mapData.js

function mapData(populationData, ratioData, serviceArray, areaServiceId, deststream) {
    // re-format the ratioData from an array to a dictionary lookup object
    // in order to do the multiplications with poplet data
    const ratioObject = dicLookup(ratioData, serviceArray);
    const writePromises = [];
    _.forEach(populationData, (populationObj) => {
      const key = `${populationObj.area_id}${populationObj.gender}${populationObj.age_band}`;
      if (ratioObject[key]) {
        const rObj = {
          areaServiceId: areaServiceId,
          areaId: populationObj.area_id,
          gender: populationObj.gender,
          year: populationObj.year,
          ageBand: populationObj.age_band,
        };
        console.log(`ratioObject has ${Object.keys(ratioObject[key]).length} keys`);
        _.forEach(ratioObject[key], (ratio, serviceId) => {
          // add serviceId field to each original poplet object
          rObj.serviceId = serviceId;
          // multiply with service ratio and persons each areaId, each demographic
          rObj.count = ratio * populationObj.persons;
          writePromises.push(new Promise((resolve, reject) => {
            if (!deststream.write(`${JSON.stringify(rObj)}\n`)) {
              // Wait for drain event
              deststream.once("drain", resolve);
            } else {
              resolve();
            }
          }));          
        });
      }
    });
    return Promise.all(writePromises);
  }