spawn for streaming processes in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the spawn function from the child_process module.

Node.js

const { [1] } = require('child_process');

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

Aexec

Bfork

Cspawn

DexecFile

Attempts:

3 left

💡 Hint

Common Mistakes

Using exec instead of spawn, which buffers output instead of streaming.

Trying to import a function that does not exist in child_process.

The spawn function is imported from the child_process module to create streaming child processes.

2fill in blank

medium

Complete the code to spawn a child process that runs the 'ls' command.

Node.js

const ls = spawn('[1]');

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

Adir

Bshow

Clist

Dls

Attempts:

3 left

💡 Hint

Common Mistakes

Using Windows command 'dir' on Unix systems.

Using a non-existent command like 'list' or 'show'.

The ls command lists directory contents on Unix-like systems and is used here with spawn.

3fill in blank

hard

Fix the error in the code to correctly handle the data event from the child process stdout stream.

Node.js

ls.stdout.on('[1]', (data) => {
  console.log(`Output: ${data}`);
});

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

Adata

Berror

Cmessage

Dclose

Attempts:

3 left

💡 Hint

Common Mistakes

Listening for 'message' which is not an event on stdout streams.

Using 'error' event to get output data.

The data event is emitted when the child process sends output data on stdout.

4fill in blank

hard

Fill both blanks to spawn a process running 'node' with arguments ['-v'] and handle its exit event.

Node.js

const proc = spawn('[1]', ['[2]']);
proc.on('exit', (code) => {
  console.log(`Process exited with code ${code}`);
});

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

Anode

B-v

C-h

Dnpm

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'npm' instead of 'node' for the command.

Using '-h' which shows help, not version.

The node command runs Node.js, and -v prints its version. The exit event tells when the process finishes.

5fill in blank

hard

Fill all three blanks to create a child process that runs 'grep' to filter 'error' from input, and handle its stdout data event.

Node.js

const grep = spawn('[1]', ['[2]']);
grep.stdout.on('[3]', (data) => {
  console.log(`Filtered output: ${data}`);
});

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

Agrep

Berror

Cdata

Dexit

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'exit' event instead of 'data' to read output.

Using wrong command or pattern strings.

grep filters input lines matching 'error'. The data event reads the filtered output stream.

Practice

1. What is the main advantage of using spawn in Node.js for running commands compared to exec?

easy

A. It automatically retries failed commands.

B. It runs commands only in the background without output.

C. It streams output live without buffering all data first.

D. It converts output to JSON format automatically.

Solution

1. Step 1: Understand spawn behavior

   spawn runs commands and streams their output as it happens, without waiting for the whole output to finish.

2. Step 2: Compare with exec

   exec buffers the entire output before returning it, which can cause delays or memory issues with large outputs.

3. Final Answer:

   It streams output live without buffering all data first. -> Option C

4. Quick Check:

   spawn streams output live = B [OK]

Hint: spawn streams output live, exec buffers all output [OK]

Common Mistakes:

• Thinking spawn retries commands automatically
• Assuming spawn hides output
• Believing spawn formats output as JSON

2. Which of the following is the correct way to import spawn from the child_process module in Node.js?

easy

A. const spawn = require('child_process').spawn;

B. import spawn from 'child_process';

C. import { spawn } from 'child_process';

D. const { spawn } = require('child_process');

Solution

1. Step 1: Identify Node.js import syntax

   Node.js commonly uses CommonJS syntax with require and destructuring to import specific functions.

2. Step 2: Check correct destructuring

   The correct way is const { spawn } = require('child_process'); to get spawn from the module.

3. Final Answer:

   const { spawn } = require('child_process'); -> Option D

4. Quick Check:

   Destructure spawn from require('child_process') = C [OK]

Hint: Use destructuring with require for spawn import [OK]

Common Mistakes:

• Using default import syntax in CommonJS
• Not destructuring spawn from the module
• Using import without enabling ES modules

3. Consider this code snippet:

const { spawn } = require('child_process');
const ls = spawn('ls', ['-l']);
ls.stdout.on('data', (data) => {
  console.log(`Output: ${data}`);
});
ls.stderr.on('data', (data) => {
  console.error(`Error: ${data}`);
});
ls.on('close', (code) => {
  console.log(`Process exited with code ${code}`);
});

What will this code do when run in a directory?

medium

A. Print the detailed list of files, errors if any, and exit code when done.

B. Only print errors and ignore normal output.

C. Buffer all output and print it after the process ends.

D. Throw a syntax error because of wrong event names.

Solution

1. Step 1: Analyze event listeners

   The code listens to stdout data events to print output live, stderr for errors, and close to know when the process ends.

2. Step 2: Understand spawn behavior

   spawn streams output, so the console logs will show file list lines as they come, errors if any, and finally the exit code.

3. Final Answer:

   Print the detailed list of files, errors if any, and exit code when done. -> Option A

4. Quick Check:

   spawn streams output and errors live = D [OK]

Hint: spawn streams stdout, stderr, and close events [OK]

Common Mistakes:

• Thinking output is buffered until process ends
• Ignoring stderr event handling
• Assuming event names are incorrect

4. What is wrong with this code snippet that uses spawn?

const { spawn } = require('child_process');
const proc = spawn('node', ['-v']);
proc.stdout.on('data', (data) => {
  console.log(data);
});
proc.on('close', (code) => {
  console.log(`Exited with ${code}`);
});

medium

A. It misses listening to the 'error' event on the process.

B. It logs a Buffer object instead of a string for stdout data.

C. It uses wrong arguments for spawn command.

D. It does not handle the 'exit' event.

Solution

1. Step 1: Check stdout data handling

   The data event provides a Buffer, so logging it directly prints a Buffer object, not a readable string.

2. Step 2: Correct usage to convert Buffer

   To print readable output, convert Buffer to string using data.toString() before logging.

3. Final Answer:

   It logs a Buffer object instead of a string for stdout data. -> Option B

4. Quick Check:

   stdout data is Buffer, needs toString() = A [OK]

Hint: Convert stdout Buffer to string before logging [OK]

Common Mistakes:

• Logging Buffer directly without conversion
• Ignoring error event handling (not critical here)
• Confusing 'close' and 'exit' events

5. You want to run a long-running command that outputs JSON lines continuously. Which approach using spawn is best to process each JSON line as it arrives without waiting for the command to finish?

hard

A. Listen to stdout 'data' events, buffer chunks, split by newline, and parse each JSON line immediately.

B. Use exec to get all output at once, then parse JSON lines after process ends.

C. Listen only to close event and parse output then.

D. Spawn the process without event listeners and read output from a file.

Solution

1. Step 1: Understand streaming JSON lines

   For continuous JSON lines, you must process output as it streams, not wait for all output.

2. Step 2: Use stdout 'data' event with buffering

   Listen to 'data' events, accumulate chunks, split by newline, and parse each JSON line immediately to handle streaming data.

3. Step 3: Why other options fail

   exec buffers all output (bad for long-running), close fires only at end, and reading from file is indirect and slower.

4. Final Answer:

   Listen to stdout 'data' events, buffer chunks, split by newline, and parse each JSON line immediately. -> Option A

5. Quick Check:

   Stream and parse JSON lines live = A [OK]

Hint: Buffer and split stdout data by newline to parse JSON live [OK]

Common Mistakes:

• Using exec for streaming output
• Parsing only after process ends
• Ignoring buffering and splitting lines