Deploying to Deno Deploy - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when you deploy a Deno script with an unhandled promise rejection?

Consider a Deno Deploy script that contains an async function which rejects a promise but does not catch the error. What will be the behavior when this script runs on Deno Deploy?

Deno

addEventListener('fetch', event => {
  event.respondWith(handleRequest(event.request));
});

async function handleRequest(request) {
  await Promise.reject(new Error('Oops!'));
  return new Response('Hello World');
}

A The rejection is ignored and the response 'Hello World' is sent successfully.

B The deployment will fail with a syntax error before running.

C The script will crash and Deno Deploy will return a 500 Internal Server Error response.

D Deno Deploy automatically retries the request until the promise resolves.

💡 Hint

Think about how unhandled promise rejections affect serverless environments.

In Deno Deploy, an unhandled promise rejection inside the request handler causes the script to fail and results in a 500 error response. The runtime does not ignore such errors or retry automatically.

📝 Syntax

intermediate

Which import statement correctly loads a third-party module in Deno Deploy?

You want to import the Oak web framework from a URL in your Deno Deploy project. Which import statement is correct?

A import { Application } from './oak';

B import Application from 'oak';

C const Application = require('oak');

D import { Application } from 'https://deno.land/x/oak@v12.5.0/mod.ts';

💡 Hint

Deno uses URL imports for third-party modules.

Deno requires full URLs with versioning for third-party modules. Option D correctly imports Oak from deno.land with a version number.

🔧 Debug

advanced

Why does this Deno Deploy script fail to respond?

This script is deployed to Deno Deploy but clients never receive a response. What is the cause?

Deno

addEventListener('fetch', event => {
  handleRequest(event.request);
});

async function handleRequest(request) {
  return new Response('Hello from Deno Deploy');
}

A The Response constructor is used incorrectly and throws a runtime error.

B The event.respondWith() method is missing, so the response is never sent.

C The fetch event listener is not supported in Deno Deploy.

D The async function handleRequest must be synchronous to work.

💡 Hint

How does Deno Deploy know what to send back to the client?

In Deno Deploy, you must call event.respondWith() with a Response or a promise resolving to a Response. Without it, the request hangs and no response is sent.

❓ lifecycle

advanced

How does Deno Deploy handle environment variables during deployment?

You set environment variables in the Deno Deploy dashboard for your project. When and how are these variables available to your script?

A Environment variables are injected at runtime and accessible via Deno.env.get() inside the script.

B Environment variables must be imported as modules before use.

C They are only available during build time and not at runtime.

D Deno Deploy does not support environment variables.

💡 Hint

Think about how serverless platforms provide secrets and config to running code.

Deno Deploy injects environment variables at runtime. You can access them using Deno.env.get('VAR_NAME') inside your deployed script.

🧠 Conceptual

expert

What is the effect of using the 'unstable' flag in Deno Deploy scripts?

You deploy a Deno script to Deno Deploy with the --unstable flag enabled. What does this allow you to do?

A Use experimental Deno APIs that are not yet stable but may change in the future.

B Enable automatic retries on failed requests in Deno Deploy.

C Allow the script to run indefinitely without timeout.

D Disable all security permissions for the script.

💡 Hint

Consider what 'unstable' means in software development.

The --unstable flag in Deno enables access to experimental APIs that are not finalized. This lets developers try new features but with the risk of breaking changes.

Practice

1. What is the main purpose of deno deploy in Deno?

easy

A. To run Deno scripts locally without internet

B. To install Deno on your computer

C. To create a new Deno project template

D. To publish your Deno app online as a serverless function

Solution

1. Step 1: Understand the command purpose

   deno deploy is used to publish apps online, not for local installs or running scripts.

2. Step 2: Match the description to the command

   Publishing as serverless functions matches the description of deno deploy.

3. Final Answer:

   To publish your Deno app online as a serverless function -> Option D

4. Quick Check:

   deno deploy = publish app online [OK]

Hint: Remember: deploy means publish online [OK]

Common Mistakes:

• Confusing deploy with install
• Thinking deploy runs code locally
• Assuming deploy creates projects

2. Which of the following is the correct command to deploy a Deno project located in the current folder?

easy

A. deno deploy --install

B. deno deploy .

C. deno run deploy

D. deno start deploy

Solution

1. Step 1: Identify the correct deploy syntax

   The command deno deploy . tells Deno to deploy the current folder project.

2. Step 2: Check other options for correctness

   Options B, C, and D are invalid commands for deployment.

3. Final Answer:

   deno deploy . -> Option B

4. Quick Check:

   Deploy current folder = deno deploy . [OK]

Hint: Use dot (.) to deploy current folder [OK]

Common Mistakes:

• Using deno run instead of deno deploy
• Adding wrong flags like --install
• Using deno start which is not a Deno command

3. Given this command run in your terminal: deno deploy https://github.com/user/myapp, what will happen?

medium

A. Deno will deploy the app from the GitHub repo URL as a serverless app

B. Deno will clone the repo locally but not deploy

C. Deno will throw a syntax error because URLs are not allowed

D. Deno will run the app locally without deploying

Solution

1. Step 1: Understand deploying from a repo URL

   Deno Deploy supports deploying directly from GitHub repo URLs.

2. Step 2: Analyze the command behavior

   The command deploys the app serverlessly from the given URL, no local clone or syntax error occurs.

3. Final Answer:

   Deno will deploy the app from the GitHub repo URL as a serverless app -> Option A

4. Quick Check:

   Deploy from URL = deploy serverless app [OK]

Hint: Deploy accepts GitHub URLs directly [OK]

Common Mistakes:

• Thinking deploy clones repo locally
• Assuming URLs cause syntax errors
• Confusing deploy with local run

4. You run deno deploy myapp but get an error saying "No such file or directory". What is the most likely cause?

medium

A. You forgot to install Deno before deploying

B. The internet connection is down

C. The folder or file named 'myapp' does not exist in the current directory

D. You need to run deno run myapp first

Solution

1. Step 1: Interpret the error message

   "No such file or directory" means the path 'myapp' is missing locally.

2. Step 2: Match error to cause

   Since deploy expects a valid path or URL, missing folder causes this error.

3. Final Answer:

   The folder or file named 'myapp' does not exist in the current directory -> Option C

4. Quick Check:

   Missing folder = No such file error [OK]

Hint: Check folder exists before deploying [OK]

Common Mistakes:

• Assuming Deno is not installed
• Blaming internet connection for local file errors
• Confusing deploy with run command

5. You want to deploy a Deno app that uses environment variables securely on Deno Deploy. Which approach is best?

hard

A. Set environment variables in Deno Deploy dashboard and access them via Deno.env.get()

B. Hardcode environment variables in your source code before deploying

C. Pass environment variables as command line arguments to deno deploy

D. Store environment variables in a local .env file and deploy it with your app

Solution

1. Step 1: Understand environment variable best practices

   Hardcoding or deploying .env files risks exposing secrets publicly.

2. Step 2: Use Deno Deploy features

   Deno Deploy dashboard allows secure environment variable setup accessed by Deno.env.get().

3. Final Answer:

   Set environment variables in Deno Deploy dashboard and access them via Deno.env.get() -> Option A

4. Quick Check:

   Secure env vars = dashboard + Deno.env.get() [OK]

Hint: Use dashboard for env vars, not code files [OK]

Common Mistakes:

• Hardcoding secrets in code
• Uploading .env files publicly
• Trying to pass env vars via deploy command