Type Coverage

Type coverage is a metric that measures the percentage of variables, properties, parameters, and return values in your code that have explicitly defined types. It's a key indicator of code quality, especially in languages that support type systems like PHP, TypeScript, and others.

Why Type Coverage Matters

High type coverage offers several benefits:

• Early Error Detection: Catches type-related errors at compile/static analysis time rather than runtime
• Better Developer Experience: Enables better IDE auto-completion and suggestions
• Self-Documenting Code: Makes code more readable and understandable
• Safer Refactoring: Reduces the risk of introducing bugs when making changes
• Improved Maintainability: Makes it easier for new developers to understand the codebase

How Type Coverage is Calculated

Type coverage is typically calculated as:

Type Coverage = (Number of Typed Elements / Total Number of Elements) × 100%

Elements include function parameters, return types, property declarations, and variable declarations, depending on the language and tool being used.

Type Coverage in PHP

In PHP, type coverage can be measured using tools like PHPStan or Psalm. These tools analyze your codebase and report on missing type declarations. With PHP 7.0 and later versions, you can add scalar type hints, return type declarations, and with PHP 7.4, property type declarations.

For PHP projects, OtterWise helps you track type coverage over time to ensure your codebase maintains good typing practices.

Related Concepts

Type coverage is related to but distinct from code coverage. While code coverage measures how much of your code is executed by tests, type coverage measures how much of your code has proper type declarations.