(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?

Answer 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 eg:

<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;
}

Answer 2

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>