Permission prompts at runtime in Deno - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Deno permission prompts

What happens when you run a Deno script that tries to read a file without the --allow-read flag?

Deno

console.log(await Deno.readTextFile("example.txt"));

A The script runs successfully and prints the file content.

B The script silently ignores the read request and prints nothing.

C Deno prompts the user at runtime to allow file read permission.

D The script throws a runtime error about missing read permission.

💡 Hint

Think about Deno's security model and how it handles permissions by default.

Deno requires explicit permission flags for file system access. Without --allow-read, it throws a runtime error denying access.

❓ component_behavior

intermediate

Behavior of Deno.permissions.request()

What is the output of this code snippet if the user denies the permission request?

Deno

const status = await Deno.permissions.request({ name: "read", path: "./data.txt" });
console.log(status.state);

A "granted"

B "prompt"

C "denied"

D Throws a runtime error

💡 Hint

Consider what the request() method returns based on user action.

If the user denies the permission prompt, request() returns a status object with state set to "denied".

📝 Syntax

advanced

Correct usage of Deno.permissions.query()

Which option correctly queries if the script has write permission to the current directory?

A await Deno.permissions.query({ permission: "write", location: "." });

B await Deno.permissions.query({ name: "write", path: "." });

C await Deno.permissions.query("write", ".");

D await Deno.permissions.query({ name: "write" });

💡 Hint

Check the official Deno permissions API for the correct object shape.

Deno.permissions.query() accepts a descriptor with name and optionally path for filesystem permissions like write. To check access to the current directory specifically, use { name: "write", path: "." }.

🔧 Debug

advanced

Debugging permission error in Deno script

A Deno script tries to listen on port 8080 but fails with a permission error. Which option explains the cause?

Deno

import { serve } from "https://deno.land/std/http/server.ts";

serve(() => new Response("Hello"), { port: 8080 });

A The script lacks <code>--allow-net</code> permission to open network ports.

B The port 8080 is already in use by another process.

C The <code>serve</code> function is deprecated and causes errors.

D The script needs <code>--allow-read</code> permission to serve HTTP.

💡 Hint

Consider what permission is needed to open network sockets in Deno.

Deno requires --allow-net to open network ports. Without it, a permission error occurs.

❓ state_output

expert

Permission state after multiple requests

Given this code, what is the final output if the user first denies and then grants the permission when prompted again?

Deno

const first = await Deno.permissions.request({ name: "read" });
const second = await Deno.permissions.request({ name: "read" });
console.log(`${first.state},${second.state}`);

A "denied,denied"

B "prompt,prompt"

C "denied,granted"

D "granted,granted"

💡 Hint

Think about how Deno caches permission decisions and how multiple requests behave.

Deno caches permission decisions. The first request() prompts the user (denied), returning "denied". The second request() returns the cached "denied" state without prompting again.

Practice

1. What does Deno.permissions.query() do in a Deno program?

easy

A. It checks if a specific permission is already granted or denied.

B. It automatically grants all permissions to the program.

C. It revokes permissions from the program at runtime.

D. It lists all permissions the program will request in the future.

Solution

1. Step 1: Understand the purpose of Deno.permissions.query()

   This method is used to check the current status of a permission, whether it is granted, denied, or prompt.

2. Step 2: Compare with other permission methods

   Unlike request() which asks for permission, query() only checks the status without changing it.

3. Final Answer:

   It checks if a specific permission is already granted or denied. -> Option A

4. Quick Check:

   Permission check = A [OK]

Hint: Query checks status without changing permissions [OK]

Common Mistakes:

• Confusing query() with request()
• Thinking query() grants permissions
• Assuming query() revokes permissions

2. Which of the following is the correct syntax to request read permission for the file data.txt in Deno?

easy

A. await Deno.permissions.request("read", "data.txt");

B. Deno.permissions.request({ permission: "read", file: "data.txt" });

C. await Deno.permissions.request({ name: "read", path: "data.txt" });

D. Deno.permissions.query({ name: "read", path: "data.txt" });

Solution

1. Step 1: Identify correct method and parameters

   The request() method takes an object with a name property for permission type and optional path for file-specific permission.

2. Step 2: Check syntax correctness

   await Deno.permissions.request({ name: "read", path: "data.txt" }); uses correct async call with an object containing name: "read" and path: "data.txt". Other options misuse parameters or use query instead of request.

3. Final Answer:

   await Deno.permissions.request({ name: "read", path: "data.txt" }); -> Option C

4. Quick Check:

   Correct request syntax = B [OK]

Hint: Use object with name and path in request() [OK]

Common Mistakes:

• Passing parameters as separate arguments
• Using query() instead of request()
• Wrong property names like permission or file

3. Consider this code snippet in Deno:

const status = await Deno.permissions.query({ name: "net" });
console.log(status.state);

If the program has not requested network permission yet, what will be printed?

medium

A. "granted"

B. "denied"

C. An error is thrown

D. "prompt"

Solution

1. Step 1: Understand default permission state

   When a permission like "net" is queried before any request, Deno returns the state "prompt" indicating user has not yet granted or denied.

2. Step 2: Confirm no error thrown on query

   Querying permission does not throw errors; it returns the current state as a string.

3. Final Answer:

   "prompt" -> Option D

4. Quick Check:

   Unrequested permission state = prompt [OK]

Hint: Unasked permissions show 'prompt' state [OK]

Common Mistakes:

• Assuming default is granted or denied
• Expecting an error on query
• Confusing query() with request() behavior

4. What is wrong with this Deno code snippet?

const permission = Deno.permissions.request({ name: "read" });
if (permission.state === "granted") {
  console.log("Access allowed");
}

medium

A. Missing await before Deno.permissions.request call.

B. Using wrong permission name "read" instead of "readFile".

C. Cannot check permission.state directly; must use a callback.

D. request() does not return a permission status object.

Solution

1. Step 1: Check async usage of request()

   The request() method returns a Promise, so it must be awaited to get the permission status object.

2. Step 2: Identify missing await impact

   Without await, permission is a Promise, so accessing permission.state is invalid and causes errors.

3. Final Answer:

   Missing await before Deno.permissions.request call. -> Option A

4. Quick Check:

   Always await request() for permission object [OK]

Hint: Always await async permission requests [OK]

Common Mistakes:

• Forgetting await on async calls
• Confusing permission names
• Trying to use callbacks instead of promises

5. You want to write a Deno script that reads a file only if the user grants permission at runtime. Which approach correctly handles permission requests and reading the file?

hard

A. const status = Deno.permissions.query({ name: "read", path: "file.txt" });
if (status.state === "granted") {
const data = Deno.readTextFileSync("file.txt");
console.log(data);
}

B. const status = await Deno.permissions.request({ name: "read", path: "file.txt" });
if (status.state === "granted") {
const data = await Deno.readTextFile("file.txt");
console.log(data);
} else {
console.log("Permission denied.");
}

C. await Deno.readTextFile("file.txt");
console.log("File read without permission check.");

D. Deno.permissions.request({ name: "read" });
const data = await Deno.readTextFile("file.txt");
console.log(data);

Solution

1. Step 1: Request permission with await and check result

   const status = await Deno.permissions.request({ name: "read", path: "file.txt" }); if (status.state === "granted") { const data = await Deno.readTextFile("file.txt"); console.log(data); } else { console.log("Permission denied."); } correctly awaits the permission request and checks if the state is "granted" before proceeding.

2. Step 2: Read file only if permission granted

   It reads the file asynchronously only after permission is granted, otherwise it informs the user permission was denied.

3. Step 3: Identify errors in other options

   const status = Deno.permissions.query({ name: "read", path: "file.txt" }); if (status.state === "granted") { const data = Deno.readTextFileSync("file.txt"); console.log(data); } uses query() which does not request permission; await Deno.readTextFile("file.txt"); console.log("File read without permission check."); reads without permission check; Deno.permissions.request({ name: "read" }); const data = await Deno.readTextFile("file.txt"); console.log(data); does not await request() and reads file regardless.

4. Final Answer:

   const status = await Deno.permissions.request({ name: "read", path: "file.txt" }); if (status.state === "granted") { const data = await Deno.readTextFile("file.txt"); console.log(data); } else { console.log("Permission denied."); } -> Option B

5. Quick Check:

   Request permission, await result, then read file [OK]

Hint: Always await request() and check state before file access [OK]

Common Mistakes:

• Using query() instead of request() to ask permission
• Not awaiting async permission requests
• Reading files without permission check