discount.js

data.map(item => {
    if (item.price > 100) {
        return {
            id: item.id,
            title: item.title,
            price: item.price * (1 - 0.2)
        };
    }
    return item;
});
console.log(data);

// result

/*
0: {id: 1, title: "Don't Waste your life ", price: 33, Author: "John Piper"}
1: {id: 2, title: "Living for a greater purpose", price: 12, Author: "JH Baritam"}
2: {id: 3, title: "The call for Mercy", price: 324}
3: {id: 4, title: "Heart Blindness", price: 440}
*/