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name: formatting-conditional-types description: Formatting rules for TypeScript conditional types. Use when writing complex conditional types to ensure debuggable, maintainable formatting with proper alignment.

Formatting Conditional Types

Goal

Enable commenting out individual cases without breaking syntax (useful for debugging).

Two Patterns

FLAT PATTERN (prefer when possible)

Use for mutually exclusive top-level cases:

• All top-level conditions start at same indentation
• Place : at end of line for each case
• Next condition returns to base indentation
• Align ? symbols vertically within each case

NESTED PATTERN (only when logic requires it)

Use when conditions are inherently nested (one inside another):

• Each nesting level increases indentation
• Align ? and : symbols at each level

Examples

Copy// ✅ GOOD - FLAT pattern for mutually exclusive cases
// Each top-level case can be commented out safely
type Result<$T> = $T extends string ? 'string'
  : $T extends number ? 'number'
  // $T extends boolean                  ? 'boolean' :  // ← Can comment out
  : $T extends undefined ? 'undefined'
  : 'unknown' // ← Final else, aligned

// ✅ GOOD - FLAT pattern with inner ternary
type LengthSlow<$S, $Acc> = $S extends `${string}${string}${string}${string}${infer __r__}`
  ? string extends __r__ // Inner ternary (nested logic)
    ? number
  : LengthSlow<__r__, [...$Acc, 0, 0, 0, 0]> // ← : at end
  : $S extends `${string}${string}${string}${infer __r__}` // ← Back to base
    ? string extends __r__ ? number
    : [...$Acc, 0, 0, 0]['length'] // ← : at end
  : $Acc['length'] // ← Final else

// ✅ GOOD - NESTED pattern (inherent logic requires it)
type DeepCheck<$S> = $S extends `${infer __c__}${infer __rest__}` ? string extends __rest__ ? number
  : __rest__ extends '' ? 1
  : __rest__ extends `${infer __c2__}${infer __r2__}` ? string extends __r2__ ? number
    : __r2__ extends '' ? 2
    : never
  : never
  : 0

// ❌ BAD - Misaligned flat pattern
type Bad<$T> = $T extends string ? 'string' // ← Alignment lost
  : $T extends number ? 'number' // ← Alignment lost
  : 'unknown'

// ❌ BAD - Using nested when flat would work
type Bad2<$T> = $T extends string ? 'string'
  : $T extends number ? 'number' // ← Unnecessary nesting
  : $T extends boolean ? 'boolean'
  : 'unknown'

Notes

• Use //dprint-ignore before long conditional types to preserve alignment
• Align on ? and : for readability