res.send for general responses in Express - Step-by-Step Execution

Concept Flow - res.send for general responses

Client sends HTTP request

↓

Express receives request

↓

Route handler runs

↓

Call res.send(data)

↓

Express sets HTTP response body

↓

Express sends response to client

↓

Client receives response

This flow shows how Express handles a client request and sends a response using res.send.

Execution Sample

Express

app.get('/hello', (req, res) => {
  res.send('Hello, friend!');
});

This code sends a simple text response 'Hello, friend!' when the client visits /hello.

Execution Table

Step	Action	Input/Value	Result/Output
1	Client sends GET request to /hello	GET /hello	Request received by Express
2	Express matches route /hello	/hello	Route handler function called
3	res.send called with 'Hello, friend!'	'Hello, friend!'	Response body set to 'Hello, friend!'
4	Express sends HTTP response	Response body: 'Hello, friend!'	Client receives response with status 200 and body
5	Request handling complete	-	Connection closed or kept alive

💡 Response sent to client, request cycle ends

Variable Tracker

Variable	Start	After res.send
res._sent	false	true
res.body	undefined	'Hello, friend!'

Key Moments - 3 Insights

Why does the client see 'Hello, friend!' after res.send is called?

Because res.send sets the response body and sends it immediately, as shown in execution_table step 3 and 4.

Can res.send send different types of data?

Yes, res.send can send strings, buffers, JSON objects, or arrays. Express converts them properly before sending.

What happens if res.send is called twice?

Only the first call sends the response. Subsequent calls are ignored or cause errors, since res._sent becomes true after the first call.

Visual Quiz - 3 Questions

Look at the execution table, what is the response body set to at step 3?

A undefined

B 'Goodbye!'

C 'Hello, friend!'

D null

💡 Hint

Check the 'Input/Value' column at step 3 in the execution_table.

At which step does Express send the response back to the client?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look for the step where 'Express sends HTTP response' in the 'Action' column.

If res.send is called twice, what happens according to variable_tracker?

A res._sent stays false

B res._sent becomes true after first call and second call is ignored

C res.body changes to second value

D Express crashes immediately

💡 Hint

See how res._sent changes from false to true after res.send in variable_tracker.

Concept Snapshot

res.send(data) sends HTTP response body to client.
Accepts strings, buffers, JSON, arrays.
Sends response immediately and ends request.
Only first res.send call works per request.
Used in route handlers to reply to requests.

Full Transcript

When a client sends a request to an Express server, Express finds the matching route handler. Inside the handler, calling res.send with some data sets the HTTP response body and sends it back to the client immediately. The client then receives this response. res.send can send different data types like strings or JSON. Only the first call to res.send sends the response; calling it again does nothing because the response is already sent. This process is shown step-by-step in the execution table and variable tracker.