Severity: warning
Category: Control flow
Stage: Stage 1

Description

JavaScript's switch statement falls through from one case clause to the next unless a break, return, throw, or continue terminates the clause. Unintentional fallthrough is a frequent source of bugs. SJS emits this warning when a non-empty case clause (one that contains at least one statement) ends without an explicit terminator and execution would implicitly fall into the next clause.

Empty case clauses (used to share a body between multiple values) are exempt, as that pattern is idiomatic and intentional:

case "a":
case "b":
  handle()   // intentional grouping — no warning
  break

Example

// ✗ warning
function grade(score: number): string {
  let label = ""
  switch (score) {
    case 5:
      label = "excellent"       // SJS-W008 — falls through to case 4
    case 4:
      label = "good"
      break
    case 3:
      label = "average"         // SJS-W008 — falls through to default
    default:
      label = "unknown"
  }
  return label
}

Suppression / Fix

Add an explicit terminator at the end of every non-empty case clause:

// ✓ correct
function grade(score: number): string {
  switch (score) {
    case 5:
      return "excellent"
    case 4:
      return "good"
    case 3:
      return "average"
    default:
      return "unknown"
  }
}

If the fallthrough is intentional (rare), annotate it with a // fallthrough comment immediately before the next case to suppress the warning:

// ✓ correct — intentional fallthrough documented
switch (phase) {
  case "init":
    setup()
    // fallthrough
  case "run":
    execute()
    break
}

Related codes

• SJS-W009 — unreachable code following a terminator statement
• SJS-E008 — break used outside of a loop or switch