TypeScript Quality Assurance & Improvement 4

Question 1

What is a 'type predicate' in TypeScript and what QA problem does it solve?

Accepted Answer

A function return type of `param is Type` that narrows the type in calling code

Answer

Type predicates (`x is MyType`) let you write custom type guard functions whose return value narrows the type in the caller's scope.

Question 2

When should you prefer `interface` over `type` alias in TypeScript for improved maintainability?

Accepted Answer

When defining object shapes that may be extended via declaration merging

Answer

Interfaces support declaration merging, which allows separate files or libraries to augment the same interface — useful for extending third-party types.

Question 3

What is the purpose of the `satisfies` operator introduced in TypeScript 4.9?

Accepted Answer

It validates an expression matches a type while preserving the narrowest inferred type

Answer

`satisfies` checks that a value conforms to a type without widening it, so you get both type safety and access to the literal/inferred type.

Question 4

Which pattern helps prevent 'boolean trap' bugs in TypeScript function signatures?

Accepted Answer

Replacing boolean parameters with discriminated union or named option objects

Answer

Replacing boolean flags with option objects or union types makes call sites self-documenting and prevents passing flags in the wrong order.

Question 5

What does `exactOptionalPropertyTypes` enforce in TypeScript 4.4+?

Accepted Answer

Setting an optional property explicitly to `undefined` is a type error unless `undefined` is in the type

Answer

`exactOptionalPropertyTypes` distinguishes between a property being absent and being explicitly set to `undefined`, catching subtle bugs in object construction.

TypeScript Practice Test

TypeScript Practice Test

TypeScript Quality Assurance & Improvement 4