Typescript Prevent Default Case

switch-case.ts

interface Box {
    height: number;
    width: number;
    color: 'RED' | 'BLUE'
}

function logColor(box: Box) {
    switch (box.color) {
        case 'RED':
            console.log(box, 'is red');
            break;
        case 'BLUE':
            console.log(box, 'is blue');
            break;
        default:
            return throwBadKind(box.color);
      
  }
}

function throwBadKind(x: never): never {
	throw new Error('Unknown type "' + (x as string) + "'");
}

Typescript will not allow data to get into the default case.

Try it yourself in playground

To see the typing error add a new color in line 4 or remove case 'BLUE' in line 12-14.