firestore_typing_web.js

db.collection('cities').get().then(function(querySnapshot) {
    querySnapshot.forEach(function(doc) {
        const data = doc.data();
        display(data);
        if (closeToMillion(data)) {
            console.log(`${dat.name} is close to reaching 1 mil citizens.`);
        }
    });
});

/**
 * Adds the given city to the HTML table. This function is type agnostic, and not
 * affected by the exact type of fields.
 */
function display(data) {
    const table = document.getElementById('cities');
    const row = table.insertRow(0);
    const cell1 = row.insertCell(0);
    const cell2 = row.insertCell(1);
    cell1.innerHTML = data.name;
    cell2.innerHTML = data.population;
}

/**
 * Checks if the current population is close to 1 million or more. Since the population
 * field can contain string values, we perform an explicit type conversion before performing
 * math operations.
 */
function closeToMillion(data) {
    const population = parseInt(data.population);
    return population + 5000 >= 1000000;
}