Function Chaining

const counter = function() {
    let i = 0;
    
    return {
        add(val) {
            i += val;
            return this;
        },
        subtract(val) {
            i -= val;
            return this;
        },
        print() {
            return i;
        }
    };
};

const outerCounter = counter();

outerCounter
    .add(2)
    .add(2)
    .add(2)
    .subtract(1)
    .print(); // 5

We need to add return this; inside the method which would return the entire object back so we could call another method. If we did not do this, it would return undefined instead.