pure function

app.js

const R = require('ramda');
const filterFood = (food) => R.gt(food.amount, 0);
const cleaner = (day) =>
  R.map((item) => R.filter(filterFood, item.foods), day.meals);
const removeZeroAmounts = (diet) => {
  const dietClone = R.clone(diet);
  const cleanDays = R.map(cleaner, dietClone.days);
  return { ...dietClone, days: cleanDays };
};

you can make this more readable, you don't need ramda

const removeZeroAmounts = (diet) => ({ ...diet, days: diet.days.map(item => item.meals.filter(x => x > 0)) })