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Severity: warning
Category: dynamic type
Stage: Stage 1

Description

When a value of type dynamic flows into a position that has a concrete type annotation (string, number, User, etc.) without a type guard or narrowing check in between, the assignment silently discards all type safety for that binding.

SJS emits this warning to surface the unsafety at the assignment site rather than letting it propagate invisibly through the codebase.

Example

// ✗ warning
const raw: dynamic = externalLib.fetch()

let name: string = raw          // SJS-W002 — no narrowing before assignment
let count: number = raw + 1     // SJS-W002 — arithmetic on dynamic assigned to number

Suppression / Fix

Narrow the dynamic value with a typeof check, instanceof check, or an explicit cast before assigning it to a typed position:

// ✓ correct — typeof guard
const raw: dynamic = externalLib.fetch()

if (typeof raw === "string") {
  let name: string = raw        // safe: raw is narrowed to string inside this branch
}
// ✓ correct — explicit cast at the JS interop boundary
const name: string = raw as string

If the position genuinely cannot be typed, change its annotation to dynamic or unknown and narrow before use downstream.

Related codes

  • SJS-W001 — implicit dynamic on unannotated positions in --strict mode
  • SJS-E004 — any is not a valid type in SJS; dynamic is the correct escape hatch
Documentation