TypeScript Quality Assurance & Improvement 3

Question 1

What is a discriminated union in TypeScript and how does it help QA?

Accepted Answer

A union where all members share a common literal property used for type narrowing

Answer

Discriminated unions use a shared literal 'tag' field so TypeScript can narrow the type in switch/if blocks, ensuring all cases are handled.

Question 2

When using Jest with TypeScript, what is the role of `ts-jest` or `babel-jest` with TypeScript preset?

Accepted Answer

They transform TypeScript files so Jest can execute them without a separate compile step

Answer

Transformers like `ts-jest` transpile TypeScript source on-the-fly so Jest can run tests without a prior build step.

Question 3

What does the `unknown` type offer over `any` for improving code quality?

Accepted Answer

unknown forces you to narrow the type before performing operations on it

Answer

Unlike `any`, `unknown` requires a type guard or assertion before you can access properties or call methods, making unsafe code explicit.

Question 4

Which TypeScript utility type would you use to make all properties of an interface optional for a partial update function?

Accepted Answer

Partial<T>

Answer

`Partial<T>` constructs a type with all properties of T set to optional, commonly used for PATCH-style update functions.

Question 5

What is the primary QA benefit of enabling `noUncheckedIndexedAccess` in tsconfig.json?

Accepted Answer

It adds `undefined` to the type of indexed array/object access results, forcing null checks

Answer

`noUncheckedIndexedAccess` makes TypeScript include `| undefined` in array and index signature types, preventing silent undefined access bugs.

TypeScript Practice Test