Exhaustive checking with never in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of using never in TypeScript for exhaustive checking?

The never type is used to ensure that all possible cases in a union type are handled. If a case is missed, TypeScript will show an error, helping catch bugs early.

intermediate

How do you use never in a switch statement for exhaustive checking?

You add a default case that assigns the variable to never. This makes TypeScript check if any case is missing and gives an error if so.

beginner

What happens if you forget to handle a case in a union type when using exhaustive checking with never?

TypeScript will produce a compile-time error, telling you that the variable is not of type never. This means you missed a case and need to handle it.

intermediate

Explain the role of the assertNever function in exhaustive checking.

The assertNever function takes a value of type never. It is called in the default case to make sure no other cases exist. If a new case is added but not handled, TypeScript will error.

beginner

Why is exhaustive checking with never useful in real projects?

It helps catch missing cases early, making code safer and easier to maintain. It prevents bugs caused by unhandled values in union types.

What type does the never type represent in TypeScript?

A A type for any value

B A type for numbers only

C A type for strings only

D A type that never occurs

never means a value that never happens, used to catch impossible cases.

In exhaustive checking, where is the never type usually used?

A In the first case of a <code>switch</code>

B In the <code>default</code> case of a <code>switch</code> statement

C Only in function return types

D In variable declarations

The never type is used in the default case to catch unhandled cases.

What does TypeScript do if a case is missing when using exhaustive checking with never?

A Runs the program anyway

B Ignores the missing case

C Shows a compile-time error

D Converts the missing case to <code>any</code>

TypeScript shows an error to help you fix the missing case.

What is the purpose of an assertNever function?

A To check that a value is of type <code>never</code>

B To convert a value to <code>never</code>

C To return a string

D To handle all cases explicitly

assertNever ensures the value is never, helping with exhaustive checks.

Which of these is a benefit of exhaustive checking with never?

A Catches missing cases early

B Makes code run faster

C Allows any type without errors

D Removes the need for <code>switch</code> statements

It helps find missing cases before running the program.

Explain how the never type helps with exhaustive checking in TypeScript.

Think about how TypeScript checks all options in a union.

Describe the role and structure of an assertNever function in exhaustive checking.

It's a helper function to make sure no cases are missed.

Practice

1. What is the main purpose of using never in exhaustive checking in TypeScript?

easy

A. To declare a variable that can hold any value

B. To allow any type of value without restrictions

C. To ignore errors during compilation

D. To ensure all possible cases are handled in a switch or conditional

Solution

1. Step 1: Understand the role of never in exhaustive checks

   never is used to represent values that should never occur, helping catch unhandled cases.

2. Step 2: Connect never usage to exhaustive case handling

   By forcing a function to accept never, TypeScript ensures all cases are covered, or else a compile error occurs.

3. Final Answer:

   To ensure all possible cases are handled in a switch or conditional -> Option D

4. Quick Check:

   Exhaustive checking = catch all cases [OK]

Hint: Use never to catch missing cases in switch statements [OK]

Common Mistakes:

• Thinking never allows any value
• Using never to ignore errors
• Confusing never with unknown type

2. Which of the following is the correct syntax for a helper function that uses never to enforce exhaustive checking?

easy

A. function assertNever(x: never): never { throw new Error('Unexpected value: ' + x); }

B. function assertNever(x: any): void { console.log(x); }

C. function assertNever(x: unknown): never { return x; }

D. function assertNever(x: never): void { return; }

Solution

1. Step 1: Identify correct parameter and return types

   The helper function must accept a parameter of type never and return never because it throws an error.

2. Step 2: Check function body matches expected behavior

   The function throws an error with the unexpected value, which is the standard pattern for exhaustive checks.

3. Final Answer:

   function assertNever(x: never): never { throw new Error('Unexpected value: ' + x); } -> Option A

4. Quick Check:

   Helper function with never param and throw = correct [OK]

Hint: Helper must take never and throw error to enforce checks [OK]

Common Mistakes:

• Using any or unknown instead of never
• Not throwing an error inside the function
• Returning void instead of never

3. What will be the output of this TypeScript code?

type Shape = 'circle' | 'square';
function area(shape: Shape) {
  switch (shape) {
    case 'circle':
      return 3.14 * 2 * 2;
    // Missing case for 'square'
    default:
      return assertNever(shape);
  }
}

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

console.log(area('square'));

medium

A. Returns 12.56

B. Throws Error: Unexpected shape: square

C. Returns undefined

D. Compilation error due to missing case

Solution

1. Step 1: Analyze switch cases and input

   The function handles 'circle' but not 'square', so at runtime, calling area('square') hits the default case.

2. Step 2: Understand assertNever behavior at runtime

   The assertNever function throws an error with the unexpected shape, so the program throws a runtime error.

3. Final Answer:

   Throws Error: Unexpected shape: square -> Option B

4. Quick Check:

   Missing case causes runtime error when called [OK]

Hint: Missing case triggers runtime error when called [OK]

Common Mistakes:

• Assuming missing case returns undefined
• Thinking it returns a number for all inputs
• Expecting compile error instead of runtime error

4. Identify the error in this TypeScript code using exhaustive checking:

type Direction = 'up' | 'down' | 'left';
function move(dir: Direction) {
  switch (dir) {
    case 'up':
      return 'Moving up';
    case 'down':
      return 'Moving down';
    default:
      return assertNever(dir);
  }
}

function assertNever(x: never): never {
  throw new Error('Unexpected direction: ' + x);
}

medium

A. Missing case for 'left' in switch statement

B. assertNever function has wrong parameter type

C. Switch statement should not have a default case

D. Function move should return void instead of string

Solution

1. Step 1: Check all union cases in switch

   The type Direction has 'up', 'down', and 'left', but 'left' is missing in the switch.

2. Step 2: Understand impact of missing case with assertNever

   Missing 'left' causes default to call assertNever with a string, which is not never, causing a compile error.

3. Final Answer:

   Missing case for 'left' in switch statement -> Option A

4. Quick Check:

   All union cases must be handled [OK]

Hint: Check all union cases explicitly to avoid errors [OK]

Common Mistakes:

• Ignoring missing union cases
• Thinking default case covers all safely
• Wrong assertNever parameter type

5. Given this code, how can you modify it to ensure exhaustive checking for all Status values?

type Status = 'success' | 'error' | 'loading';

function handleStatus(status: Status) {
  switch (status) {
    case 'success':
      return 'Done';
    case 'error':
      return 'Failed';
    // Missing 'loading' case
    default:
      return assertNever(status);
  }
}

function assertNever(x: never): never {
  throw new Error('Unexpected status: ' + x);
}

hard

A. Change the type Status to include only 'success' and 'error'

B. Remove the default case and rely on TypeScript errors

C. Add a case for 'loading' in the switch statement

D. Change assertNever parameter type to any

Solution

1. Step 1: Identify missing union case in switch

   The 'loading' case is missing, so the switch is not exhaustive.

2. Step 2: Add missing case to ensure all Status values are handled

   Adding case 'loading': with appropriate return ensures exhaustive checking and no runtime errors.

3. Final Answer:

   Add a case for 'loading' in the switch statement -> Option C

4. Quick Check:

   All union members must have cases [OK]

Hint: Add missing union cases to switch for full coverage [OK]

Common Mistakes:

• Removing default case without adding missing cases
• Changing type to avoid handling all cases
• Using any instead of never in assertNever