With all these useful array methods you learned about, it's important to understand how you can combine them. Let's take map() and reduce() as an example:

const originalArray = [{price: 10.99}, {price: 5.99}, {price: 29.99}];
const transformedArray = originalArray.map(obj => obj.price); // produces [10.99, 5.99, 29.99]
const sum = transformedArray.reduce((sumVal, curVal) => sumVal + curVal, 0); // => 46.97

Of course, you could skip the map step and just add the extraction logic to reduce():

const originalArray = [{price: 10.99}, {price: 5.99}, {price: 29.99}];
const sum = originalArray.reduce((sumVal, curVal) => sumVal + curVal.price, 0); // => 46.97

But let's say you have a more complex extraction logic and hence want to split this into multiple method calls. Or you have a re-usable map function which you want to be able to use in different places of your app. Then you can still write the initial example in a more concise way if you leverage method chaining:

const originalArray = [{price: 10.99}, {price: 5.99}, {price: 29.99}];
const sum = originalArray.map(obj => obj.price)
    .reduce((sumVal, curVal) => sumVal + curVal, 0); // => 46.97

We call .reduce() directly on the result of map() (which produces an array, that's why this is possible). Hence we can avoid storing the mapped array in a separate constant or variable that we might not need in any other place.