The array of objects will be filtered based on a object.property value.

array-of-objects-filter-by-property

people = [
    {name: 'Mary', gender: 'girl'},
    {name: 'Paul', gender: 'boy'},
    {name: 'John', gender: 'boy'},
    {name: 'Lisa', gender: 'girl'},
    {name: 'Bill', gender: 'boy'},
    {name: 'Maklatura', gender: 'girl'}
]

var numBoys = people.reduce(function (n, person) {
    return n + (person.gender == 'boy');
}, 0);

console.log(numBoys);