JavaScript Mermaid

Handling Mermaid Diagram Rendering Errors

Sometimes, the most challenging bugs are not in the core logic, but in the tooling around it. Recently, while working on the diagram rendering service for a project, I encountered an issue where invalid Mermaid syntax would cause the entire process to hang. Here's how I tackled it.

The Problem

The diagram rendering service, used for generating visualizations, relied on an external script to process Mermaid syntax. When the AI (integrated to suggest diagram syntax) returned invalid syntax, the rendering script would enter a 15-second timeout loop, with no error feedback. This was far from ideal, both for the user experience and for debugging.

The Solution

The primary goal was to immediately catch parsing errors instead of waiting for a timeout. This involved:

  1. Implementing Error Handling: The rendering script was modified to include error handling specifically for Mermaid parse failures.
  2. Immediate Exit: Upon encountering a parse error, the script now exits immediately with a specific exit code (2).
  3. Improved Error Messaging: The DiagramRenderService was updated to provide more informative error messages to the user, indicating the nature of the problem.
  4. Adding Tests: Integration and unit tests were added to ensure the robustness of the error handling, covering both successful rendering and invalid syntax scenarios.

Here's an illustrative example of how the error handling was implemented in the rendering script (using Node.js):

try {
  // Attempt to render the Mermaid diagram
  mermaid.render('diagram', mermaidCode, (svgCode) => {
    // ... process the svgCode ...
  });
} catch (error) {
  console.error('Mermaid syntax error:', error.message);
  process.exit(2); // Exit with error code 2
}

This code snippet demonstrates how a try...catch block is used to capture any errors during the Mermaid rendering process. If an error occurs (e.g., due to invalid syntax), the error message is logged, and the script exits with a non-zero exit code.

The Takeaway

Robust error handling is crucial for any service, especially when dealing with external tools or user-generated content. By implementing specific error handling, providing clear error messages, and adding comprehensive tests, we significantly improved the reliability and user experience of the diagram rendering service.

Handling Mermaid Diagram Rendering Errors
Gerardo Ruiz

Gerardo Ruiz

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