Find how many ingredients you need to buy

recipe.js

//
// I always like an opportunity to use `Set` in JS.
// There should be `difference` and `intersection` operators
//   in new environments, but my Node didn't have them,
//   so we're doing it slightly more laboriously.
//   (`node -v` = 20.16.0)
//

const recipe = ['eggs', 'flour', 'sugar', 'butter']
const pantry = ['sugar', 'butter', 'milk']

const both = new Set([...recipe, ...pantry])
Array.from(both).length - pantry.length
// => 2