Case statements without a break will execute their body and then fallthrough to the next case or default block and execute this block as well. While this is sometimes intentional, many times the developer has forgotten to add a break statement, intending only for a single case statement to be executed. This rule enforces that you either end each case statement with a break statement or an explicit comment that fallthrough was intentional. The fallthrough comment must contain one of fallthrough, falls through or fall through.

Invalid:

switch (myVar) {
  case 1:
    console.log("1");

  case 2:
    console.log("2");
}
// If myVar = 1, outputs both `1` and `2`.  Was this intentional?

Valid:

switch (myVar) {
  case 1:
    console.log("1");
    break;

  case 2:
    console.log("2");
    break;
}
// If myVar = 1, outputs only `1`

switch (myVar) {
  case 1:
    console.log("1");
    /* falls through */
  case 2:
    console.log("2");
}
// If myVar = 1, intentionally outputs both `1` and `2`