Download URLs in Firebase - Mini Project: Build & Apply

Firebase Storage Download URLs

📖 Scenario: You are building a simple web app that stores images in Firebase Storage. You want to get the download URLs for these images so users can view them.

🎯 Goal: Learn how to create a list of image file names, configure Firebase Storage, generate download URLs for each image, and complete the setup to access these URLs.

📋 What You'll Learn

Create a list of image file names

Initialize Firebase Storage reference

Generate download URLs for each image file

Store the URLs in a list for later use

💡 Why This Matters

🌍 Real World

Getting download URLs is essential to display or share files stored in Firebase Storage in web or mobile apps.

💼 Career

Understanding how to generate and manage download URLs is a common task for cloud developers working with Firebase or similar cloud storage services.

1

Create a list of image file names

Create a list called imageFiles with these exact strings: "photo1.jpg", "photo2.jpg", "photo3.jpg".

Firebase

# Create a list called imageFiles with the exact file names
# Your code here

Hint

Use square brackets to create a list (array) and include the file names as strings.

2

Initialize Firebase Storage reference

Create a constant called storage and set it to getStorage() from Firebase. Then create a constant called storageRef and set it to ref(storage).

Firebase

const imageFiles = ["photo1.jpg", "photo2.jpg", "photo3.jpg"];
// Initialize Firebase Storage reference
// Your code here

Hint

Use getStorage() to get the storage service and ref(storage) to get the root reference.

3

Generate download URLs for each image file

Create an empty array called downloadURLs. Use a for loop with variable fileName to iterate over imageFiles. Inside the loop, create a reference called fileRef using ref(storageRef, fileName). Then use getDownloadURL(fileRef) and push the resulting promise into downloadURLs.

Firebase

import { getStorage, ref, getDownloadURL } from "firebase/storage";

const imageFiles = ["photo1.jpg", "photo2.jpg", "photo3.jpg"];
const storage = getStorage();
const storageRef = ref(storage);

// Create downloadURLs array and fill it with promises for each file
// Your code here

Hint

Use a for...of loop to go through each file name and create a reference for it.

4

Complete setup to access download URLs

Use Promise.all(downloadURLs) and assign it to a constant called allURLs. This will wait for all download URL promises to resolve.

Firebase

import { getStorage, ref, getDownloadURL } from "firebase/storage";

const imageFiles = ["photo1.jpg", "photo2.jpg", "photo3.jpg"];
const storage = getStorage();
const storageRef = ref(storage);

const downloadURLs = [];
for (const fileName of imageFiles) {
    const fileRef = ref(storageRef, fileName);
    downloadURLs.push(getDownloadURL(fileRef));
}

// Wait for all download URLs to resolve
// Your code here

Hint

Use Promise.all() to handle multiple promises and get all URLs together.

Practice

1. What does the getDownloadURL() method in Firebase Storage do?

easy

A. It deletes a file from Firebase Storage.

B. It returns a public URL to access the stored file.

C. It uploads a file to Firebase Storage.

D. It lists all files in a storage bucket.

Solution

1. Step 1: Understand the purpose of getDownloadURL()

   This method is used to get a URL that anyone can use to download or view the file stored in Firebase Storage.

2. Step 2: Compare with other storage actions

   Uploading, deleting, or listing files are different methods. getDownloadURL() specifically returns the file's public URL.

3. Final Answer:

   It returns a public URL to access the stored file. -> Option B

4. Quick Check:

   getDownloadURL() = public file URL [OK]

Hint: Remember: getDownloadURL() gives you the file's public link [OK]

Common Mistakes:

• Confusing getDownloadURL() with upload or delete methods
• Thinking it lists files instead of returning a URL
• Assuming it modifies the file instead of providing access

2. Which of the following is the correct syntax to get a download URL for a file reference fileRef in Firebase Storage?

easy

A. const url = fileRef.getDownloadURL();

B. const url = getDownloadURL(fileRef);

C. const url = await getDownloadURL(fileRef);

D. const url = fileRef.downloadURL();

Solution

1. Step 1: Recognize that getDownloadURL returns a Promise

   Since getDownloadURL() is asynchronous, you must use await or .then() to get the URL.

2. Step 2: Check correct usage with file reference

   The correct usage is calling getDownloadURL(fileRef) as a function with the file reference passed in, and awaiting the result.

3. Final Answer:

   const url = await getDownloadURL(fileRef); -> Option C

4. Quick Check:

   Use await with getDownloadURL(fileRef) [OK]

Hint: Use await with getDownloadURL(fileRef) to get URL [OK]

Common Mistakes:

• Calling getDownloadURL as a method on fileRef without await
• Forgetting to await the Promise
• Using incorrect method names like downloadURL()

3. What will the following code output if the file exists and permissions allow access?

import { getDownloadURL } from 'firebase/storage';

async function showURL(fileRef) {
  try {
    const url = await getDownloadURL(fileRef);
    console.log(url);
  } catch (error) {
    console.log('Error:', error.message);
  }
}

medium

A. It logs undefined.

B. It logs 'Error: File not found'.

C. It logs 'Error: Permission denied'.

D. It logs the public download URL of the file.

Solution

1. Step 1: Understand the try block with getDownloadURL

   If the file exists and permissions are correct, getDownloadURL resolves with the URL string.

2. Step 2: Analyze the catch block usage

   The catch block only runs if there is an error like missing file or denied permission, which is not the case here.

3. Final Answer:

   It logs the public download URL of the file. -> Option D

4. Quick Check:

   File exists + permission OK = logs URL [OK]

Hint: Try block logs URL if file exists and accessible [OK]

Common Mistakes:

• Assuming error logs even when file is accessible
• Confusing error messages with success output
• Ignoring async/await behavior

4. Given this code snippet, what is the main issue that will cause an error?

const url = getDownloadURL(fileRef);
console.log(url);

medium

A. getDownloadURL returns a Promise, so url is not the actual URL string.

B. fileRef is not defined.

C. getDownloadURL is not imported.

D. console.log cannot print URLs.

Solution

1. Step 1: Identify getDownloadURL return type

   getDownloadURL() returns a Promise, not the URL string directly.

2. Step 2: Understand why console.log prints a Promise

   Without awaiting or using .then(), url holds a Promise object, so console.log prints the Promise, not the URL.

3. Final Answer:

   getDownloadURL returns a Promise, so url is not the actual URL string. -> Option A

4. Quick Check:

   Missing await means url is Promise, not string [OK]

Hint: Always await getDownloadURL to get the URL string [OK]

Common Mistakes:

• Forgetting that getDownloadURL returns a Promise
• Assuming console.log prints resolved Promise value
• Ignoring async/await syntax requirements

5. You want to display an image from Firebase Storage in your web app using its download URL. Which approach correctly handles errors if the file is missing or access is denied?

import { getDownloadURL } from 'firebase/storage';

async function displayImage(fileRef) {
  try {
    const url = await getDownloadURL(fileRef);
    document.getElementById('img').src = url;
  } catch (error) {
    document.getElementById('img').alt = 'Image not available';
  }
}

hard

A. This code correctly sets the image source or alt text on error.

B. This code will crash because getDownloadURL is synchronous.

C. This code does not handle errors and will leave the image blank.

D. This code sets the image source before getting the URL.

Solution

1. Step 1: Check async/await usage with getDownloadURL

   The code correctly awaits the download URL before setting the image source.

2. Step 2: Verify error handling in catch block

   If an error occurs (file missing or permission denied), the catch block sets the alt text to inform users the image is unavailable.

3. Final Answer:

   This code correctly sets the image source or alt text on error. -> Option A

4. Quick Check:

   Await + try/catch = proper URL loading and error handling [OK]

Hint: Use try/catch with await getDownloadURL to handle errors [OK]

Common Mistakes:

• Not awaiting getDownloadURL causing undefined src
• Ignoring error handling leaving broken images
• Setting image src before URL is available