Tooling, Testing & Linting

Question 1

What is ESLint?

Answer 1

ESLint is a static code analysis tool for identifying problematic patterns in JavaScript/TypeScript code.

Question 2

What is Prettier?

Answer 2

Prettier is an opinionated code formatter that enforces consistent style by parsing and re-printing code.

Question 3

Why use ESLint with TypeScript?

Answer 3

ESLint helps catch bugs and enforce best practices by analyzing TypeScript syntax and types using plugins like @typescript-eslint.

Question 4

What is ts-jest?

Answer 4

ts-jest is a Jest transformer that allows TypeScript code to be tested directly using Jest without a separate compilation step.

Question 5

What are typing mocks in Jest?

Answer 5

Typing mocks refers to giving proper TypeScript types to mocked functions or modules, avoiding ‘any’ and ensuring type safety.

Question 6

What are source maps in TypeScript?

Answer 6

Source maps map compiled JavaScript back to the original TypeScript source for easier debugging.

Question 7

What is the advantage of using ESLint and Prettier together?

Answer 7

ESLint catches code quality issues, while Prettier ensures consistent formatting—together they improve code readability and maintainability.

Question 8

What is a disadvantage of integrating ESLint, Prettier, and TypeScript?

Answer 8

It requires additional setup, and configuration conflicts may arise if not carefully managed.

Question 9

What is a best practice when using ESLint with TypeScript?

Answer 9

Use the @typescript-eslint plugin and extend recommended configs to catch both JS and TS issues.

Question 10

What is a use case for ts-jest?

Answer 10

Unit testing TypeScript code without compiling manually, enabling fast feedback and tight integration with Jest features.

Question 11

What is a performance implication of ts-jest?

Answer 11

While ts-jest simplifies testing, it can slow down test execution due to on-the-fly transpilation.

Question 12

What is the architectural impact of using ESLint and Prettier?

Answer 12

Enforces consistent coding standards and prevents code smell across large codebases or teams.

Question 13

What is a real-world tradeoff with strict lint rules?

Answer 13

Overly strict rules may hinder developer productivity or require excessive boilerplate suppression.

Question 14

What’s a common interview question around ts-jest?

Answer 14

How do you configure Jest to work with TypeScript using ts-jest?

Question 15

What is a gotcha when mocking TypeScript modules?

Answer 15

Mocked modules may lose type safety unless properly typed with jest.Mocked or manual typings.

Question 16

How do source maps help debugging?

Answer 16

They allow debuggers to show TypeScript code instead of transpiled JavaScript when setting breakpoints or reading stack traces.

Question 17

What is a common ESLint rule for TypeScript?

Answer 17

@typescript-eslint/no-explicit-any warns against using the ‘any’ type, encouraging stronger typing.

Question 18

What is the advantage of using Prettier over manual formatting?

Answer 18

It automates formatting decisions, removing debates and making code reviews more focused on logic.

Question 19

What is the benefit of typing mocks in tests?

Answer 19

Maintains type safety in unit tests, reducing runtime bugs and improving test readability.

Question 20

What is a best practice when mocking TypeScript classes?

Answer 20

Use interfaces or utility types like jest.Mocked<T> to retain type integrity.</T>

Question 21

What’s a potential issue with source maps in production?

Answer 21

They may expose original source code if not excluded from public builds, posing a security risk.

Question 22

What is a good way to integrate ESLint and Prettier?

Answer 22

Use eslint-config-prettier to turn off ESLint rules that conflict with Prettier, and run both via scripts or pre-commit hooks.

Question 23

What’s the impact of tooling on team collaboration?

Answer 23

Consistent linting and formatting prevent merge conflicts and ensure uniform code across contributors.

Question 24

What’s an example of a Jest mock with types?

Answer 24

jest.mock(‘./api’); const mockedApi = jest.mocked(api);

Question 25

What does the 'sourceMap' compiler option do?

Answer 25

It generates .map files that allow mapping transpiled code to the original TypeScript source during debugging.

Question 26

What is a best practice for maintaining source maps?

Answer 26

Only enable source maps for development and debugging builds, not in production.

Question 27

What is a tradeoff of using strict ts-jest settings?

Answer 27

Improves type safety but may require more verbose setup and configuration in test files.

Question 28

What’s a disadvantage of using Jest with large TS projects?

Answer 28

Startup time can increase, and you may need to optimize with isolatedModules or skipLibCheck.

Question 29

What’s the architectural implication of using strict linting rules?

Answer 29

They enforce clean code principles and reduce technical debt over time.

Question 30

What’s a benefit of running ESLint and Prettier in CI/CD?

Answer 30

It guarantees code quality and formatting before code is merged or deployed.

Question 31

What is a common mistake with ts-jest config?

Answer 31

Forgetting to set 'preset: ts-jest' or misconfiguring tsconfig for test environments.

Question 32

What is a potential gotcha when combining ESLint and Prettier?

Answer 32

Conflicting rules can lead to redundant or contradictory formatting unless properly integrated.

Question 33

What does 'jest.mocked()' do?

Answer 33

It provides type-safe access to mocked modules/functions when using TypeScript with Jest.

Question 34

What is the role of @typescript-eslint/parser?

Answer 34

It allows ESLint to parse TypeScript code so that rules can be applied correctly.

Question 35

What is the role of @typescript-eslint/eslint-plugin?

Answer 35

It provides TypeScript-specific linting rules for ESLint.

Question 36

How can you debug Jest tests in VSCode?

Answer 36

Use a launch config with 'jest' and enable 'sourceMaps' in tsconfig to debug TypeScript test files.

Question 37

What is a common interview question about ESLint in TypeScript?

Answer 37

How do you set up a TypeScript project with ESLint and Prettier to ensure code quality?

Question 38

What’s a good way to enforce formatting on commit?

Answer 38

Use tools like Husky + lint-staged to run ESLint and Prettier automatically before commits.

Question 39

What is the benefit of integrating type-aware linting?

Answer 39

It provides smarter rules that understand TypeScript types, catching errors plain ESLint can’t.

Question 40

Why might tests fail when using source maps incorrectly?

Answer 40

Stack traces may not align with actual code, making it hard to trace bugs or test issues.

Question 41

What is a drawback of using mocks in every test?

Answer 41

It may hide integration issues and overfit tests to implementation details rather than behavior.