Serving JavaScript files in Flask - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the Flask class.

Flask

from flask import [1]

Drag options to blanks, or click blank then click option'

Ajsonify

Brender_template

Crequest

DFlask

Attempts:

3 left

💡 Hint

Common Mistakes

Importing render_template instead of Flask

Using incorrect module name

The Flask class is imported from the flask module to create the app.

2fill in blank

medium

Complete the code to create a Flask app instance.

Flask

app = [1](__name__)

Drag options to blanks, or click blank then click option'

AFlask

Brequest

Crender_template

Djsonify

Attempts:

3 left

💡 Hint

Common Mistakes

Calling render_template instead of Flask

Forgetting to pass __name__

Creating the app instance requires calling Flask with __name__.

3fill in blank

hard

Fix the error in the route decorator to serve JavaScript files.

Flask

@app.route('/static/[1]')

Drag options to blanks, or click blank then click option'

Apath:filename

Bfilename

C<path:filename>

D<filename>

Attempts:

3 left

💡 Hint

Common Mistakes

Using angle brackets incorrectly

Missing 'path:' prefix

Using <path:filename> allows Flask to capture the full path including folders.

4fill in blank

hard

Fill both blanks to send a JavaScript file from the static folder.

Flask

return [1]('static', filename=[2])

Drag options to blanks, or click blank then click option'

Asend_from_directory

Bsend_file

Cfilename

Dfile

Attempts:

3 left

💡 Hint

Common Mistakes

Using send_file without folder path

Passing wrong variable name for filename

send_from_directory sends files from a folder; filename is the file name variable.

5fill in blank

hard

Fill all three blanks to define a route that serves JavaScript files correctly.

Flask

@app.route('/js/[1]')
def serve_js([2]):
    return [3]('static/js', filename=[2])

Drag options to blanks, or click blank then click option'

A<filename>

Bfile

Csend_from_directory

Dsend_file

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatching route and function parameter names

Using send_file without folder path

The route captures 'filename', the function parameter is 'file', and send_from_directory serves the file from 'static/js'.

Practice

1. In Flask, where should you place your JavaScript files so they can be served correctly to the browser?

easy

A. Inside the static folder

B. Inside the templates folder

C. In the root project directory

D. Inside the instance folder

Solution

1. Step 1: Understand Flask static file serving

   Flask automatically serves files placed in the static folder to the browser.

2. Step 2: Identify correct folder for JavaScript files

   JavaScript files must be inside the static folder to be accessible via URL.

3. Final Answer:

   Inside the static folder -> Option A

4. Quick Check:

   Static folder = JavaScript files served [OK]

Hint: Always put JS files in the static folder for Flask to serve them [OK]

Common Mistakes:

• Placing JS files in templates folder
• Putting JS files in root directory
• Expecting instance folder to serve static files

2. Which of the following is the correct way to link a JavaScript file named app.js located in the static folder inside your Flask HTML template?

easy

A. <script src="/templates/app.js"></script>

B. <script src="/static/app.js"></script>

C. <script href="/static/app.js"></script>

D. <link src="/static/app.js">

Solution

1. Step 1: Use correct HTML tag for JavaScript

   The <script> tag with the src attribute is used to link JS files.

2. Step 2: Use correct path to static folder

   Flask serves static files under the /static/ URL prefix, so path should be /static/app.js.

3. Final Answer:

   <script src="/static/app.js"></script> -> Option B

4. Quick Check:

   Script tag + /static path = correct link [OK]

Hint: Use

Common Mistakes:

• Using <link> instead of <script>
• Using href instead of src in <script>
• Linking to /templates folder

3. Given this Flask route and folder structure, what will happen when the browser requests /static/script.js?

app.py:
from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
    return 'Hello'

Folder structure: - static/ - script.js - app.py

medium

A. The home route will return 'Hello' instead

B. A 404 Not Found error will occur

C. Flask will raise a runtime error

D. The file script.js will be served automatically

Solution

1. Step 1: Understand Flask static file serving default behavior

   Flask automatically serves files inside the static folder at the URL path /static/filename.

2. Step 2: Check if route conflicts with static path

   The / route does not conflict with /static/script.js, so static file is served.

3. Final Answer:

   The file script.js will be served automatically -> Option D

4. Quick Check:

   Static folder files served automatically at /static/ [OK]

Hint: Files in static folder served at /static/filename URL [OK]

Common Mistakes:

• Thinking Flask needs explicit route for static files
• Confusing root route with static file path
• Expecting 404 without static folder

4. You wrote this Flask route to serve a JavaScript file manually:

@app.route('/js/app.js')
def serve_js():
    return open('static/app.js').read()

But the browser shows the JavaScript code as plain text instead of running it. What is the problem?

medium

A. The response is missing the correct Content-Type header for JavaScript

B. The file path 'static/app.js' is incorrect

C. You cannot serve JavaScript files manually in Flask

D. The route path should be '/static/app.js' instead of '/js/app.js'

Solution

1. Step 1: Check how Flask serves static files

   Flask sets the Content-Type header to 'application/javascript' automatically for static files.

2. Step 2: Identify missing Content-Type in manual response

   Returning raw file content without setting Content-Type causes browser to treat it as plain text.

3. Final Answer:

   The response is missing the correct Content-Type header for JavaScript -> Option A

4. Quick Check:

   Set Content-Type to application/javascript for JS files [OK]

Hint: Always set Content-Type header when serving JS manually [OK]

Common Mistakes:

• Not setting Content-Type header
• Assuming Flask auto-sets headers in manual routes
• Using wrong file path without checking

5. You want to serve multiple JavaScript files from a custom folder named assets/js instead of the default static folder. Which Flask code snippet correctly serves these files at URL path /js/<filename>?

hard

A. @app.route('/js/<filename>')
def serve_js(filename):
return open(f'assets/js/{filename}').read()

B. from flask import send_file

@app.route('/js/<filename>')
def serve_js(filename):
return send_file(f'assets/js/{filename}')

C. from flask import send_from_directory

@app.route('/js/<filename>')
def serve_js(filename):
return send_from_directory('assets/js', filename)

D. @app.route('/js/<filename>')
def serve_js(filename):
return redirect(f'/static/{filename}')

Solution

1. Step 1: Use send_from_directory to serve files from custom folder

   Flask's send_from_directory safely serves files from any folder with correct headers.

2. Step 2: Map URL path to custom folder

   The route /js/<filename> calls send_from_directory('assets/js', filename) to serve files from assets/js.

3. Final Answer:

   Use send_from_directory with assets/js folder -> Option C

4. Quick Check:

   send_from_directory + custom folder = correct serving [OK]

Hint: Use send_from_directory to serve files outside static folder [OK]

Common Mistakes:

• Using send_file without setting headers
• Reading file content without headers
• Redirecting to static folder without files there