Mapping from one data structure to another

task.js

function process (input) {
  
  // Reduce over input to process an object where the key is the location and the value is the products
  const tree = input
  .reduce((output, o) => {
    o.locations.forEach(location => {
      if (output[location] === undefined) {
        output[location] = [];
      }

      output[location].push(o.name);
    });

    return output;
  }, {});

  // Format the tree into an array of objects
  const output = [];

  for(const location in tree) {
    if (tree.hasOwnProperty(location)) {
      output.push({ location, names: tree[location] });
    }
  }

  return output;
};

module.exports = process;

task.spec.js

const process = require('./process');

describe('Converting an array of products and their locations', () => {

  test('returns an array of location objects', () => {
    const input = [
      { name: 'Product 1', locations: ['Paris', 'France', 'Europe'] },
      { name: 'Product 2', locations: ['Bordeux', 'France', 'Europe'] },
      { name: 'Product 3', locations: ['Edinburgh', 'Scotland', 'Europe'] },
    ];

        // Output
    const expectedOutput = [
      {
        location: 'Paris',
        names: ['Product 1'],
      },
      {
        location: 'Bordeux',
        names: ['Product 2'],
      },
      {
        location: 'Edinburgh',
        names: ['Product 3'],
      },
      {
        location: 'Scotland',
        names: ['Product 3'],
      },
      {
        location: 'France',
        names: ['Product 1', 'Product 2'],
      },
      {
        location: 'Europe',
        names: ['Product 1', 'Product 2', 'Product 3'],
      }
    ];

    expect(process(input)).toEqual(output);
  });
});