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Severity: error
Category: null-safety
Stage: Stage 0 (prototype)

Description

All types in SJS are non-nullable by default. Assigning null or undefined to a type T without opting into nullability (the T? suffix) is a type error.

Example

// ✗ error
let name: string = null      // SJS-E001

function greet(user: string) {
  return `Hello, ${user}`
}
greet(undefined)             // SJS-E001

Fix

Append ? to make the type nullable, then narrow before use:

// ✓ correct
let name: string? = null

function greet(user: string?): string {
  if (user === null) return "Hello, stranger"
  return `Hello, ${user}`
}

Or, if the value must always be present, ensure the assignment can never be null.

Related codes

  • SJS-E003 — property access on possibly-null value
  • SJS-E011 — non-null assertion (!) is banned
Documentation