File system permissions (--allow-read, --allow-write) in Deno - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to read a file using Deno with the correct permission flag.

Deno

const data = await Deno.readTextFile('example.txt');
console.log(data);
// Run with deno run [1] script.ts

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

A--allow-env

B--allow-write

C--allow-net

D--allow-read

Attempts:

3 left

💡 Hint

Common Mistakes

Using --allow-write instead of --allow-read

Forgetting to add any permission flag

Using network permission --allow-net by mistake

To read files in Deno, you must run the script with the --allow-read flag to grant read permission.

2fill in blank

medium

Complete the code to write text to a file using Deno with the correct permission flag.

Deno

await Deno.writeTextFile('output.txt', 'Hello Deno!');
// Run with deno run [1] script.ts

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

A--allow-net

B--allow-read

C--allow-write

D--allow-run

Attempts:

3 left

💡 Hint

Common Mistakes

Using --allow-read instead of --allow-write

Not adding any permission flag

Confusing with network or run permissions

Writing files requires the --allow-write permission flag in Deno.

3fill in blank

hard

Fix the error in the command to allow both reading and writing files.

Deno

deno run [1] script.ts

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

A--allow-read --allow-write

B--allow-write

C--allow-read

D--allow-net

Attempts:

3 left

💡 Hint

Common Mistakes

Using only one permission flag

Combining flags without space

Using unrelated permission flags

To allow both reading and writing, you must include both flags: --allow-read and --allow-write.

4fill in blank

hard

Fill both blanks to correctly check if the script has read and write permissions.

Deno

const hasRead = await Deno.permissions.query({ name: '[1]' });
const hasWrite = await Deno.permissions.query({ name: '[2]' });

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

Aread

Bwrite

Cnet

Drun

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'net' or 'run' instead of 'read' or 'write'

Mixing up the order of permissions

Using permission names with --allow- prefix

The permission names for checking are 'read' and 'write' for file system access.

5fill in blank

hard

Fill all three blanks to request read permission, write permission, and then log if both are granted.

Deno

const readPerm = await Deno.permissions.request({ name: '[1]' });
const writePerm = await Deno.permissions.request({ name: '[2]' });
if (readPerm.state === '[3]' && writePerm.state === 'granted') {
  console.log('Permissions granted');
}

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

Aread

Bwrite

Cgranted

Ddenied

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for 'denied' instead of 'granted'

Using wrong permission names

Not comparing permission states correctly

You request 'read' and 'write' permissions, and check if the state is 'granted' to confirm access.

Practice

1. What does the --allow-read flag do when running a Deno program?

easy

A. Allows the program to read files or folders specified.

B. Allows the program to write or modify files.

C. Blocks the program from accessing the network.

D. Runs the program without any permissions.

Solution

1. Step 1: Understand the purpose of --allow-read

   This flag grants permission to read files or folders.

2. Step 2: Compare with other flags

   --allow-write is for writing, not reading. Network access is unrelated.

3. Final Answer:

   Allows the program to read files or folders specified. -> Option A

4. Quick Check:

   --allow-read = read permission [OK]

Hint: Read permission means accessing files, not changing them [OK]

Common Mistakes:

• Confusing read permission with write permission
• Thinking it controls network access
• Assuming it grants all permissions

2. Which of the following is the correct way to run a Deno script app.ts with permission to write files?

easy

A. deno run app.ts --allow-write

B. deno run app.ts --allow-read

C. deno --allow-write run app.ts

D. deno run --allow-write app.ts

Solution

1. Step 1: Recall Deno command syntax

   The flags come immediately after deno run and before the script name.

2. Step 2: Identify correct flag placement

   deno run --allow-write app.ts is correct; options A and C have wrong order, D uses wrong flag.

3. Final Answer:

   deno run --allow-write app.ts -> Option D

4. Quick Check:

   Flags go after deno run [OK]

Hint: Flags come right after 'deno run', before script name [OK]

Common Mistakes:

• Placing flags after the script name
• Mixing up --allow-read and --allow-write
• Putting flags before 'run'

3. Consider this Deno script readfile.ts that reads data.txt and prints its content. What happens if you run deno run readfile.ts without any flags?

medium

A. The script creates data.txt if missing.

B. The script runs and prints the file content.

C. The script throws a permission error.

D. The script runs but prints nothing.

Solution

1. Step 1: Understand default permissions

   Without --allow-read, Deno denies file reading for safety.

2. Step 2: Predict script behavior

   Trying to read data.txt without permission causes a runtime error.

3. Final Answer:

   The script throws a permission error. -> Option C

4. Quick Check:

   No --allow-read means no file reading [OK]

Hint: No read flag means reading files causes error [OK]

Common Mistakes:

• Assuming Deno allows reading by default
• Thinking the file is created automatically
• Believing the script prints empty content silently

4. You have a Deno script that writes to log.txt. Running deno run --allow-read write.ts causes a permission error when writing. What is the fix?

medium

A. Add --allow-write flag when running the script.

B. Remove --allow-read flag and run without flags.

C. Change the script to read instead of write.

D. Run the script with --allow-net flag.

Solution

1. Step 1: Identify the permission needed

   Writing files requires --allow-write, not just --allow-read.

2. Step 2: Apply correct flag

   Run the script with --allow-write to allow file writing.

3. Final Answer:

   Add --allow-write flag when running the script. -> Option A

4. Quick Check:

   Write permission needs --allow-write [OK]

Hint: Write needs --allow-write, not --allow-read [OK]

Common Mistakes:

• Using only --allow-read for writing
• Removing flags thinking it grants all permissions
• Confusing network permission with file permission

5. You want to run a Deno script that reads from input.txt and writes to output.txt. Which command correctly grants the minimum permissions needed?

hard

A. deno run --allow-net script.ts

B. deno run --allow-read=./input.txt --allow-write=./output.txt script.ts

C. deno run --allow-write=./input.txt --allow-read=./output.txt script.ts

D. deno run --allow-read --allow-write script.ts

Solution

1. Step 1: Understand fine-grained permissions

   Using --allow-read=path and --allow-write=path limits access to specific files.

2. Step 2: Match permissions to files

   Read permission for input.txt and write permission for output.txt is needed.

3. Step 3: Check options

   deno run --allow-read=./input.txt --allow-write=./output.txt script.ts correctly assigns read to input and write to output. deno run --allow-read --allow-write script.ts grants full access, which is more than needed. deno run --allow-write=./input.txt --allow-read=./output.txt script.ts swaps permissions. deno run --allow-net script.ts is unrelated.

4. Final Answer:

   deno run --allow-read=./input.txt --allow-write=./output.txt script.ts -> Option B

5. Quick Check:

   Grant only needed file permissions [OK]

Hint: Use path-limited flags to grant minimal permissions [OK]

Common Mistakes:

• Granting full read/write when only specific files needed
• Swapping read and write paths
• Using unrelated flags like --allow-net