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Severity: error
Category: type-check
Stage: Stage 1

Description

TypeScript mapped types ({ [P in keyof T]: ... }) are not valid SJS syntax. They generate types implicitly from other types through metaprogramming, making shapes harder to audit and the type system harder to reason about statically.

Spell out the shape you need explicitly.

Example

// ✗ error
type Optional<T> = {
  [P in keyof T]?: T[P]    // SJS-E006
}

Fix

Declare the explicit object type:

// ✓ correct — explicit optional shape
type OptionalUser {
  id?: number
  name?: string
  email?: string
}

For library code that genuinely needs parameterised shapes, restructure as explicit generic object types with optional members.

Related codes

  • SJS-E005 — intersection types are also banned
  • SJS-E008 — conditional types are also banned
Documentation