Why is this an issue?

The requirement for a final default clause is defensive programming. The clause should either take appropriate action, or contain a suitable comment as to why no action is taken.

Noncompliant code example

switch (param) {  //missing default clause
  case 0:
    doSomething();
    break;
  case 1:
    doSomethingElse();
    break;
}

switch (param) {
  default: // default clause should be the last one
    error();
    break;
  case 0:
    doSomething();
    break;
  case 1:
    doSomethingElse();
    break;
}

Compliant solution

switch (param) {
  case 0:
    doSomething();
    break;
  case 1:
    doSomethingElse();
    break;
  default:
    error();
    break;
}

Exceptions

The rule ignores switch statements where the discriminant is a TypeScript union and there is a case branch for each constituent.

type Season = 'Spring' | 'Summer' | 'Fall' | 'Winter';
let season: Season;
switch (season) {
  case 'Spring':
    wakeUp();
    break;
  case 'Summer':
    getOut();
    break;
  case 'Fall':
    saveFood();
    break;
  case 'Winter':
    sleep();
    break;
}

The same applies for TypeScript enums.

enum Direction {
  Up,
  Down
}

let dir: Direction;
switch (dir) {
  case Direction.Up:
    getUp();
    break;
  case Direction.Down:
    getDown();
    break;
}

Resources

• MITRE, CWE-478 - Missing Default Case in Switch Statement