Code for the event handler of a PubNub Block that returns the base64 representation of the QR code of an URL

event-handler.js

/*
  SAMPLE PAYLOAD
  {
    "url": "https://google.com"
  }
*/
// https://gist.github.com/jonleighton/958841
const base64ArrayBuffer = function(arrayBuffer) {
    var base64    = '';
    var encodings = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';

    var bytes         = new Uint8Array(arrayBuffer);
    var byteLength    = bytes.byteLength;
    var byteRemainder = byteLength % 3;
    var mainLength    = byteLength - byteRemainder;

    var a, b, c, d;
    var chunk;

    // Main loop deals with bytes in chunks of 3
    for (var i = 0; i < mainLength; i = i + 3) {
        // Combine the three bytes into a single integer
        chunk = (bytes[i] << 16) | (bytes[i + 1] << 8) | bytes[i + 2];

        // Use bitmasks to extract 6-bit segments from the triplet
        a = (chunk & 16515072) >> 18; // 16515072 = (2^6 - 1) << 18
        b = (chunk & 258048)   >> 12; // 258048   = (2^6 - 1) << 12
        c = (chunk & 4032)     >>  6; // 4032     = (2^6 - 1) << 6
        d = chunk & 63;               // 63       = 2^6 - 1

        // Convert the raw binary segments to the appropriate ASCII encoding
        base64 += encodings[a] + encodings[b] + encodings[c] + encodings[d];
    }

    // Deal with the remaining bytes and padding
    if (byteRemainder == 1) {
        chunk = bytes[mainLength];

        a = (chunk & 252) >> 2; // 252 = (2^6 - 1) << 2

        // Set the 4 least significant bits to zero
        b = (chunk & 3)   << 4; // 3   = 2^2 - 1

        base64 += encodings[a] + encodings[b] + '==';
     } else if (byteRemainder == 2) {
        chunk = (bytes[mainLength] << 8) | bytes[mainLength + 1];

        a = (chunk & 64512) >> 10; // 64512 = (2^6 - 1) << 10
        b = (chunk & 1008)  >>  4; // 1008  = (2^6 - 1) << 4

        // Set the 2 least significant bits to zero
        c = (chunk & 15)    <<  2; // 15    = 2^4 - 1
        base64 += encodings[a] + encodings[b] + encodings[c] + '=';
    }

  return base64;
};

// Declare the Event Handler with the export syntax. The incoming message is called request
export default (request) => {
    
    const xhr = require("xhr");
    const console = require('console');
    const db = require('kvstore');
    const queryStringCodec = require('codec/query_string');
    
    try {
        const url = request.message.url;
        const apiParams = queryStringCodec.stringify({
            data: url,
            size: '150x150',
        });
        console.log(apiParams);
        return db.get(url).then((obj) => {
            let promise;
            
            if (!obj) {
                console.log(`The QR code for ${url} does not exist. Creating it...`);
                promise = new Promise((resolve) => 
                    xhr.fetch(`https://api.qrserver.com/v1/create-qr-code/?${apiParams}`, {})
                    .then((response) => {
                        //console.log(response);
            
                        const base64 = base64ArrayBuffer(response["$buffer"].data);
                        const newObj = {
                            imgBase64: base64,
                            contentType: response.headers["content-type"],
                        };
                        db.set(url, newObj).catch((err) => {
                            console.log("An error occured saving the object.", err);
                        });
                        
                        resolve(newObj);
                    })
                );
            } else {
                console.log(`The QR code for ${url} already exists...`);
                promise = Promise.resolve(obj);
            }
            
            return promise.then((qrObj) => {
                request.message.qrCode = qrObj;
                
                return request.ok();
            });
        });

  } catch (e) {
    console.error('Uncaught exception:', e);
  }
};