current-temperatures.js

// Use your OpenWeatherMap API KEY
// Set the current weather data API URL
const API_KEY = 'YOUR_API_KEY_HERE';
const API_URL = `https://api.openweathermap.org/data/2.5/weather?appid=${API_KEY}&units=metric`;

// Set the list of cities
const CITIES = [
  'London', 'Tokyo', 'Melbourne', 'Vancouver',
  'Lagos', 'Berlin', 'Paris', 'Johannesburg',
  'Chicago', 'Mumbai', 'Cairo', 'Beijing'
];

/**
 * Fetches the current temperature of a city (in °C).
 * @param {string} city The city to fetch its current temperature
 * @returns {Promise} A promise that is fulfilled with an array of the format [city, temp]
 */
const fetchTempForCity = city => {
  // Append the encoded city name to the API URL and make a request with `fetch()`
  // From the JSON response, get the value of the `main.temp` property
  // Return a promise that is fulfilled with an array of the format [city, temp]
  // For example: ['Lagos', 29.28]
  
  return fetch(`${API_URL}&q=${encodeURIComponent(city)}`)
    .then(response => response.json())
    .then(data => [ city, data.main.temp || null ]);
}

/**
 * Fetches the current temperatures of multiple cities (in °C).
 * @param {Array} cities The cities to fetch their current temperatures
 * @returns {Promise} A promise that is fulfilled with an object map of each city against temp
 */
const fetchTempForCities = cities => {
  // Use `Array.prototype.map()` to create a Promise for each city's current temperature.
  // Use `Promise.all()` to execute all the promises in parallel (as a single batch).
  // When all the promises have been settled, use `Array.prototype.reduce()` to construct
  // an object map of each city in the list of cities against its current temperature (in °C).
  // Return a promise that is fulfilled with the object map
  
  return Promise.all(cities.map(fetchTempForCity))
    .then(temps => {
      return temps.reduce((data, [ city, temp ]) => {
        return { ...data, [city]: Number.isFinite(temp) ? temp.toFixed(2) * 1 : null };
      }, {});
    });
}

// Fetch the current temperatures for all the cities listed in CITIES
// `console.log()` the data on fulfillment and `console.error()` on rejection
fetchTempForCities(CITIES)
  .then(console.log, console.error);