comma-operator.js

// the 3 line reduce callback
[{key: 'USA', value: […]}, {key: 'Germany', value: […]}, {key: null, value: []}]
  .filter(item => item.key)
  .map(item => {
    item.key = item.key.toUpperCase();
    return item;
  })
  .reduce((accumulator, item) => {
    accumulator[item.key] = item.value;
    return accumulator;
  }, {});

// could turn into
[{key: 'USA', value: […]}, {key: 'Germany', value: […]}, {key: null, value: []}]
  .filter(item => item.key)
  .map(item => (item.key = item.key.toLowerCase(), item))
  .reduce((accumulator, item) => (accumulator[item.key] = item.value, accumulator), {});


// -> {USA: […], GERMANY: […]}