alternative if you are not using ng-repeat and you don't want to filter the data multiple times.

angular.module('ageBracketApp', ['ageBracketApp.controllers']);
angular.module('ageBracketApp.controllers', []).controller('ageBracketController', ['$scope',
  function($scope) {

    $scope.ageBrackets = [];

    $scope.people = [{
      name: "Lucy",
      age: "18"
    }, {
      name: "Michael",
      age: "24"
    }, {
      name: "Lisa",
      age: "46"
    }];

    angular.forEach($scope.people, function(value, key) {
      $scope.ageBrackets[value.age] = $scope.ageBrackets[value.age] + 1 || 1;
    });

    $scope.addPerson = function() {
      var age = Math.floor(Math.random() * (123 - 1 + 1)) + 1; // random between 1 and 123
      $scope.people.push({
        name: 'Person ' + ($scope.people.length + 1),
        age: age
      });
      $scope.ageBrackets[age] = $scope.ageBrackets[age] + 1 || 1;

    };

    $scope.ageBracketCount = function(min, max) {
      max = max || Number.MAX_SAFE_INTEGER;
      return $scope.ageBrackets.filter(function(value, index, array) {
        return index >= min && index <= max;
      }).reduce(function(a, b) {
        return a + b;
      }, 0);
    };
  }
]);

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.23/angular.min.js"></script>
<div ng-app="ageBracketApp" ng-controller="ageBracketController">
  {{people}}
  <hr />18+: {{ageBracketCount(18)}}
  <br />18-25: {{ageBracketCount(18,25)}}
  <br />26-35: {{ageBracketCount(26,35)}}
  <br />40-: {{ageBracketCount(0,40)}}
  <hr />
  <button ng-click="addPerson()">
    Add person
  </button>
</div>

Mike Jerred · Accepted Answer · 2016-04-26 01:42:33Z

1

You can use a library like lodash: _($scope.people).filter(x => x.age > 18 && x.age <= 25).count()

Alternatively, if you are already using the filtered list in an ng-repeat you can assign it as a variable and get the length e.g.:

<ul>
    <li ng-repeat="item in people | filter:filterFunction as results"></li>
</ul>
<p>{{results.length}}</p>

function filterFunction(item) {
    return item.age > 18 && item.age <= 25;
}

answered Apr 26, 2016 at 1:42

Mike Jerred

10.7k5 gold badges25 silver badges48 bronze badges

1 Comment

mariocatch Over a year ago

Note, aliasing as is available in Angular 1.3 and greater.

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