Environment variable permissions in Deno - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Deno's environment variable access

What happens when a Deno script tries to access an environment variable without the proper permission flag?

Deno

console.log(Deno.env.get('MY_SECRET'));

A The script prints the environment variable value if it exists.

B The script throws a PermissionDenied error at runtime.

C The script returns undefined without any error.

D The script crashes with a syntax error.

💡 Hint

Deno requires explicit permission flags to access environment variables.

Deno enforces security by requiring the --allow-env flag to access environment variables. Without it, accessing Deno.env.get() throws a PermissionDenied error.

❓ state_output

intermediate

Output when accessing environment variables with partial permissions

Given the following Deno script and running it with deno run --allow-env=MY_VAR script.ts, what is the output?

Deno

console.log(Deno.env.get('MY_VAR'));
console.log(Deno.env.get('OTHER_VAR'));

A "value1" followed by a PermissionDenied error

B "value1" followed by undefined

C undefined followed by undefined

D PermissionDenied error on the first line

💡 Hint

The --allow-env flag can specify which variables are accessible.

The flag --allow-env=MY_VAR allows access only to MY_VAR. Accessing OTHER_VAR causes a PermissionDenied error.

🔧 Debug

advanced

Identify the cause of a PermissionDenied error

Why does the following Deno script throw a PermissionDenied error when run with deno run script.ts?

Deno

const token = Deno.env.get('API_TOKEN');
console.log(`Token: ${token}`);

A The environment variable API_TOKEN is not set in the system.

B Deno does not support environment variables.

C The script has a syntax error in the template literal.

D The script lacks the <code>--allow-env</code> permission flag.

💡 Hint

Check the permissions required to access environment variables in Deno.

Deno requires the --allow-env flag to access environment variables. Without it, accessing Deno.env.get() throws a PermissionDenied error regardless of whether the variable is set.

📝 Syntax

advanced

Correct usage of environment variable access with permission flags

Which command correctly runs a Deno script that accesses environment variables named USER and PATH?

A deno run --allow-env=USER PATH script.ts

B deno run --allow-env script.ts

C deno run --allow-env=USER,PATH script.ts

D deno run --allow-env=USER;PATH script.ts

💡 Hint

Check how to specify multiple environment variables in the permission flag.

The --allow-env flag accepts a comma-separated list of variable names. Option C correctly uses --allow-env=USER,PATH.

❓ component_behavior

expert

Behavior of environment variable access in a Deno script with dynamic permission requests

Consider a Deno script that tries to read an environment variable and requests permission dynamically using Deno.permissions.request({ name: 'env', variable: 'SECRET' }). What is the expected behavior if the user denies the permission request?

Deno

async function getSecret() {
  const status = await Deno.permissions.request({ name: 'env', variable: 'SECRET' });
  if (status.state === 'granted') {
    return Deno.env.get('SECRET');
  } else {
    return 'Permission denied';
  }
}

getSecret().then(console.log);

A The script prints the value of SECRET if permission is granted; otherwise, it prints 'Permission denied'.

B The script throws a runtime error if permission is denied.

C The script ignores the permission and always prints the SECRET value.

D The script crashes immediately without any output.

💡 Hint

Deno's permissions API allows dynamic permission requests and returns status objects.

The Deno.permissions.request() method returns a status object. If the user denies permission, the script can handle it gracefully and avoid runtime errors by checking the state.

Practice

1. What does the --allow-env flag do in Deno?

easy

A. It encrypts environment variables.

B. It grants permission to access environment variables.

C. It sets environment variables automatically.

D. It disables environment variable access.

Solution

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

   This flag is used to allow a Deno program to read environment variables, which are otherwise blocked by default for security.

2. Step 2: Identify the correct description

   Among the options, only granting permission to access environment variables matches the flag's purpose.

3. Final Answer:

   It grants permission to access environment variables. -> Option B

4. Quick Check:

   Permission flag for env vars = D [OK]

Hint: Remember: --allow-env means allow environment access [OK]

Common Mistakes:

• Thinking it disables environment access
• Confusing it with setting variables
• Assuming it encrypts variables

2. Which of the following is the correct way to allow access only to the environment variable API_KEY in Deno?

easy

A. deno run --allow-env=API_KEY script.ts

B. deno run --allow-env API_KEY script.ts

C. deno run --allow-env API_KEY=script.ts

D. deno run --allow-env:API_KEY script.ts

Solution

1. Step 1: Recall the syntax for environment variable permission

   Deno uses --allow-env=VAR_NAME to allow access only to a specific variable.

2. Step 2: Match the correct syntax

   deno run --allow-env=API_KEY script.ts correctly uses --allow-env=API_KEY. Other options misuse spaces, equal signs, or colons.

3. Final Answer:

   deno run --allow-env=API_KEY script.ts -> Option A

4. Quick Check:

   Specific env var permission uses = [OK]

Hint: Use = to specify variable after --allow-env [OK]

Common Mistakes:

• Using space instead of = after --allow-env
• Adding extra characters like : or = in wrong places
• Confusing argument order

3. Given this command: deno run --allow-env=HOME script.ts and this code inside script.ts:

console.log(Deno.env.get("HOME"));
console.log(Deno.env.get("PATH"));

What will be the output?

medium

A. Prints undefined for both variables.

B. Prints HOME variable value, then prints PATH variable value.

C. Throws a permission error on accessing PATH.

D. Prints HOME variable value, then prints undefined.

Solution

1. Step 1: Understand permission scope

   The command allows access only to the HOME environment variable, not PATH.

2. Step 2: Predict output for each variable

   Accessing HOME prints its value. Accessing PATH returns undefined because permission is not granted.

3. Final Answer:

   Prints HOME variable value, then prints undefined. -> Option D

4. Quick Check:

   Allowed var prints value, others undefined [OK]

Hint: Only allowed env vars return values; others give undefined [OK]

Common Mistakes:

• Assuming all env vars print values
• Expecting a permission error instead of undefined
• Confusing variable names

4. You run deno run --allow-env=USER script.ts but inside script.ts you try to access Deno.env.get("HOME"). What will happen?

medium

A. It returns undefined without error.

B. It prints the HOME variable value.

C. It throws a runtime permission error.

D. It crashes the program.

Solution

1. Step 1: Check permission granted

   Only USER variable access is allowed, not HOME.

2. Step 2: Behavior when accessing disallowed env var

   Deno returns undefined for disallowed environment variables instead of throwing errors.

3. Final Answer:

   It returns undefined without error. -> Option A

4. Quick Check:

   Disallowed env vars return undefined, no error [OK]

Hint: Disallowed env vars return undefined, no crash [OK]

Common Mistakes:

• Expecting a permission error
• Thinking it prints the value anyway
• Assuming program crashes

5. You want to run a Deno script that needs access to API_KEY and DB_PASS environment variables only. Which command correctly grants minimal permissions?

hard

A. deno run --allow-env=API_KEY script.ts && deno run --allow-env=DB_PASS script.ts

B. deno run --allow-env script.ts

C. deno run --allow-env=API_KEY,DB_PASS script.ts

D. deno run --allow-env=API_KEY DB_PASS script.ts

Solution

1. Step 1: Understand how to specify multiple env vars

   Deno allows multiple variables by comma-separating them after --allow-env=.

2. Step 2: Evaluate each option

   deno run --allow-env=API_KEY,DB_PASS script.ts correctly uses comma separation. deno run --allow-env script.ts grants all env vars (too broad). deno run --allow-env=API_KEY script.ts && deno run --allow-env=DB_PASS script.ts runs script twice separately (inefficient). deno run --allow-env=API_KEY DB_PASS script.ts syntax is invalid.

3. Final Answer:

   deno run --allow-env=API_KEY,DB_PASS script.ts -> Option C

4. Quick Check:

   Multiple vars comma-separated after --allow-env= [OK]

Hint: Use commas to list multiple env vars after --allow-env= [OK]

Common Mistakes:

• Granting all env vars instead of specific ones
• Running script multiple times instead of once
• Using spaces instead of commas