Serving static files in Deno

Introduction

Serving static files lets your web server send images, styles, or scripts to users. It helps show pictures or style your pages.

You want to show a logo or picture on your website.

You need to send CSS files to style your webpage.

You want to provide JavaScript files for interactive features.

You want to share downloadable files like PDFs or documents.

Syntax

Deno

import { serve } from "https://deno.land/std@0.201.0/http/server.ts";
import { serveFile } from "https://deno.land/std@0.201.0/http/file_server.ts";

serve(async (req) => {
  const url = new URL(req.url);
  if (url.pathname === "/") {
    return await serveFile(req, "./index.html");
  } else {
    return await serveFile(req, `.${url.pathname}`);
  }
});

Use serveFile to send a file as a response.

Check the request URL to decide which file to send.

Examples

Sends the style.css file when requested.

Deno

return await serveFile(req, "./style.css");

Serves a specific image file when the URL matches.

Deno

if (url.pathname === "/image.png") {
  return await serveFile(req, "./image.png");
}

Serves any file requested by its path, assuming it exists in the folder.

Deno

serve(async (req) => {
  const url = new URL(req.url);
  return await serveFile(req, `.${url.pathname}`);
});

Sample Program

This program starts a server that sends index.html when the root URL is visited. For other paths, it tries to send the matching file. If the file is missing, it returns a 404 error.

Deno

import { serve } from "https://deno.land/std@0.201.0/http/server.ts";
import { serveFile } from "https://deno.land/std@0.201.0/http/file_server.ts";

serve(async (req) => {
  const url = new URL(req.url);
  if (url.pathname === "/") {
    return await serveFile(req, "./index.html");
  } else {
    try {
      return await serveFile(req, `.${url.pathname}`);
    } catch {
      return new Response("File not found", { status: 404 });
    }
  }
});

Important Notes

Make sure the files you want to serve are in the correct folder.

Use try-catch to handle missing files gracefully.

Static files include images, CSS, JavaScript, and HTML files.

Summary

Serving static files sends images, styles, and scripts to users.

Use serveFile with the request and file path.

Check the URL path to decide which file to send.

Practice

1. What is the main purpose of serving static files in a Deno web server?

easy

A. To send images, styles, and scripts to the user's browser

B. To execute server-side scripts dynamically

C. To connect to a database and fetch data

D. To compile TypeScript files on the server

Solution

1. Step 1: Understand what static files are

   Static files include images, CSS stylesheets, and JavaScript files that do not change dynamically.

2. Step 2: Identify the purpose of serving static files

   Serving static files means sending these files from the server to the user's browser so the webpage looks and works correctly.

3. Final Answer:

   To send images, styles, and scripts to the user's browser -> Option A

4. Quick Check:

   Static files = images, styles, scripts [OK]

Hint: Static files are fixed resources sent to browsers [OK]

Common Mistakes:

• Confusing static files with dynamic server code
• Thinking static files run on the server
• Mixing static files with database operations

2. Which of the following is the correct way to import the function to serve static files in Deno?

easy

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

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

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

D. import { serveFile } from "https://deno.land/std/http/file_server.ts";

Solution

1. Step 1: Identify the correct module for serving static files

   The function serveFile is provided by the file.ts module in Deno's standard HTTP library.

2. Step 2: Match the import path

   The correct import path is "https://deno.land/std/http/file.ts" for serveFile.

3. Final Answer:

   import { serveFile } from "https://deno.land/std/http/file.ts"; -> Option C

4. Quick Check:

   serveFile import = file.ts [OK]

Hint: serveFile comes from file.ts in std/http [OK]

Common Mistakes:

• Using server.ts instead of file.ts
• Importing from fs module incorrectly
• Wrong URL path for the import

3. Given this Deno server code snippet, what will be the response when a user requests /image.png?

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

serve(async (req) => {
  const url = new URL(req.url);
  if (url.pathname === "/image.png") {
    return await serveFile(req, "./static/image.png");
  }
  return new Response("Not Found", { status: 404 });
});

medium

A. The server throws a runtime error because serveFile is used incorrectly

B. The server sends the file located at ./static/image.png to the user

C. The server returns a 404 Not Found response

D. The server sends an empty response with status 200

Solution

1. Step 1: Check the URL path condition

   The code checks if the requested path is exactly "/image.png".

2. Step 2: Understand serveFile usage

   If the path matches, serveFile sends the file "./static/image.png" as the response.

3. Final Answer:

   The server sends the file located at ./static/image.png to the user -> Option B

4. Quick Check:

   Path matches "/image.png" so serveFile sends file [OK]

Hint: Check URL path then serveFile sends matching file [OK]

Common Mistakes:

• Assuming 404 is returned for existing path
• Thinking serveFile causes error here
• Ignoring the URL pathname check

4. Identify the error in this Deno code snippet for serving a static CSS file:

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

serve(async (req) => {
  const url = new URL(req.url);
  if (url.pathname = "/style.css") {
    return await serveFile(req, "./static/style.css");
  }
  return new Response("Not Found", { status: 404 });
});

medium

A. The server function is not awaited

B. serveFile is missing an await keyword

C. The import path for serveFile is incorrect

D. The assignment operator (=) is used instead of comparison (===) in the if condition

Solution

1. Step 1: Check the if condition syntax

   The condition uses a single equals sign (=), which assigns instead of compares.

2. Step 2: Understand the impact of assignment in if

   Using = in if always evaluates to true, causing incorrect logic and possible bugs.

3. Final Answer:

   The assignment operator (=) is used instead of comparison (===) in the if condition -> Option D

4. Quick Check:

   Use === for comparison, not = [OK]

Hint: Use === for comparison, not = assignment in conditions [OK]

Common Mistakes:

• Confusing = with === in if statements
• Forgetting to await serveFile (though it is awaited here)
• Incorrect import paths

5. You want to serve multiple static files (HTML, CSS, JS) from a folder named public in Deno. Which approach correctly serves any requested file from that folder, falling back to a 404 if the file doesn't exist?

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

serve(async (req) => {
  const url = new URL(req.url);
  try {
    // Which line correctly serves the file?
    return await ???;
  } catch {
    return new Response("Not Found", { status: 404 });
  }
});

hard

A. serveFile(req, `./public${url.pathname}`)

B. serveFile(req, `./public/index.html`)

C. serveFile(req, `./public/style.css`)

D. serveFile(req, `./public/${url.searchParams.get('file')}`)

Solution

1. Step 1: Understand dynamic file serving

   To serve any file requested, the server must use the URL path to build the file path inside the public folder.

2. Step 2: Match the URL pathname to the file path

   Using `./public${url.pathname}` correctly maps the request path to the file location.

3. Final Answer:

   serveFile(req, `./public${url.pathname}`) -> Option A

4. Quick Check:

   Dynamic path = ./public + url.pathname [OK]

Hint: Use url.pathname appended to folder path for dynamic files [OK]

Common Mistakes:

• Serving only a fixed file like index.html
• Using query parameters instead of pathname
• Hardcoding a single file path