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Functional Programming Patterns With RamdaJS!

Bang for Your Buck: Solution Review

Explore a practical review of RamdaJS functional programming techniques by filtering items under a max price, sorting by rating, and selecting top entries. Learn how to apply point-free style and use comparators like descend and ascend to create readable, functional sorting functions.

We'll cover the following...

Review

Let’s review the steps

  1. Get everything maxPrice or less
  2. Sort by rating
  3. Get the top 3

Here’s a vanilla solution

Javascript (babel-node)
import menu from './menu';
const getTop3MealsFor = (maxPrice, menu) => menu
    .filter((item) => item.price <= maxPrice)
    .sort((a, b) => b.rating - a.rating)
    .slice(0, 3);
const result = getTop3MealsFor(12, menu);
console.log({ result });

Ramda has slice and sort functions. Using them lets us pipe the entire function to make it point-free.

Javascript (babel-node)
import { pipe, slice, sort } from 'ramda';
import menu from './menu';
const getTop3MealsFor = pipe(
  (maxPrice, menu) => menu.filter((item) => item.price <= maxPrice),
  sort((a, b) => b.rating - a.rating),
  slice(0, 3)
);
const result = getTop3MealsFor(12, menu);
console.log({ result });

We can point-free up filter's predicate function using propSatisfies. It tests an object’s property against a function to return true or false.

Javascript (babel-node)
import { gte, pipe, propSatisfies, slice, sort } from 'ramda';
import menu from './menu';
const getTop3MealsFor = pipe(
  (maxPrice, menu) => menu.filter(propSatisfies(gte(maxPrice), 'price')),
  sort((a, b) => b.rating - a.rating),
  slice(0, 3)
);
const result = getTop3MealsFor(12, menu);
console.log({ result });

I personally wouldn’t do that, but it’s fun to experiment.

Comparators

I would, however, play around with comparators. These are functions that compare data for sorting.

Ramda’s descend function is a descending comparator, meaning a function that sorts entries from biggest to smallest. The flip side would be R.ascend, an ascending comparator.

We can break the sorting logic into a function

const byPrice = descend(prop('rating'));

Then use it like so

sort(byPrice);

It reads like a sentence!

Javascript (babel-node)
import { descend, gte, pipe, prop, propSatisfies, slice, sort } from 'ramda';
import menu from './menu';
const byPrice = descend(prop('rating'));
const getTop3MealsFor = pipe(
  (maxPrice, menu) => menu.filter(propSatisfies(gte(maxPrice), 'price')),
  sort(byPrice),
  slice(0, 3)
);
const result = getTop3MealsFor(12, menu);
console.log({ result });