Diagnosing and Fixing 'Undefined Variable' Errors in Sass

The Problem: The Undefined Variable Compilation Error

When the Sass compiler cannot locate a variable declaration before it is called in a stylesheet, it halts the build process with a specific error: Error: Undefined variable: $variable-name. This prevents the CSS from generating and often breaks the entire CI/CD pipeline.

The core issue is almost always a scope or sequence problem: the compiler is processing a line of code that references a symbol it has not yet encountered in its current memory state.

Diagnostic Matrix: Common Causes

Symptom	Likely Cause	Scope Impact
Variable exists in project but not in current file	Missing @use or @import	Global/Module
Variable is declared lower in the same file	Incorrect declaration order	Local
Variable is in a module but called as a global	Namespace missing (Sass Module System)	Module
Variable is wrapped in an @if block	Conditional exclusion	Conditional
Variable is completely missing from codebase	Typo or missing definition	N/A

Step-by-Step Resolution Process

Follow these checks in order. Stop once the error is resolved.

1. Global Existence Check

Perform a project-wide search for the exact variable name (including the $ prefix). If the variable does not exist anywhere in the source code, you have a typo or a missing definition.

• Fix: Define the variable in your configuration or theme partial (e.g., _variables.scss).

2. Sequence and Order Verification

Sass processes files linearly. If a variable is declared on line 50 but used on line 10, the compiler will fail.

• Fix: Move the variable declaration to the top of the file or into a separate partial that is imported at the very beginning of your main entry point.

3. Import and Module Mapping

If the variable is defined in _colors.scss but used in _buttons.scss, the latter must have access to the former. The method depends on your Sass version (assumed v1.23+ for module support).

For the Module System (@use):
If you use @use 'colors';, the variables are not global. They are namespaced to the filename.

// _colors.scss
$primary: #007bff;

// _buttons.scss
@use 'colors';
.btn { color: colors.$primary; } // Correct: uses namespace
.btn { color: $primary; }       // Error: Undefined variable

• Fix: Prefix the variable with the module name (colors.$primary) or use an alias: @use 'colors' as c; then call c.$primary.

4. Conditional Logic Audit

Check if the variable is defined inside an @if or @mixin block that is not being triggered during the current build configuration.

• Fix: Provide a fallback value outside the conditional block or ensure the condition evaluates to true for the target environment.

Verification and Testing

To verify the fix, run the compiler from your terminal. If using the Dart Sass CLI, use the --trace flag to pinpoint the exact file and line causing the failure:

# Run in project root with appropriate permissions
sass --trace style.scss style.css

Expected Result: The command completes without an "Undefined variable" error, and the resulting CSS file contains the compiled hex or pixel values instead of the variable names.

Rollback Procedure

If the changes to imports or namespaces cause "Duplicate variable" errors or break other stylesheets:

1. Revert the @use or @import statements to their previous state.
2. If you renamed a variable to fix a typo, use a global search-and-replace to ensure all references are updated simultaneously.

When to Escalate

If the variable is clearly declared and imported correctly, but the error persists, the issue is likely outside the Sass syntax. Escalate to a build engineer if:

• The error only occurs in CI/CD but not locally (indicates a stale build cache).
• Running npm ls sass reveals a version mismatch between local and server environments.
• The error persists after deleting node_modules and performing a clean npm install.