TypeScript Quality Assurance & Improvement 5

Question 1

What is the purpose of `@ts-expect-error` compared to `@ts-ignore` in TypeScript?

Accepted Answer

`@ts-expect-error` causes a compile error if the next line does NOT have a type error, making suppression intentional

Answer

`@ts-expect-error` fails if the suppressed line becomes valid, so stale suppressions are caught — unlike `@ts-ignore` which silently ignores all errors.

Question 2

Which TypeScript configuration option helps detect dead code by flagging unused local variables?

Accepted Answer

noUnusedLocals

Answer

`noUnusedLocals: true` causes the compiler to emit errors for declared local variables that are never read, helping remove dead code.

Question 3

What problem does the `Record<K, V>` utility type solve compared to an index signature `{ [key: string]: V }`?

Accepted Answer

Record constrains keys to a specific union type, catching invalid key access at compile time

Answer

`Record<'a'|'b', number>` ensures only the keys `'a'` and `'b'` are valid, while a plain index signature allows any string, providing weaker guarantees.

Question 4

In TypeScript, what is a 'branded type' and why is it useful for QA?

Accepted Answer

A nominal type pattern using intersection with a unique symbol to prevent mixing structurally identical types

Answer

Branded types add a phantom property to distinguish types that are structurally identical, such as `UserId` vs `OrderId`, both being `string` underneath.

Question 5

What does `isolatedModules: true` in tsconfig.json enforce and why is it a good QA practice?

Accepted Answer

It ensures each file can be safely transpiled independently without cross-file type info, catching re-export-only patterns

Answer

`isolatedModules` errors on constructs like `export { MyType }` that require type information to distinguish from value exports, ensuring compatibility with transpilers like Babel or esbuild.

TypeScript Practice Test

TypeScript Practice Test

TypeScript Quality Assurance & Improvement 5