Exhaustive checking with never in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable with type never.

Typescript

let errorValue: [1];

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

Aunknown

Bany

Cvoid

Dnever

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'any' instead of 'never' because 'any' allows any value.

Confusing 'void' with 'never'.

The never type represents values that never occur. It is used for variables that never hold any value.

2fill in blank

medium

Complete the function signature to indicate it never returns.

Typescript

function fail(message: string): [1] {
  throw new Error(message);
}

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Avoid

Bstring

Cnever

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'void' which means returns nothing but does return.

Using 'string' or 'undefined' which are incorrect here.

The function throws an error and never returns normally, so its return type is never.

3fill in blank

hard

Fix the error in the exhaustive check function by completing the switch default case.

Typescript

function assertNever(x: never): never {
  throw new Error("Unexpected value: " + [1]);
}

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AJSON.stringify(x)

Bx

Ctypeof x

DString(x)

Attempts:

3 left

💡 Hint

Common Mistakes

Using typeof x which returns a string describing the type, not the value.

Using JSON.stringify(x) which may cause runtime errors if x is never.

The parameter x is of type never, so passing it to the error message directly is correct.

4fill in blank

hard

Fill both blanks to complete the exhaustive check in the switch statement.

Typescript

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

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

function [2](x: never): never {
  throw new Error('Unexpected shape: ' + x);
}

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AassertNever

BcheckNever

DfailNever

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the function call and declaration.

Not calling the function in the default case.

The function assertNever is called in the default case to ensure all cases are handled.

5fill in blank

hard

Fill all three blanks to complete the exhaustive check with a switch and assertNever function.

Typescript

type Result = { type: 'success', value: number } | { type: 'error', message: string };

function handleResult(result: Result): string {
  switch (result.type) {
    case 'success':
      return `Value is ${result.value}`;
    case 'error':
      return `Error: ${result.message}`;
    default:
      return [1]([2]); // call assertNever with result
  }
}

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

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

AassertNever

Bresult

Dfail

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the wrong variable to the function.

Using a different function name in call and declaration.

The default case calls assertNever with the variable result to ensure all cases are handled.