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BUG002-switch-fallthrough — Switch case falls through

PropertyValue
DimensionMaintainability
SeverityMedium
ConfidenceHigh
CWECWE-484
LanguagesJavaScript, TypeScript

What it detects​

A case (or default:) clause inside a switch statement that:

  1. Is not the last clause in its switch, and
  2. Has at least one statement in its body, and
  3. Does not end with a terminating statement (break, return, throw, continue), and
  4. Has no ESLint-style fallthrough comment immediately before the next case label.

The control falls through to the next clause silently — almost always a typo for a missing break.

Carve-outs​

Empty case grouping​

Empty consequents are the idiomatic way to apply the same body to multiple case values and are not flagged:

// NOT flagged — intentional grouping
switch (x) {
  case 1:
  case 2:
  case 3:
    doMulti();
    break;
}

// falls through comment​

Following ESLint no-fallthrough, a comment matching /falls?\s*through/i on the line immediately before the next case label silences the finding. Both line comments and block comments work:

switch (x) {
  case 1:
    doA();
    // falls through
  case 2:
    doB();
    break;
}

switch (x) {
  case 1:
    doA();
    /* fallthrough */
  case 2:
    doB();
    break;
}

Why it matters​

CWE-484 (Omitted Break Statement in Switch) is a textbook source of bugs. The omitted break is invisible and the code still compiles, but the following case body runs anyway and silently changes behavior.

// Bug: forgot break — `doB()` always runs when x === 1
switch (x) {
  case 1:
    doA();
  case 2:
    doB();
    break;
}

Examples — flagged​

// Plain fallthrough
switch (x) {
  case 1:
    doA();
  case 2:
    doB();
    break;
}

// Multiple cascading falls
switch (x) {
  case 1:
    doA();
  case 2:
    doB();
  case 3:
    doC();
    break;
}

// `default:` in the middle, falling through
switch (x) {
  case 1:
    doA();
    break;
  default:
    doDefault();
  case 2:
    doB();
    break;
}

// Block body without terminator
switch (x) {
  case 1: {
    doA();
  }
  case 2:
    doB();
    break;
}

Examples — not flagged​

// Terminators stop fallthrough
switch (x) {
  case 1:
    doA();
    break;
  case 2:
    return doB();
  case 3:
    throw new Error('c');
}

// Last case can't fall through
switch (x) {
  case 1:
    doA();
    break;
  case 2:
    doB();  // last case
}

// Empty grouping
switch (x) {
  case 1:
  case 2:
    doMulti();
    break;
}

// Comment carve-out
switch (x) {
  case 1:
    doA();
    // falls through
  case 2:
    doB();
    break;
}

Fix guidance​

Add a terminating statement at the end of the case body, or — if the fallthrough is intentional — add a // falls through comment immediately before the next case label.

// Before (bug):
switch (x) {
  case 1:
    doA();
  case 2:
    doB();
    break;
}

// After (fix):
switch (x) {
  case 1:
    doA();
    break;
  case 2:
    doB();
    break;
}

Implementation​

References​