JSFiddle - React, Tailwind, and code Playground

JavaScript

const adder = {
    sum: 0,
    add: (numbers) => {
        numbers.forEach(n => {
            console.log(this)
            adder.sum += n;
        });
    }
};

adder.add([1,2,3]);
console.log(adder.sum === 6)
console.log(adder.sum)