Employee Average Salaries | Victor Savkin's Functional TypeScript

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/*.js

employee_salaries.ts

// https://gist.githubusercontent.com/robertpenner/f02bb38553adb09ec09e2e265c01c750/raw/9f0f7bfaabebeb375e774c528bfc5089f52a80d7/employee_salaries.ts
// https://vsavkin.com/functional-typescript-316f0e003dc6

class Employee {
  constructor(public name: string, public salary: number) {}
}

class Department {
  constructor(public employees: Employee[]) {}

  works(employee: Employee): boolean {
    return this.employees.indexOf(employee) > -1
  }
}

type Predicate<T> = (t: T) => boolean

function and<T>(p1 : Predicate<T>, p2: Predicate<T>) {
  return (v) => p1(v) && p2(v)
}

function andAll<T>(predicates: Predicate<T>[]): Predicate<T> {
  return predicates.reduce((p1, p2) => and(p1, p2), (v) => true)
}

function filteredSalaries(employees: Employee[], p: Predicate<Employee>): number[] {
  return employees.filter(p).map(e => e.salary)
}

function average(nums: number[]): number {
  const total = nums.reduce((a,b) => a + b, 0)
  return total / nums.length
}

function averageFilteredSalary(employees: Employee[], p: Predicate<Employee>): number {
  return average(filteredSalaries(employees, p))
}

const employees = [
  new Employee("Jim", 100),
  new Employee("John", 200),
  new Employee("Liz", 120),
  new Employee("Penny", 30)
]

const sales = new Department([employees[0], employees[1]])

const salaryGt50 = (employee) => employee.salary > 50
const worksForSales = (employee) => sales.works(employee)

function technique1() {
  const p = andAll([salaryGt50, worksForSales])
  const salesAverageSalaryOver50 = averageFilteredSalary(employees, p)
  console.log('technique1: salesAverageSalaryOver50:', salesAverageSalaryOver50)
}

function technique2() {
  // No need for array of predicates.
  const p = and(salaryGt50, worksForSales)
  const salesAverageSalaryOver50 = averageFilteredSalary(employees, p)
  console.log('technique2: salesAverageSalaryOver50:', salesAverageSalaryOver50)
}

function technique3() {
  // Inline the averageFilteredSalary as it was very small now.
  const p = and(salaryGt50, worksForSales)
  const salesAverageSalaryOver50 = average(employees.filter(p).map(e => e.salary))
  console.log('technique3: salesAverageSalaryOver50:', salesAverageSalaryOver50)
}

technique1()
technique2()
technique3()