Error in console when running on Flutter web: Refused to execute script from localhost/main.js, because MIME type ('text/html') is not executable

ONLINECAST

2 min read

·

05-10-2024

8

0

Share

Flutter Web: Fixing the "Refused to execute script" Error

Problem: You're trying to run your Flutter web app, but you encounter an error in the browser console that reads: "Refused to execute script from localhost/main.js, because MIME type ('text/html') is not executable."

Simplified: Essentially, your web browser is trying to run a JavaScript file (main.js) but is confused because it thinks the file is a regular HTML page. This happens when the server isn't properly configured to serve JavaScript files with the correct MIME type.

Scenario: Let's imagine you have a basic Flutter web app and are running it using flutter run -d chrome. You see the following error in the browser's developer console:

Refused to execute script from 'localhost:60612/main.js' because its MIME type ('text/html') is not executable, and strict MIME type checking is enabled.

Analysis: This error occurs because the web server (usually a development server provided by Flutter) is sending the main.js file with the incorrect MIME type (text/html) instead of application/javascript. This misconfiguration prevents the browser from recognizing main.js as an executable script.

Resolution: Here's how you can fix this:

1. Identify the Development Server: Flutter uses a built-in development server for web apps. You might need to identify the server process running and its configuration files.

2. Modify Server Configuration: You need to adjust the server configuration to serve JavaScript files correctly. The specific steps depend on the server you are using.

   • For built-in Flutter Dev Server:

     • Look for a configuration file named something like flutter_web.yaml or flutter_tools.yaml in your Flutter project's root directory.
     • Find the section related to web server configuration and modify the MIME type mapping for .js files. Here's an example of how to do it in flutter_tools.yaml:

     flutter:
       web:
         # ... other configurations
         static_files:
           - pattern: "*.js"
             mime_type: application/javascript
     

   • For Custom Servers (like Apache or Nginx):

     • You'll need to modify the server configuration files (e.g., httpd.conf for Apache or nginx.conf for Nginx) to include a MIME type directive for .js files. Here's an example for Apache:

     # ... other directives
     AddType application/javascript .js
     

3. Restart the Server: After making changes to the server configuration, restart the server to apply the changes.

Additional Insights:

• MIME Type: MIME (Multipurpose Internet Mail Extensions) type defines the nature of a file. It tells the browser how to handle a specific file, such as an image, a document, or a script.
• Strict MIME Type Checking: Modern browsers have strict MIME type checking enabled. This means they will only execute scripts if they are served with the correct MIME type.

Resources:

• Flutter Web Documentation
• MIME Type Registry
• Apache Web Server Documentation
• Nginx Web Server Documentation

By following these steps, you can fix the "Refused to execute script" error and successfully run your Flutter web application.