Deno standard library overview

Introduction

The Deno standard library gives you ready-to-use tools to build your programs easily and safely.

When you want to handle files or directories in your Deno app.

When you need to work with dates, times, or formatting.

When you want to create HTTP servers or clients without extra setup.

When you want to parse or format data like JSON, CSV, or YAML.

When you want to write tests or check code style in your project.

Syntax

Deno

import { moduleName } from "https://deno.land/std@version/module/path.ts";

Always import modules directly from the URL with the version number to keep your code stable.

The standard library is organized into folders like fs (file system), http, datetime, testing, and more.

Examples

This imports a function to read a file as a string from the file system.

Deno

import { readFileStr } from "https://deno.land/std@0.203.0/fs/mod.ts";

This imports a function to create a simple HTTP server.

Deno

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

This imports a function to format dates and times.

Deno

import { format } from "https://deno.land/std@0.203.0/datetime/mod.ts";

Sample Program

This program uses the Deno standard library to create a simple HTTP server that listens on port 8000. When you visit http://localhost:8000/, it shows a friendly message.

Deno

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

const server = serve({ port: 8000 });
console.log("HTTP server running on http://localhost:8000/");

for await (const req of server) {
  req.respond({ body: "Hello from Deno standard library!" });
}

Important Notes

The Deno standard library is always improving, so check the official site for updates.

Using version numbers in imports helps avoid breaking changes in your code.

Many modules in the standard library are designed to work well together for common tasks.

Summary

The Deno standard library provides useful tools for everyday programming tasks.

Import modules by URL with version numbers to keep your code stable.

It covers many areas like file handling, HTTP, dates, and testing.

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