Demo: Workshop Attendance Feed

A workshop sends its attendance as a JSON string. After parsing it into an array of records, group visits by topic while counting each email address only once.

const attendanceText = '[{"topic":"Audio","email":"[email protected]"},{"topic":"Lights","email":"[email protected]"},{"topic":"Audio","email":"[email protected]"}]';
const visits = JSON.parse(attendanceText);
const visitsByTopic = new Map();
const attendeeEmails = new Set();
for (const visit of visits) {
  visitsByTopic.set(visit.topic, (visitsByTopic.get(visit.topic) ?? 0) + 1);
  attendeeEmails.add(visit.email);
}
console.log(`${visitsByTopic.get("Audio")} visits | ${attendeeEmails.size} people`);

JSON is a string until JSON.parse() produces an array you can iterate. A Map stores a count for each topic; ?? 0 means “use zero only if the lookup returns null or undefined,” so it supplies the starting count for a new key. A Set stores distinct emails. Three visits therefore represent only two people.

Checkpoint

Run the example, then inspect which step counts visits and which deduplicates people. The exercise applies the same pipeline to parcel scans rather than attendance.