Permission prompts at runtime in Deno - Performance & Optimization

Performance: Permission prompts at runtime

MEDIUM IMPACT

This affects page load speed and user interaction responsiveness by delaying script execution until permissions are granted.

Requesting file system access permission in a Deno script

Deno

if ((await Deno.permissions.query({ name: 'read', path: '/path/to/file.txt' })).state !== 'granted') {
  await Deno.permissions.request({ name: 'read', path: '/path/to/file.txt' });
}
const data = await Deno.readTextFile('/path/to/file.txt');

Explicitly requesting permission before file access avoids unexpected blocking and improves interaction flow.

📈 Performance GainReduces unexpected blocking, improving INP and user experience

Requesting file system access permission in a Deno script

Deno

const data = await Deno.readTextFile('/path/to/file.txt');

This code assumes permission is granted upfront, causing the runtime to block and prompt unexpectedly, delaying execution.

📉 Performance CostBlocks script execution until user responds, increasing INP by hundreds of milliseconds

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Implicit permission request during operation	N/A	0	0	[X] Bad
Explicit permission request before operation	N/A	0	0	[OK] Good

Rendering Pipeline

Permission prompts pause JavaScript execution, blocking the main thread and delaying rendering and interaction readiness.

→JavaScript Execution

→Interaction Readiness

⚠️ BottleneckJavaScript Execution blocking due to permission prompt

Core Web Vital Affected

INP

This affects page load speed and user interaction responsiveness by delaying script execution until permissions are granted.

Optimization Tips

1Request permissions before critical operations to avoid blocking execution.

2Avoid implicit permission requests during user interactions to reduce input delay.

3Monitor scripting blocks in DevTools Performance panel to detect permission prompt delays.

Performance Quiz - 3 Questions

What is the main performance impact of requesting permissions at runtime in Deno?

A It increases the number of DOM nodes, causing more reflows.

B It adds large files to the bundle size, slowing down load time.

C It blocks JavaScript execution until the user responds, increasing input delay.

D It causes layout shifts that affect visual stability.

DevTools: Performance

How to check: Record a performance profile while running the Deno script and look for long tasks or scripting blocks caused by permission prompts.

What to look for: Look for long scripting tasks that pause execution and increase Interaction to Next Paint (INP) metric.

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