typescript-variables

Variables

Apply the TypeScript Style Guide's variable conventions in the context of the current task.

Workflow

1. Inspect the consuming repository's conventions and configuration.
2. Let explicit repository conventions take precedence over this opinionated guidance.
3. Apply, review, or explain only the guidance relevant to the task.
4. State important tradeoffs when the appropriate choice depends on context or judgment.

Boundaries

• Keep TypeScript and ESLint responsible for checks they can enforce automatically.
• Do not introduce unrelated TypeScript Style Guide conventions merely because this skill is active.

Related Guidance

Application State

For detailed guidance on states that require different data, use typescript-discriminated-unions when it is available.

Variables

Const Assertion

Strive to declare constants using the const assertion as const:

Constants are used to represent values that are not meant to change, ensuring reliability and consistency in a codebase. Const assertions preserve literal types and infer readonly properties.

• Type Narrowing - Using as const ensures that literal values (e.g., numbers, strings) are treated as exact values instead of generalized types like number or string.
• Readonly Properties - Objects and arrays get readonly properties, so TypeScript catches direct mutations.

Examples:

• Objects

  // ❌ Avoid
  const FOO_LOCATION = { x: 50, y: 130 }; // Type { x: number; y: number; }
  FOO_LOCATION.x = 10;
  
  // ✅ Use
  const FOO_LOCATION = { x: 50, y: 130 } as const; // Type '{ readonly x: 50; readonly y: 130; }'
  FOO_LOCATION.x = 10; // Error
  

• Arrays

  // ❌ Avoid
  const BAR_LOCATION = [50, 130]; // Type number[]
  BAR_LOCATION.push(10);
  
  // ✅ Use
  const BAR_LOCATION = [50, 130] as const; // Type 'readonly [50, 130]'
  BAR_LOCATION.push(10); // Error
  

• Template Literals

  // ❌ Avoid
  const RATE_LIMIT = 25;
  const RATE_LIMIT_MESSAGE = `Max number of requests/min is ${RATE_LIMIT}.`; // Type string
  
  // ✅ Use
  const RATE_LIMIT = 25;
  const RATE_LIMIT_MESSAGE = `Max number of requests/min is ${RATE_LIMIT}.` as const; // Type 'Max number of requests/min is 25.'