DICTIONARY.js

function compress(column) {
  column.compression = 'DICTIONARY';
  const dictionary = [];
  const indexes = {};

  column.values = column.values.map(value => {
    if (value === null) return null;
    // add string to dictionary if there is no such string yet
    if (indexes[value] === undefined) {
      indexes[value] = dictionary.length;
      dictionary.push(value);
    }
    // return value index
    return indexes[value];
  });

  return Object.assign(column, { dictionary });
}