noFallthroughCasesInSwitch

A forgotten break, caught

Flags a `case` that does work and then runs into the next one, while still permitting the deliberate empty fallthrough.

Since: TypeScript 1.8
strict: Not in strict
In your tsconfig: "noFallthroughCasesInSwitch": true

The same snippet both times. Only the option changed.

With it off

"noFallthroughCasesInSwitch": false

export function pick(n: 1 | 2): string {
  switch (n) {
    case 1:
      const label = 'one'
    case 2:
      return 'two'
  }
}

Compiles clean

With it on

"noFallthroughCasesInSwitch": true

export function pick(n: 1 | 2): string {
  switch (n) {
    case 1:
      const label = 'one'
    case 2:
      return 'two'
  }
}

Emits TS7029

Why the compiler bothers

Stacked empty cases are a real idiom and the check knows it: only a clause with statements in it is reported, so `case 1: case 2: return x` stays legal. That distinction is what makes the flag usable — a check that banned all fallthrough would be turned off within a week, and a check nobody runs catches nothing.

Takeaway

Empty fallthrough stays legal, which is the only reason this one survives in a real codebase.

Where to go next

Try it in the Lab

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