Permission prompts at runtime in Deno

Introduction

Permission prompts let your program ask the user for access to files, network, or other resources while it runs. This keeps the computer safe by controlling what the program can do.

When your program needs to read or write files on the user's computer.

When your program wants to connect to the internet or a local network.

When your program needs to access environment variables or run subprocesses.

When you want to make sure your program only uses permissions when really needed.

When you want to improve security by asking for permissions at the right time.

Syntax

Deno

const status = await Deno.permissions.query({ name: "read", path: "/path/to/file" });

if (status.state === "granted") {
  // permission granted, safe to read file
} else {
  // permission denied or prompt needed
}

You use Deno.permissions.query() to check if permission is already granted.

Permission names include read, write, net, env, and run.

Examples

Check if the program can read any files without specifying a path.

Deno

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

Ask the user for permission to access the network for example.com.

Deno

const netStatus = await Deno.permissions.request({ name: "net", host: "example.com" });
console.log(netStatus.state);

Request permission to write to a specific file path.

Deno

const writeStatus = await Deno.permissions.request({ name: "write", path: "/tmp/data.txt" });
if (writeStatus.state === "granted") {
  // safe to write to /tmp/data.txt
}

Sample Program

This program asks the user for permission to read a file named example.txt. If allowed, it reads and prints the file content. Otherwise, it shows a denial message.

Deno

async function checkAndReadFile() {
  const permission = await Deno.permissions.request({ name: "read", path: "./example.txt" });

  if (permission.state === "granted") {
    const data = await Deno.readTextFile("./example.txt");
    console.log("File content:", data);
  } else {
    console.log("Permission to read file was denied.");
  }
}

checkAndReadFile();

Important Notes

Permissions can be granted, denied, or prompt (waiting for user decision).

Always handle the case when permission is denied to avoid program crashes.

Request only the permissions you need, when you need them, to keep users safe and trustful.

Summary

Permission prompts help control what your program can access at runtime.

Use Deno.permissions.query() to check and Deno.permissions.request() to ask for permissions.

Handle permission results carefully to keep your program safe and user-friendly.

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