transform_discount.js

function transform(input) {
  return input
    .filter(havePrice)
    .map(withDiscount)
}

const havePrice = (product) => Boolean(product.price)

function withDiscount(product) {
  switch (product.savingsType) {
    case 'total':
      return withTotalDiscount(product)
    case 'percent':
      return withPercentDiscount(product)
    default:
      console.error(`Error: product with unknow savingType '${product.savingsType}'`)
  }
}

const withTotalDiscount = ({ price, savings, kind }) => ({
  basePrice: `${price} €`,
  description: `Vous économisez ${savings} € par rapport au prix initial`,
  price: `${price - savings} €`,
  savings: `${savings} €`,
  title: kindToTitle(kind)
})

const withPercentDiscount =({ price, savings, kind }) => {
  const totalSavings = price * (savings / 100)
  return {
    basePrice: `${price} €`,
    description: `Vous économisez ${totalSavings} € par rapport au prix initial`,
    price: `${price - totalSavings} €`,
    savings: `${savings} %`,
    title: kindToTitle(kind)
  }
}

const kindToTitle = (discountKind) => {
  const titleMap = {
    'reductionVoucher': 'Bon de réduction',
    'sales': 'Soldes'
  }
  return titleMap[discountKind] || 'Reduction inconnue'
}

const products = [
  {
    price: 3000,
    kind: 'reductionVoucher',
    savings: 300,
    savingsType: 'total',
  },
  {
    price: undefined,
    kind: 'sales',
    savings: 10,
    savingsType: 'percent',
  },
  {
    price: 3000,
    kind: 'sales',
    savings: 15,
    savingsType: 'percent',
  }
]

console.log(transform(products))

transform_list.js

// Note: https://twitter.com/jaffathecake/status/1213077702300852224 ;-)
function transform(input) {
  return input.reduce((acc, val) => {
    const [key, ...arr] = val
    acc[key] = arr
    return acc
  }, {})
}

const input = [
  ["key1", 1, 2, 3, 4],
  ["key2", 4, 5, 6, 7]
]

const transformed = transform(input)
console.log(transformed)