Watching files for changes in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the built-in module for watching files.

Node.js

const fs = require('[1]');

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

Afs

Bpath

Cfs-extra

Dfilewatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-existent module name like 'filewatch'.

Confusing 'path' module with 'fs'.

The built-in Node.js module to watch files is fs. You import it using require('fs').

2fill in blank

medium

Complete the code to watch a file named 'example.txt' for changes.

Node.js

fs.[1]('example.txt', (eventType, filename) => {
  if (filename) {
    console.log(`${filename} file changed`);
  }
});

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

AreadFile

Bwatch

Copen

DwriteFile

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'readFile' which reads file content once.

Using 'writeFile' which writes data to a file.

The fs.watch method watches for changes on a file or directory.

3fill in blank

hard

Fix the error in the callback parameter name to correctly detect the changed file.

Node.js

fs.watch('data.json', (eventType, [1]) => {
  if (filename) {
    console.log(`${filename} was modified`);
  }
});

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

AfileName

Bfile

Cfilename

Dfile_path

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'fileName' with uppercase N which is undefined.

Using a different variable name than inside the function.

The callback parameter for the filename is filename (all lowercase). It must match the variable used inside the function.

4fill in blank

hard

Fill both blanks to watch a directory named 'logs' and log the event type and filename.

Node.js

fs.[1]('logs', ([2], filename) => {
  console.log(`Event: ${eventType}, File: ${filename}`);
});

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

Awatch

Bchange

CeventType

Dopen

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'change' as a method name instead of 'watch'.

Using 'change' as a parameter name instead of 'eventType'.

Use fs.watch to watch the directory. The first callback parameter is eventType which tells what happened.

5fill in blank

hard

Fill all three blanks to watch 'config.json', check if the event is 'rename', and log the filename.

Node.js

fs.watch('config.json', ([1], [2]) => {
  if (eventType === [3]) {
    console.log(`File ${filename} was renamed.`);
  }
});

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

AeventType

Bfilename

C'rename'

D'change'

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the order of parameters.

Checking eventType against 'change' instead of 'rename'.

The callback parameters are eventType and filename. To check if the file was renamed, compare eventType to the string 'rename'.

Practice

1. What is the main purpose of using fs.watch in Node.js?

easy

A. To read the content of a file once

B. To write data to a file

C. To monitor changes in files or directories and react automatically

D. To delete a file from the system

Solution

1. Step 1: Understand the function of fs.watch

   fs.watch is designed to watch for changes in files or directories, triggering events when changes occur.

2. Step 2: Compare with other file operations

   Reading, writing, or deleting files are different operations handled by other functions like fs.readFile, fs.writeFile, or fs.unlink.

3. Final Answer:

   To monitor changes in files or directories and react automatically -> Option C

4. Quick Check:

   Watching files = react to changes [OK]

Hint: Remember: watch means observe changes, not read or write [OK]

Common Mistakes:

• Confusing watching with reading file content
• Thinking it deletes or modifies files
• Assuming it returns file data immediately

2. Which of the following is the correct syntax to watch a file named example.txt using fs.watch?

easy

A. fs.watch('example.txt', function(event) { console.log(event); });

B. fs.watch('example.txt', (eventType, filename) => { console.log(eventType); });

C. fs.watchFile('example.txt', (eventType, filename) => { console.log(filename); });

D. fs.watchFile('example.txt', function(filename) { console.log(filename); });

Solution

1. Step 1: Identify correct function and parameters

   fs.watch takes the filename and a callback with two parameters: eventType and filename.

2. Step 2: Check callback parameter correctness

   fs.watch('example.txt', (eventType, filename) => { console.log(eventType); }); correctly uses fs.watch with the right callback signature. The other options either use fs.watchFile or incorrect callback parameters.

3. Final Answer:

   fs.watch('example.txt', (eventType, filename) => { console.log(eventType); }); -> Option B

4. Quick Check:

   Correct syntax = fs.watch('example.txt', (eventType, filename) => { console.log(eventType); }); [OK]

Hint: Remember: fs.watch callback has (eventType, filename) parameters [OK]

Common Mistakes:

• Using fs.watchFile instead of fs.watch
• Using wrong callback parameters
• Omitting filename parameter in callback

3. What will be the output when running this code if test.txt is modified?

const fs = require('fs');
fs.watch('test.txt', (eventType, filename) => {
  if (filename) {
    console.log(`${filename} file changed with event: ${eventType}`);
  } else {
    console.log('filename not provided');
  }
});

medium

A. test.txt file changed with event: change

B. filename not provided

C. SyntaxError

D. No output

Solution

1. Step 1: Understand the callback behavior on file change

   When test.txt changes, fs.watch triggers the callback with eventType usually as 'change' and filename as 'test.txt'.

2. Step 2: Analyze the conditional output

   Since filename is provided, the code logs the filename and event type message.

3. Final Answer:

   test.txt file changed with event: change -> Option A

4. Quick Check:

   File changed event logs filename and event [OK]

Hint: If filename exists, output shows file and event type [OK]

Common Mistakes:

• Assuming filename is always undefined
• Expecting no output on file change
• Confusing eventType values

4. Identify the error in this code snippet that watches a file and logs changes:

const fs = require('fs');
fs.watch('log.txt', (event, file) => {
  console.log(file + ' changed');
});

medium

A. The file name string should be an absolute path

B. Missing error handling for fs.watch

C. fs.watch cannot watch files, only directories

D. Callback parameters are incorrect; should be (eventType, filename)

Solution

1. Step 1: Recall fs.watch callback signature

   fs.watch's callback takes two arguments: the first is the event type (conventionally eventType), the second is the filename (conventionally filename). The code uses (event, file), which mismatches the standard names.

2. Step 2: Identify the issue from options

   Callback parameters are incorrect; should be (eventType, filename) correctly states that the callback parameters are incorrect and should use (eventType, filename).

3. Final Answer:

   Callback parameters are incorrect; should be (eventType, filename) -> Option D

4. Quick Check:

   Correct callback params = (eventType, filename) [OK]

Hint: Use (eventType, filename) as callback parameters for clarity [OK]

Common Mistakes:

• Using non-standard callback parameter names
• Thinking fs.watch only works on directories
• Assuming relative path is invalid

5. You want to watch a directory logs and print the name of any new file created inside it. Which code snippet correctly achieves this?

hard

A. fs.watch('logs', (eventType, filename) => { if (eventType === 'rename' && filename) { console.log(`New file: ${filename}`); } });

B. fs.watch('logs', (eventType) => { if (eventType === 'change') { console.log('File changed'); } });

C. fs.watchFile('logs', (curr, prev) => { console.log('Directory changed'); });

D. fs.watch('logs', () => { console.log('Something changed'); });

Solution

1. Step 1: Understand event types for directory watching

   When watching a directory, the rename event indicates a file was added or removed.

2. Step 2: Check for filename and event type

   fs.watch('logs', (eventType, filename) => { if (eventType === 'rename' && filename) { console.log(`New file: ${filename}`); } }); checks for rename event and ensures filename is provided before logging the new file name, which is correct.

3. Step 3: Evaluate other options

   fs.watch('logs', (eventType) => { if (eventType === 'change') { console.log('File changed'); } }); only checks for change event and does not handle new files specifically. fs.watchFile('logs', (curr, prev) => { console.log('Directory changed'); }); uses fs.watchFile which is for files, not directories. fs.watch('logs', () => { console.log('Something changed'); }); logs on any change but does not specify new files.

4. Final Answer:

   fs.watch('logs', (eventType, filename) => { if (eventType === 'rename' && filename) { console.log(`New file: ${filename}`); } }); -> Option A

5. Quick Check:

   Use 'rename' event and check filename for new files [OK]

Hint: Use 'rename' event to detect new files in directory [OK]

Common Mistakes:

• Using 'change' event to detect new files
• Using fs.watchFile for directories
• Not checking if filename is provided