Long polling and Server-Sent Events in HLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a long polling request from the client.

HLD

function startLongPolling() {
  fetch('/events')
    .then(response => response.[1]())
    .then(data => {
      console.log('Received:', data);
      startLongPolling();
    });
}

Drag options to blanks, or click blank then click option'

AarrayBuffer

Bjson

Cblob

Dtext

Attempts:

3 left

💡 Hint

Common Mistakes

Using json() when the server sends plain text.

Not calling the function recursively to continue polling.

The long polling response is usually plain text or JSON. Using text() reads the response as plain text, which is common for long polling.

2fill in blank

medium

Complete the server-side pseudo-code to send an SSE event.

HLD

function sendSSE(response, data) {
  response.write('data: ' + [1] + '\n\n');
}

Drag options to blanks, or click blank then click option'

Adata

BJSON.stringify(data)

Cdata.toString()

DString(data)

Attempts:

3 left

💡 Hint

Common Mistakes

Sending raw objects without stringifying.

Missing the double newline to end the event.

Server-Sent Events send data as text lines. Using JSON.stringify(data) ensures complex data is sent as a JSON string.

3fill in blank

hard

Fix the error in the SSE client code to properly listen for messages.

HLD

const evtSource = new EventSource('/events');
evtSource.[1] = function(event) {
  console.log('Message:', event.data);
};

Drag options to blanks, or click blank then click option'

Aonerror

Bonopen

Conmessage

Donconnect

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'onopen' which fires when connection opens, not for messages.

Using 'onerror' which fires on errors, not messages.

The onmessage event handler listens for incoming SSE messages.

4fill in blank

hard

Fill both blanks to complete the long polling server logic that waits for new data before responding.

HLD

function handleLongPolling(request, response) {
  waitForNewData(() => {
    response.writeHead(200, { 'Content-Type': 'text/plain' });
    response.[1](newData);
    response.[2]();
  });
}

Drag options to blanks, or click blank then click option'

Aend

Bwrite

Csend

Dflush

Attempts:

3 left

💡 Hint

Common Mistakes

Calling end() before write(), causing no data sent.

Using send() which is not a standard Node.js response method.

The server writes the new data with write() and then ends the response with end() to complete the request.

5fill in blank

hard

Fill all three blanks to complete the SSE server setup with correct headers and keep-alive logic.

HLD

function setupSSE(response) {
  response.writeHead(200, {
    'Content-Type': [1],
    'Cache-Control': [2],
    'Connection': [3]
  });
}

Drag options to blanks, or click blank then click option'

A'text/event-stream'

B'no-cache'

C'keep-alive'

D'application/json'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'application/json' as Content-Type for SSE.

Omitting Cache-Control header causing caching issues.

Not setting Connection to keep-alive causing connection close.

SSE requires Content-Type as text/event-stream, disables caching with no-cache, and keeps the connection alive with keep-alive.