Never type and unreachable code in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a function that never returns.

Typescript

function fail(): [1] {
  throw new Error('This function never returns');
}

Drag options to blanks, or click blank then click option'

Anever

Bvoid

Cany

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'void' instead of 'never' for functions that throw errors.

Using 'any' which is too general.

The never type is used for functions that never return, such as those that always throw an error.

2fill in blank

medium

Complete the code to make the switch statement exhaustive using the never type.

Typescript

type Color = 'red' | 'green' | 'blue';

function checkColor(c: Color) {
  switch(c) {
    case 'red':
      return 'Red';
    case 'green':
      return 'Green';
    case 'blue':
      return 'Blue';
    default:
      const _exhaustiveCheck: [1] = c;
      return _exhaustiveCheck;
  }
}

Drag options to blanks, or click blank then click option'

Anever

Bvoid

Cany

Dunknown

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'any' which disables type checking.

Not handling the default case at all.

Using never in the default case ensures the switch is exhaustive. If a new color is added, TypeScript will warn.

3fill in blank

hard

Fix the error in the function that should never return but currently returns a value.

Typescript

function errorFunction(): never {
  [1] 'Error occurred';
}

Drag options to blanks, or click blank then click option'

Areturn

Bthrow

Cconsole.log

Dyield

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'return' with a string, which contradicts the 'never' return type.

Using 'console.log' which does not stop execution.

The function must throw an error to never return. Using throw stops execution and matches the never type.

4fill in blank

hard

Fill both blanks to create an exhaustive check function using the never type.

Typescript

function assertNever(x: [1]): [2] {
  throw new Error('Unexpected value: ' + x);
}

Drag options to blanks, or click blank then click option'

Anever

Bstring

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' as parameter type which allows unexpected values.

Using 'void' as return type which implies function can return normally.

The parameter type is never and the function return type is also never because it always throws an error.

5fill in blank

hard

Fill all three blanks to handle all cases and use the never type for unreachable code.

Typescript

type Shape = { kind: 'circle', radius: number } | { kind: 'square', size: number };

function area(s: Shape): number {
  switch(s.kind) {
    case 'circle':
      return Math.PI * s.radius * s.radius;
    case 'square':
      return s.size * s.size;
    default:
      return [1]([2]);
  }
}

function [3](x: never): never {
  throw new Error('Unexpected shape: ' + x);
}

Drag options to blanks, or click blank then click option'

AassertNever

Bs

Cnever

DhandleError

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong function name in the default case.

Passing the wrong variable to the function.

Not using the never type in the function parameter.

The assertNever function is called with s in the default case to ensure all cases are handled. It accepts a never type and throws an error.