ESP32 DHT22 weather web server

/*
 * ESP32 weather web server: a DHT22 on GPIO 4, and a web page that shows the
 * temperature and humidity, served by the ESP32 itself.
 *
 *   DHT22   VCC -> 3V3, DATA -> GPIO 4 (D4), GND -> GND
 *           (the module has its own pull-up on DATA; a bare sensor needs
 *           a 10 k resistor from DATA to 3V3)
 *
 * Press Run, wait for "open http://..." on the Serial Monitor, then open the
 * Web tab. The page reloads itself every 10 seconds. /data answers the same
 * reading as JSON, for another program to read.
 *
 * # Reading the sensor without stalling the server
 *
 * A DHT22 can only be read once every two seconds, and a read takes a few
 * milliseconds. So loop() reads it on a timer, keeps the last good reading,
 * and serves that. A request never waits for the sensor, and a failed read
 * (NaN) keeps the last good value instead of showing "nan" on the page.
 *
 * The network is SIMULATED: there is no 802.11, no TCP and no real internet,
 * and the only thing that can reach this server is the Web tab in this page.
 * On a real ESP32 the same sketch serves any phone or laptop on your WiFi.
 */
#include <WiFi.h>
#include <WebServer.h>
#include <DHT.h>

const char *SSID = "Mokxi-Lab";
const char *PASSWORD = "breadboard";

const int DHT_PIN = 4;
DHT dht(DHT_PIN, DHT22);
WebServer server(80);

float temperatureC = NAN;
float humidity = NAN;
unsigned long readAt = 0;
unsigned long readings = 0;

void readSensor()
{
    float t = dht.readTemperature();
    float h = dht.readHumidity();
    if (isnan(t) || isnan(h)) {
        Serial.println("DHT22 read failed: check the DATA wire");
        return;
    }
    temperatureC = t;
    humidity = h;
    readings++;
    Serial.print("temperature ");
    Serial.print(temperatureC, 1);
    Serial.print(" C, humidity ");
    Serial.print(humidity, 1);
    Serial.println(" %");
}

void handleRoot()
{
    String page("<!doctype html><html><head><meta charset=\"utf-8\">");
    page += "<meta name=\"viewport\" content=\"width=device-width, initial-scale=1\">";
    page += "<meta http-equiv=\"refresh\" content=\"10\">";
    page += "<title>ESP32 weather station</title></head><body>";
    page += "<h1>ESP32 weather station</h1>";
    if (isnan(temperatureC)) {
        page += "<p>Waiting for the first reading...</p>";
    } else {
        page += "<p>Temperature: <b>";
        page += String(temperatureC, 1);
        page += " &deg;C</b> (";
        page += String(temperatureC * 9.0 / 5.0 + 32.0, 1);
        page += " &deg;F)</p>";
        page += "<p>Humidity: <b>";
        page += String(humidity, 1);
        page += " %</b></p>";
    }
    page += "<p><a href=\"/\">Refresh</a> &middot; <a href=\"/data\">JSON</a></p>";
    page += "</body></html>";
    server.send(200, "text/html", page);
}

void handleData()
{
    String json("{\"temperature\":");
    json += isnan(temperatureC) ? String("null") : String(temperatureC, 1);
    json += ",\"humidity\":";
    json += isnan(humidity) ? String("null") : String(humidity, 1);
    json += ",\"readings\":";
    json += String(readings);
    json += "}";
    server.send(200, "application/json", json);
}

void setup()
{
    Serial.begin(115200);
    dht.begin();

    WiFi.begin(SSID, PASSWORD);
    Serial.print("joining ");
    Serial.print(SSID);
    while (WiFi.status() != WL_CONNECTED) {
        Serial.print(".");
        delay(250);
    }
    Serial.println();
    Serial.print("open http://");
    Serial.print(WiFi.localIP());
    Serial.println("/ in the Web tab");

    server.on("/", handleRoot);
    server.on("/data", handleData);
    server.begin();
    readSensor();
    readAt = millis();
}

void loop()
{
    server.handleClient();
    if (millis() - readAt >= 2000) {
        readAt = millis();
        readSensor();
    }
    delay(2);
}