JS - from procedural to functional

js-procedural-to-functional.js

const developers = [
 {
   id: 1,
   name: 'John Doe',
   age: 29,
   sex: 'male',
   level: 'senior',
   earnings: [
     {
       month: 'February',
       year: 2021,
       amount: 12500
     }, 
     {
       month: 'March',
       year: 2021,
       amount: 12000
     }, 
     {
       month: 'April',
       year: 2021,
       amount: 13100
     }
   ]
},
{
   id: 2,
   name: 'Peter Johnson',
   age: 27,
   sex: 'male',
   level: 'mid',
   earnings: [
     {
       month: 'February',
       year: 2021,
       amount: 9800
     }, 
     {
       month: 'March',
       year: 2021,
       amount: 8600
     }, 
     {
       month: 'April',
       year: 2021,
       amount: 10000
     }

   ]
 },
 {
   id: 3,
   name: 'Jane Doe',
   age: 22,
   sex: 'female',
   level: 'mid',
   earnings: [
     {
       month: 'February',
       year: 2021,
       amount: 10450
     }, 
     {
       month: 'March',
       year: 2021,
       amount: 11340
     }, 
     {
       month: 'April',
       year: 2021,
       amount: 11050
     }

   ]
 },
 {
   id: 4,
   name: 'Mary Jane',
   age: 35,
   sex: 'female',
   level: 'senior',
   earnings: [
     {
       month: 'February',
       year: 2021,
       amount: 14600
     }, 
     {
       month: 'March',
       year: 2021,
       amount: 15230
     }, 
     {
       month: 'April',
       year: 2021,
       amount: 14200
     }

   ]
 },
 {
   id: 5,
   name: 'Bob Taylor',
   age: 19,
   sex: 'male',
   level: 'junior',
   earnings: [
     {
       month: 'February',
       year: 2021,
       amount: 6700
     }, 
     {
       month: 'March',
       year: 2021,
       amount: 5900
     }, 
     {
       month: 'April',
       year: 2021,
       amount: 6230
     }
   ]
},
{
   id: 6,
   name: 'Ted Talker',
   age: 48,
   sex: 'male',
   level: 'senior',
   earnings: [
     {
       month: 'February',
       year: 2021,
       amount: 18450
     }, 
     {
       month: 'March',
       year: 2021,
       amount: 17660
     }, 
     {
       month: 'April',
       year: 2021,
       amount: 17995
     }
   ]
 }
]


// Procedural (imperative - requires lodash/fp)
const JUNIOR_AVERAGE_SALARY = 7000
const MID_AVERAGE_SALARY = 10000
const SENIOR_AVERAGE_SALARY = 13000

for(let developer of developers) {
  let lastThreeMonthsTotalEarnings = 0

  for(let earning of developer.earnings) {
    lastThreeMonthsTotalEarnings += earning.amount
  }

  developer.averageSalary = lastThreeMonthsTotalEarnings / developer.earnings.length

  if(developer.level === 'junior') {
    if(developer.averageSalary === JUNIOR_AVERAGE_SALARY) {
      developer.averagePosition = 'equal'
    } else if(developer.averageSalary > JUNIOR_AVERAGE_SALARY) {
      developer.averagePosition = 'above'
    } else {
      developer.averagePosition = 'below'
    }
  }

  if(developer.level === 'mid') {
    if(developer.averageSalary === MID_AVERAGE_SALARY) {
      developer.averagePosition = 'equal'
    } else if(developer.averageSalary > MID_AVERAGE_SALARY) {
      developer.averagePosition = 'above'
    } else {
      developer.averagePosition = 'below'
    }
  }

  if(developer.level === 'senior') {
    if(developer.averageSalary === SENIOR_AVERAGE_SALARY) {
      developer.averagePosition = 'equal'
    } else if(developer.averageSalary > SENIOR_AVERAGE_SALARY) {
      developer.averagePosition = 'above'
    } else {
      developer.averagePosition = 'below'
    }
  }
}

// Functiona (declarative)

import { pipe, get, reduce, map, curry } from 'lodash/fp'

const AVERAGE_SALARIES = {
  junior: 7000,
  mid: 10000,
  senior: 13000
}

const AVERAGE_POSITIONS = {
  equal: 'equal',
  above: 'above',
  below: 'below'
}

function appendSalaryInfo(developers) {
  return pipe(
   map(developer => pipe(
       appendAverageSalary,
       appendAveragePosition,       
     )(developer))
  )(developers)
}

function getAveragePosition(developer) {
  const { averageSalary, level } = developer
  const averageSalaryReference = get(level, AVERAGE_SALARIES)

  if(averageSalary === averageSalaryReference) {
     return AVERAGE_POSITIONS.equal
  } else if(averageSalary > averageSalaryReference) {
     return AVERAGE_POSITIONS.above
  } else {
    return AVERAGE_POSITIONS.below
  }
}

function calculateAverageSalary(developer) {
  const earnings = get('earnings', developer)
  
  return pipe(
    reduce((result, { amount }) => result += amount, 0),
    curry(calculateAverage)(earnings.length)
  )(earnings)
}

function calculateAverage(length, total) {
  return total / length
  
}

function appendAverageSalary(developer) {
  const averageSalary = calculateAverageSalary(developer)
  return {
    ...developer,
    averageSalary
  }
}

function appendAveragePosition(developer) {
  const averagePosition = getAveragePosition(developer)
  return {
    ...developer,
    averagePosition
  }
}