Working script to get vaccine availability alert on console as soon as a slot opens up in https://selfregistration.cowin.gov.in/

cowin_vaccination.js

// This javascript code will alert you whenever some slot opens up on the given date within the given pincodes.
// This script will run every 2 seconds and check for availability of slots.

// Steps to use
// 1. Update pincodes,age,district_id,vaccine & vaccine_date. 
// 2. Login to https://selfregistration.cowin.gov.in/
// 3. Right Click on the website
// 4. Click on Inspect
// 5. Switch to the Console Tab on the recently opened Inspect window
// 6. Copy paste the contents of this entire file - cowin_vaccination.js
// 7. Press Enter


var sleepNow = (delay) => new Promise((resolve) => setTimeout(resolve, delay))
var trialCounter = 1;

// Change the pincodes as per your city
var pincodes = [411001,411028,411057,410506,410501,411027,411017,411006,411033,411035,411041,412115,411044,411011,411027,411026,411001,411038,411018];

// Change the age as per your requirement
var age = 18; //45,18

// Change the distric id as per the city you are living in
var district_id = 363;

// Change the vaccine as per your need. Check comments on how to get distric id. 
var vaccine = "COVAXIN"; // COVISHIELD,COVAXIN

// change the date to the date of slot booking
var vaccine_date = "29-05-2021"; 

// Enter dose = 1 for first dose, dose = 2 for second dose
var dose = 1;

function httpGet(theUrl)
{
    var xmlHttp = new XMLHttpRequest();
    xmlHttp.open( "GET", theUrl, false ); // false for synchronous request
    xmlHttp.send( null );
    return xmlHttp.responseText;
}

async function fetchByDistrict() {
    console.log("Check: ", trialCounter++);
    url = "https://cdn-api.co-vin.in/api/v2/appointment/sessions/public/calendarByDistrict?district_id="+district_id+"&date="+vaccine_date;
    	
	try {
	  centers = JSON.parse(httpGet(url)).centers;
	} catch(e) {
	  console.log("error");
	}
			
	for (i in centers) { 
		center = centers[i];
		 for (j in center.sessions) {
			session = center.sessions[j];
			//console.log(session);
			
			var available_capacity = 0;
			if(dose==1)
				available_capacity = session.available_capacity_dose1;
			else if(dose==2)
				available_capacity = session.available_capacity_dose2;
				
			if (session.min_age_limit == age && session.available_capacity > 0 && session.vaccine == vaccine && pincodes.includes(center.pincode)) {
				console.log("Vaccines available at : ", center.pincode, center.name, session.available_capacity);
				var audio = new Audio('https://media.geeksforgeeks.org/wp-content/uploads/20190531135120/beep.mp3');
				audio.play();
			}
        }	 
	} 
	await sleepNow(2000);
    fetchByDistrict();  
}

fetchByDistrict();

Those looking for 2nd dose can replace "session.available_capacity" on line 55 and 56 with "session.available_capacity_dose2"