Recipe scaler

index.js

const R = require('ramda')

const recipie = {
  ingredients: [{
    name: 'butter',
    weight: 100
  }, {
    name: 'salt',
    weight: 25
  }]
}


const largest = R.reduce(R.max, -Infinity)
const largestBy = key => R.compose(
  largest,
  R.chain(R.props([key]))
)

const toRatio = R.curry((list, largest) =>
  R.map(row =>
    R.assoc('ratio', R.divide(row.weight, largest), row),
    list
  ))

const ratioFrom = key => R.ap(
  toRatio,
  largestBy(key)
)

const scaleBy = name => weight => R.ap(
  list => factor => R.map(R.evolve({
    weight: R.multiply(factor)
  }), list),
  R.compose(
    R.divide(weight),
    R.prop('weight'),
    R.find(R.propEq(name))
  )
)

R.compose(
  R.over(R.lensProp('ingredients'), scaleBy('butter')(15)),
  R.over(R.lensProp('ingredients'), ratioFrom('weight')),
)(recipie)

// Object
//   ingredients: Array (2 items)
//     0: Object {name: "butter", ratio: 1, weight: 15}
//     1: Object {name: "salt", ratio: 0.25, weight: 3.75}