Debugging Pug Mixin Invocation Errors

The Problem: Mixin Compilation Failures

When working with Pug templates, mixin errors typically manifest as hard failures during the compilation phase rather than runtime errors in the browser. These errors stop the rendering process entirely, often leaving you with a stack trace that points to the line where the mixin is called, rather than where it is defined.

The most common symptoms are Mixin [name] is not defined or Wrong number of arguments for mixin [name]. Because Pug processes templates as a stream, these errors usually stem from scope visibility, incorrect file inclusion, or signature mismatches.

Diagnostic Mapping

Use this table to match your error message to the likely architectural cause.

Error Message	Likely Cause	Diagnostic Focus
Mixin [name] is not defined	Missing definition or scope isolation	Include paths and definition order
Wrong number of arguments	Signature mismatch	Parameter count vs. argument count
Maximum call stack size exceeded	Circular dependency	Recursive mixin calls

Step-by-Step Resolution Process

Follow these checks in order to isolate the failure. These steps assume you are using Pug 3.x.

1. Verify Definition and Inclusion

Pug requires that a mixin be defined before it is invoked, or that the file containing the definition is explicitly brought into the current context.

• Check the local file: Ensure the mixin name() block appears above the +name() call.
• Check external files: If the mixin resides in a separate file (e.g., _mixins.pug), verify you have used the include keyword.

Example of correct inclusion:

// layout.pug
include ./partials/_mixins.pug

body
  +userCard('Jane Doe', 'Developer')

2. Validate the Argument Signature

If the mixin is found but fails during invocation, compare the definition signature with the call site. Pug expects the number of passed arguments to align with the declared parameters unless default values are provided.

• Count parameters: Count the variables inside the mixin name(param1, param2) parentheses.
• Count arguments: Count the values passed in +name(val1, val2).

Fix: If the arguments vary by use case, implement default values in the definition to prevent "Wrong number of arguments" errors.

// Definition with default value
mixin alert(message, type = 'info')
  div(class=type) = message

// Both of these calls will now succeed
+alert('Operation successful')
+alert('System error', 'danger')

3. Inspect Scope and Nesting

Mixins defined inside other blocks or conditional logic may not be accessible globally. If a mixin is nested within a specific block that isn't executed or is scoped to a different template fragment, the compiler will report it as undefined.

• Check nesting: Ensure the mixin is defined at the top level of the file or within a globally included partial.
• Check inheritance: If using extends, remember that mixins defined in a child template are not available in the parent layout.

4. Trace for Circular References

If the compiler crashes with a stack overflow or a "Maximum call stack size exceeded" error, you likely have a circular reference where Mixin A calls Mixin B, which then calls Mixin A.

• Map the calls: Trace the execution path of the mixin.
• Fix: Refactor the shared logic into a separate helper function or a non-recursive mixin.

Verification and Testing

To verify the fix, run the Pug compiler from your terminal. Replace template.pug with your actual file path:

# Run as a user with read permissions to the template directory
# Risk: This will overwrite the output HTML file
pug template.pug

Expected Result: The command should complete without outputting a stack trace and generate a valid HTML file. If the error persists, check that your include paths are relative to the file being compiled, not the project root.

Escalation Criteria

If the steps above do not resolve the issue, escalate the problem if you encounter the following:

• The mixin is defined and included, but the error only occurs when using specific extends hierarchies.
• The error message changes after updating the Pug version, suggesting a breaking change in the compiler.
• The mixin works in isolation but fails when integrated into a larger build pipeline (e.g., Gulp or Webpack).