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.