Level 10

This is Hollywood we are calling you

Solution

We see an error, it says: Missing parameter ?callbackDomain.
So let's add it to our url.

http://18.198.170.188/this-is-hollywood-we-are-calling-you?callbackDomain=google.com

Now we have another error:

  Could not connect to your endpoint at
  https://google.com/add
  ?a=304200972448620530645069240248291931164
  &b=32662422308137202040109741665765070137

So the service will call /add endpoint with a and b url-parameters. Each of them contains BigInt. And I assume we should add them together and send it back to the server.

So let's write simpe server that adds two big numbers together.

{
  "name": "level10",
  "version": "1.0.0",
  "description": "Service for sum a and b",
  "main": "app.js",
  "scripts": {
    "start": "node app.js",
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "sierikov",
  "license": "MIT",
  "dependencies": {
    "express": "^4.17.1"
  }
}

So I have only express. Now we need app.js file.

Init the express and take port to run on from the enviroment:

const express = require("express");

const app = express();
const port = process.env.PORT || 443;

Activate listening:

app.listen(port, () => {
  console.log(`Server is running on port: ${port}`);
});

Define endpoint wich tage a and b and send the anwser as string back to the server.

app.get("/add", function (req, res) {
  let a = req.query?.a
  let b = req.query?.b
  res.send(`${a + b}`);
  console.log(`Got a: ${a}, b: ${b}`);
});

But, we have trouble that JS will round our numbers in exponential form.

app.get("/add", function (req, res) {
  let a = BigInt(req.query?.a);
  let b = BigInt(req.query?.b);
  res.send(`${a + b}`);
  console.log(`Got a: ${a}, b: ${b}`);
});

So we need to wrap our a and b with BigInt constructor.

Let's check if this works.

1. Run the server npm start
2. Execute http://localhost:443/add?a=2&b=4
3. Got 6

Here you can download the full source code.

We cannot give just IP adress of our network as callbackDomain

We need to get one.

I have found three services to expose our local Node.js app to the world:

• localtunnel.me - i will use this one
• ngrok
• Zeit's now

Install: npm install -g localtunnel
Start the app: npm start
Expose: lt --port 443

We get perfect-swan-72.loca.lt and will pass it to the website.

  Calling https://perfect-swan-72.loca.lt/add?a=27850060344159548346000990344376658296
  &b=89380160078435183319458195098204483639

  Congratulations, you returned the correct sum
  27850060344159548346000990344376658296 + 89380160078435183319458195098204483639
  === 117230220422594731665459185442581141935

  Find the next level at lets-go-to-the-movies.mp4

So our result url will be:
http://18.198.170.188:80/lets-go-to-the-movies.mp4

If we go to this url we will see Level 11 ( task / solution )

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