Never type and unreachable code in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the never type in TypeScript?

The never type represents values that never occur. It is used for functions that never return or variables that cannot have any value.

beginner

When does TypeScript consider code to be unreachable?

Code is unreachable if it can never be executed, for example, code after a function that always throws an error or an infinite loop.

beginner

What happens if a function has a return type of never?

It means the function never finishes normally. It either throws an error or runs forever, so it never returns a value.

intermediate

Why is the never type useful for exhaustive checks?

It helps catch missing cases in code like switch statements by making sure all possibilities are handled, otherwise TypeScript shows an error.

beginner

Give an example of unreachable code in TypeScript.

Example: <pre>function example() { return; console.log('unreachable'); }</pre> The <code>console.log</code> line is unreachable because the function returns before it.

What does the never type represent in TypeScript?

A A value that never occurs

B A value that can be any type

C A value that is optional

D A value that is null

The never type means a value that never happens, like a function that never returns.

Which of these is an example of unreachable code?

A Code inside an if statement

B Code inside a function

C Code inside a loop

D Code after a return statement

Code after a return statement is unreachable because the function stops running at return.

What will a function with return type never do?

A Return a string

B Never return normally

C Return a number

D Return null

A never function never returns normally; it throws or loops forever.

How does TypeScript help with unreachable code?

A It shows an error or warning for unreachable code

B It converts unreachable code to comments

C It deletes unreachable code automatically

D It ignores unreachable code

TypeScript warns you when code can never be reached, helping you find mistakes.

Why use never in exhaustive checks?

A To allow missing cases

B To ignore errors

C To catch missing cases at compile time

D To make code run faster

Using never forces you to handle all cases, so TypeScript catches missing ones.

Explain what the never type means and give an example of a function that returns never.

Think about functions that never finish normally.

Describe what unreachable code is and why TypeScript warns about it.

Consider what happens after a return statement.