Deno standard library overview - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Core purpose of Deno standard library modules

What is the main reason Deno provides a standard library separate from third-party modules?

A To replace all third-party modules with official ones

B To offer audited, secure, and stable utilities that work well with Deno's runtime

C To provide only UI components for building web apps

D To limit developers to only built-in features without external code

💡 Hint

Think about why a runtime would want a trusted set of tools.

Deno's standard library offers trusted, well-maintained utilities that integrate smoothly with its runtime. It is not meant to replace all third-party modules or limit developers but to provide a solid foundation.

❓ component_behavior

intermediate

Behavior of Deno's standard library 'fs' module function

What does the function `existsSync` from Deno's standard library 'fs' module return when called with a path that does not exist?

Deno

import { existsSync } from "https://deno.land/std/fs/mod.ts";

const result = existsSync("./nonexistent-file.txt");
console.log(result);

A Returns undefined

B true

C Throws a runtime error

D false

💡 Hint

Think about what a function named 'existsSync' should indicate.

The function `existsSync` checks synchronously if a file or directory exists at the given path. It returns false if the path does not exist, without throwing an error.

📝 Syntax

advanced

Correct import statement for Deno standard library 'http' server

Which import statement correctly imports the HTTP server from Deno's standard library version 0.203.0?

A import { serve } from "https://deno.land/std@0.203.0/http/server.ts";

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

C import serve from "https://deno.land/std@0.203.0/http/server.ts";

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

💡 Hint

Pay attention to the version pinning and import syntax.

Option A correctly pins the version and uses named import syntax. Option A lacks version pinning which is discouraged. Option A uses default import which is invalid here. Option A uses 'latest' which is not a valid version tag.

🔧 Debug

advanced

Identify the error in this Deno standard library usage

What error will this code produce when run in Deno?

Deno

import { readJson } from "https://deno.land/std/fs/mod.ts";

async function main() {
  const data = await readJson("./data.json");
  console.log(data);
}

main();

A TypeError: readJson is not a function

B ReferenceError: data is not defined

C SyntaxError: Unexpected await outside async function

D No error, prints JSON content

💡 Hint

Consider where 'await' can be used in JavaScript/TypeScript.

The code uses 'await' inside a non-async function, causing a SyntaxError: Unexpected await outside async function.

❓ state_output

expert

Output of Deno standard library 'path' join with absolute and relative parts

What is the output of this code snippet?

Deno

import { join } from "https://deno.land/std/path/mod.ts";

const result = join("/home/user", "../docs", "./file.txt");
console.log(result);

A "/home/docs/file.txt"

B "/home/user/docs/file.txt"

C "/home/user/../docs/./file.txt"

D "/docs/file.txt"

💡 Hint

Think about how join resolves '..' and '.' in paths.

The join function normalizes the path by resolving .. and . segments. So, joining /home/user, ../docs, and ./file.txt results in /home/docs/file.txt.

Practice

1. What is the main purpose of the Deno standard library?

easy

A. To provide ready-to-use modules for common programming tasks

B. To replace the Deno runtime environment

C. To serve as a database management system

D. To compile TypeScript code into JavaScript

Solution

1. Step 1: Understand the role of a standard library

   A standard library offers pre-built tools and modules to simplify common programming tasks.

2. Step 2: Identify the Deno standard library's purpose

   The Deno standard library provides modules for file handling, HTTP, dates, testing, and more, helping developers avoid writing everything from scratch.

3. Final Answer:

   To provide ready-to-use modules for common programming tasks -> Option A

4. Quick Check:

   Deno standard library = ready-to-use modules [OK]

Hint: Standard libraries offer common tools, not runtime or compilers [OK]

Common Mistakes:

• Confusing the standard library with the runtime environment
• Thinking it compiles code
• Assuming it manages databases

2. Which is the correct way to import a module from the Deno standard library with versioning?

easy

A. import { serve } from "https://deno.land/std@0.200.0/http/server.ts";

B. import serve from "deno.land/std/http/server.ts";

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

D. import * as serve from "https://deno.land/std@latest/http/server.ts";

Solution

1. Step 1: Check the import URL format for Deno standard library

   The URL must include the version number after @ to ensure stability, e.g., @0.200.0.

2. Step 2: Verify the import syntax

   Correct syntax uses named imports with curly braces and a full URL including version.

3. Final Answer:

   import { serve } from "https://deno.land/std@0.200.0/http/server.ts"; -> Option A

4. Quick Check:

   Versioned URL + named import = import { serve } from "https://deno.land/std@0.200.0/http/server.ts"; [OK]

Hint: Always include @version in URL for stable imports [OK]

Common Mistakes:

• Omitting the version number in the URL
• Using incorrect import syntax without braces
• Using 'latest' instead of a fixed version

3. What will the following code output?

import { format } from "https://deno.land/std@0.200.0/datetime/mod.ts";
const date = new Date("2024-06-15T12:00:00Z");
console.log(format(date, "yyyy-MM-dd"));

medium

A. 06-15-2024

B. 2024-06-15

C. 15-06-2024

D. 2024/06/15

Solution

1. Step 1: Understand the format function usage

   The format function formats a Date object according to the pattern string provided.

2. Step 2: Analyze the pattern "yyyy-MM-dd"

   "yyyy" means 4-digit year, "MM" means 2-digit month, "dd" means 2-digit day, separated by dashes.

3. Final Answer:

   2024-06-15 -> Option B

4. Quick Check:

   Date formatted as yyyy-MM-dd = 2024-06-15 [OK]

Hint: Match format pattern exactly to output date string [OK]

Common Mistakes:

• Mixing day and month order
• Using slashes instead of dashes
• Ignoring the format string

4. Identify the error in this Deno standard library import:

import { assertEquals } from "https://deno.land/std/testing/asserts.ts";
assertEquals(1 + 1, 2);

medium

A. Incorrect import syntax, should use default import

B. assertEquals is not a valid function

C. Missing version number in the import URL

D. assertEquals requires async function

Solution

1. Step 1: Check the import URL for versioning

   The URL lacks a version number after @, which is required for stable imports in Deno.

2. Step 2: Confirm the function and syntax

   assertEquals is a valid named export and the import syntax is correct; no async needed here.

3. Final Answer:

   Missing version number in the import URL -> Option C

4. Quick Check:

   Version number missing in URL = Missing version number in the import URL [OK]

Hint: Always check for @version in import URLs [OK]

Common Mistakes:

• Assuming assertEquals is invalid
• Thinking default import is required
• Believing assertEquals needs async

5. You want to create a simple HTTP server using Deno's standard library that responds with "Hello World". Which code snippet correctly uses the standard library's HTTP module with versioning?

hard

A. import { serve } from "https://deno.land/std@0.200.0/http/server.ts";
serve(() => Response.json({message: "Hello World"}));

B. import serve from "https://deno.land/std/http/server.ts";
serve(() => new Response("Hello World"));

C. import { serve } from "https://deno.land/std@0.200.0/http/server.ts";
serve(request => Response.text("Hello World"));

D. import { serve } from "https://deno.land/std@0.200.0/http/server.ts";
serve(() => new Response("Hello World"));

Solution

1. Step 1: Verify import URL and syntax

   import { serve } from "https://deno.land/std@0.200.0/http/server.ts"; serve(() => new Response("Hello World")); correctly imports serve with a versioned URL and uses named import syntax.

2. Step 2: Check the serve function usage

   serve expects a function returning a Response object; new Response("Hello World") is correct.

3. Step 3: Evaluate other options

   import serve from "https://deno.land/std/http/server.ts"; serve(() => new Response("Hello World")); lacks versioning; import { serve } from "https://deno.land/std@0.200.0/http/server.ts"; serve(request => Response.text("Hello World")); uses incorrect Response method; import { serve } from "https://deno.land/std@0.200.0/http/server.ts"; serve(() => Response.json({message: "Hello World"})); returns JSON, not plain text.

4. Final Answer:

   import { serve } from "https://deno.land/std@0.200.0/http/server.ts"; serve(() => new Response("Hello World")); -> Option D

5. Quick Check:

   Versioned import + correct Response = import { serve } from "https://deno.land/std@0.200.0/http/server.ts"; serve(() => new Response("Hello World")); [OK]

Hint: Use versioned import and new Response("text") for HTTP server [OK]

Common Mistakes:

• Omitting version number in import URL
• Using incorrect Response methods
• Wrong import syntax