Check out example codes for "res.write prints html tags as text in express". It will help you in understanding the concepts better.

Code Example 1

const express = require("express");
const https = require("https");
const app = express();

const url =
  "https://api.openweathermap.org/data/2.5/weather?q=London,uk&units=metric&appid=0333cb6bfed722ca09f1062ec1ea9ca1";
app.get("/", (req, res) => {
  https.get(url, response => {
    response.on("data", data => {
      const weatherData = JSON.parse(data);
      const temp = weatherData.main.temp;
      const { description, icon } = weatherData.weather[0];
      const imageURL = `http://openweathermap.org/img/wn/${icon}@2x.png`;

      res.set("Content-Type", "text/html");
      //OR
      res.setHeader("Content-Type", "text/html");

      res.send(`
      <h3>The weather is currently ${description}</h3>
      <img src="${imageURL}">
      <h1>The temperature in London is <span>${temp}</span> ° Celsius.</h1>
      `);
    });
  });
  //res.send('server is up!!!');
});

app.listen(3000, () => {
  console.log("Server started!!!");
});

Learn ReactJs, React Native from akashmittal.com