/credit-card-demo-api

Primary LanguageJavaScript

Section 1

Question No. 1: What is the result of this code in the console:

for (var i = 0; i < 100; i++) {
  setTimeout(function () {
    console.log(i);
  }, 200);
}

The result of this code in console is:

100
100
100
...
100

100 is printed in the console 100 times. Reason: This happens because of "closures." When setTimeout stores the function and attaches the timer to it, it remembers the reference to i. So every time the value of when i is being logged, it is pointing to the same reference of i which has already been incremented to 100 before the timer function is executed after 2ms.

Question No. 2: What is the result of this code in the console:

(async function () {
  function waitForMe() {
    return new Promise(function (resolve, reject) {
      setTimeout(function () {
        resolve('hello');
      }, 200);
    });
  }
  const result = await waitForMe();
  console.log(result);
})();

The result of the this code in console is:

hello

Reason: waitForMe is enclosed in the self calling function. When the code is executed the self invoking function (also known as IIFE) is called. So when the execution reaches the point where waitForMe is called, as waitForMe returns a promise, it waits for waitForMe to resolve or reject, waitForMe returns a resolved value of 'hello' after 2ms which is logged to the console.

Question No. 3: Explain the difference:

var a;
let b;
const c;

Differences:

Var Let Const
Function scoped Block Scoped Block Scoped
Can pe updated/re-assigned or re-declared within the scope Can only be updated/re-assigned Can't be updated/reassigned
Hoisted with a default value Hoisted but not initialized Hoisted but not initialized

Question No. 4: What is the result of this code in the console: We got a module "test.js":

var arr = [];
module.exports = arr;

We got a module using it:

var test = require('./test');
test.push('hello');
console.log(JSON.stringify(test));

We got another module using it later:

var test = require('./test');
test.push('another');
console.log(JSON.stringify(test));

Explanation: From test module arr is being exported but it is being assigned to a separate memory location by the modules using it respectively. So if we run hello module, the log would be:

["hello"]

Similarly, if we run another module the log would be:

["another"]

It won't get pushed to the same array because the references are different.

Section 2

Assuming you have Node.js and Postgres installed on your system, follow the following steps to run the server: Note: env file is being pushed with the project for demo purposes. Clone this repo using to your local system.

git clone <repolink>

Install the dependencies using:

npm i

Run the server using:

npm run start

Answers About Section 2

Choice of Encryption Algorithm and the reason behind it:

I chose aes-256-ctr to encrypt ccv and credit card number.

Reasons:

  1. It's virtually impossible to brute force it. [https://www.n-able.com/blog/aes-256-encryption-algorithm#:~:text=AES%20256%20is%20virtually%20impenetrable,encryption%20system%20is%20entirely%20secure.]
  2. High performance [https://en.wikipedia.org/wiki/Advanced_Encryption_Standard]
  3. Comes out of the box with node so it's easier to implement.

Choice of Implementation of Luhn Algo:

I used npm to install luhn algo implementation

Reasons:

  1. Didn't want to reinvent the wheel.
  2. Got things done faster
  3. Used by the community so it's tried and tested.