Keywords: SyntaxError | Express Routing | AngularJS
Abstract: This article provides a detailed analysis of the common JavaScript error SyntaxError: expected expression, got '<', focusing on issues caused by improper Express routing configuration. Through practical code examples, it explains how to correctly configure Express routes to avoid returning HTML content for JavaScript file requests, while introducing routing handling strategies for AngularJS single-page applications and error debugging methods. The article also discusses the fundamental differences between HTML tags and character escaping, offering comprehensive solutions for developers.
Error Phenomenon and Background Analysis
During Node.js and Express framework development, developers often encounter console errors like SyntaxError: expected expression, got '<'. This error typically occurs when the browser attempts to parse a JavaScript file but receives HTML content instead. Semantically, the browser expects valid JavaScript expressions but actually receives content starting with HTML tags (such as <!DOCTYPE html>), causing the JavaScript engine to fail in proper parsing.
Root Cause Investigation
Analyzing the routing configuration in the original code:
app.all('*', function (req, res) {
res.sendFile(__dirname+'/index.html')
})This code uses the app.all('*') pattern to match all request paths. Regardless of whether the client requests HTML files, CSS stylesheets, JavaScript scripts, or other resources, the server returns the content of the index.html file. When the browser requests JavaScript files like angular.min.js or jquery.js, the server returns HTML content from index.html instead of the expected JavaScript code.
Solution Implementation
The correct approach is to dynamically determine the return content based on the request path. Here is an improved code example:
// Static file serving middleware
app.use(express.static(__dirname + '/public'));
// API route handling
app.get('/api/data', function(req, res) {
res.json({message: 'API response data'});
});
// Single-page application route fallback
app.get('*', function(req, res) {
// Check if the request is for static resources
if (req.path.includes('.') && !req.path.endsWith('/')) {
// If it's a file request and not a directory, return 404
res.status(404).send('File not found');
} else {
// Otherwise return the main page for front-end routing
res.sendFile(__dirname + '/index.html');
}
});AngularJS Integration Considerations
In AngularJS single-page applications, special attention must be paid to the coordination between front-end and back-end routing. AngularJS's ng-view directive relies on the front-end routing system, while Express needs to correctly handle various resource requests:
// Configure static resource directories
app.use('/js', express.static(__dirname + '/js'));
app.use('/css', express.static(__dirname + '/css'));
app.use('/lib', express.static(__dirname + '/lib'));
// AngularJS route fallback handling
app.get('*', function(req, res) {
// Exclude requests with known file extensions
const excludedExtensions = ['.js', '.css', '.png', '.jpg', '.ico'];
const hasExtension = excludedExtensions.some(ext => req.path.endsWith(ext));
if (hasExtension) {
res.status(404).send('Resource not found');
} else {
res.sendFile(__dirname + '/index.html');
}
});Error Debugging and Prevention
When encountering the SyntaxError: expected expression, got '<' error, the following debugging steps can be taken:
- Check the Network tab in the browser developer tools to confirm the actual response content of JavaScript files
- Verify that Express routing configuration correctly distinguishes between static resource requests and page requests
- Use middleware to log request information, aiding in diagnosing routing matching issues
- Ensure that external resource paths referenced in HTML files are correct
Best Practices Summary
To avoid such errors, developers should: clearly distinguish between static resource serving and dynamic route handling; properly configure Express middleware order; implement appropriate route fallback mechanisms in single-page applications; regularly check the correctness of front-end resource reference paths. By following these practices, the occurrence probability of SyntaxError: expected expression, got '<' errors can be significantly reduced, improving application stability.