justJoinItReport.ts

import Fetch from 'node-fetch';

interface JJITOffer {
  title: string;
  street: string;
  city: string;
  country_code: string;
  address_text: string;
  marker_icon: string;
  workplace_type: string;
  company_name: string;
  company_url: string;
  company_size: string;
  experience_level: string;
  latitude: string;
  longitude: string;
  published_at: string;
  remote_interview: boolean;
  id: string;
  employment_types: EmploymentType[];
  company_logo_url: string;
  skills: Skill[];
  remote: boolean;
}

interface EmploymentType {
  type: "mandate_contract" | "permanent" | "b2b";
  salary: null | { from: number; to: number; currency: "pln" | "usd" | "eur" };
}

interface EmploymentTypePln {
  type: "mandate_contract" | "permanent" | "b2b";
  salary: null | { from: number; to: number; currency: "pln" };
}

interface Skill {
  name: string;
  level: number;
}

const notNil = <T>(el: T | null | undefined): el is T => el != null;
const average = (ss: Array<number | undefined | null>) => {
  const filtered = ss.filter(notNil);
  return filtered.reduce((a, b) => a + b, 0) / filtered.length;
}
const by = <Key extends string, Obj extends {[K in Key]: unknown}>(arr: readonly (Obj | null | undefined)[], key: Key) => arr.map(el => el?.[key]);

const hasSalary = (o: JJITOffer) => o.employment_types.some(t => !!t.salary);
const requiresJavaScript = (o: JJITOffer) => o.skills.some(skill => skill.name.toLowerCase() === 'javascript')
const levelJunior = (o: JJITOffer) => o.experience_level === 'junior'
const takeEmploymentTypes = (o: JJITOffer) => o.employment_types;
const convertCurrency = (e: EmploymentType): EmploymentTypePln | null => {
  switch(e.salary?.currency) {
    case 'pln':
      return e as EmploymentTypePln;
    case 'eur':
      return {...e, salary: { to: e.salary.to * 4.54, from: e.salary.from * 4.54, currency: 'pln' }}
    case 'eur':
      return {...e, salary: { to: e.salary.to * 4.00, from: e.salary.from * 4.00, currency: 'pln' }}
  }
  return null;
}

const res = await Fetch('https://justjoin.it/api/offers');
const offers = await res.json() as readonly JJITOffer[];

const salariesByEmploymentType =
  offers
    .filter(levelJunior)
    .filter(hasSalary)
    .filter(requiresJavaScript)
    .flatMap(takeEmploymentTypes)
    .map(convertCurrency)
    .filter(notNil)
    .reduce((acc: Partial<Record<EmploymentTypePln['type'], EmploymentTypePln['salary'][]>>, el) => {
      acc[el.type] = acc[el.type] || [];
      acc[el.type]!.push(el.salary);
      return acc;
    }, {});

const result =
  Object.fromEntries(
    Object.entries(salariesByEmploymentType).map(([type, salaries]) => {
      const low = average(by(salaries, 'from'));
      const high = average(by(salaries, 'to'));
      const avg = (low + high) / 2;
      return [type, { low: low.toFixed(2), high: high.toFixed(2), avg: avg.toFixed(2) }] as const;
    })
  );

console.log(result);