Using Reduce to Count the Array in Javascript

NodeJs / Javascript

Let’s say, we have an array in Javascript, and we want to group the elements, ount their occurences and store the final results in a key-value dictionary.

Input Array:

const cars = ['BMW', 'Audi', 'Audi', 'Benz', 'Benz', 'Tesla', 'BMW', 'Toyota'];

Output JSON-like key-value pairs i.e. mapping:

{ BMW: 2, Audi: 2, Benz: 2, Tesla: 1, Toyota: 1 }

We can add a functoin count by extending the Array’s prototype. We will implement it using the Array’s reduce the array into a single value. The reduce takes two parameters, the first one is the function(previousValue, currentValue, currentIndex, currentArray), and the second value is the initial value.

Array.prototype.count = function() {
    return this.reduce(function(obj, name) {
        obj[name] = obj[name] ? ++obj[name] : 1;
        return obj;
    }, {});
}

We pass the initial value, {} i.e. an empty JSON-object, then in the reduced-function, we will update the frequency of the current element in the dictionary and return the updated object for next iteration.

–EOF (The Ultimate Computing & Technology Blog) —