Question 1

Why use TypeScript for test automation?

Accepted Answer

TypeScript adds static types to JavaScript, catching typos, wrong arguments and missing awaits at compile time instead of during a long test run. It also gives strong autocompletion for page objects, fixtures and API payloads.

Question 2

What are the basic types in TypeScript?

Accepted Answer

Primitives are string, number, boolean, bigint, symbol, null and undefined, plus arrays, tuples, objects, and special types any, unknown, void and never. Type inference often lets you skip explicit annotations.

Question 3

What is the difference between type and interface?

Accepted Answer

Both describe object shapes. Interfaces can be extended and merged through declaration merging, while type aliases can also represent unions, tuples, primitives and mapped types.

Question 4

What is the difference between any and unknown?

Accepted Answer

any disables type checking entirely, while unknown accepts any value but forces you to narrow it before use. Prefer unknown for API responses and caught errors.

Question 5

Explain union and intersection types.

Accepted Answer

A union (A | B) means a value can be one of several types, while an intersection (A & B) combines all properties of multiple types into one. Unions model options, intersections compose shapes.

Question 6

What are enums, and should you use them?

Accepted Answer

Enums define a named set of constants and emit a runtime object. Many teams now prefer string literal unions or as const objects because they are erasable, tree-shakable and work with Node's type stripping.

Question 7

What do optional (?), readonly and non-null assertion (!) mean?

Accepted Answer

? marks a property or parameter as possibly undefined, readonly prevents reassignment after creation, and ! tells the compiler a value is not null or undefined. Use ! sparingly because it bypasses safety.

Question 8

What are generics and how do you use them in automation?

Accepted Answer

Generics let functions, classes and types work with many types while preserving type information. In automation they type API clients, test data builders and helper utilities.

Question 9

Which utility types do you use most?

Accepted Answer

Partial, Required, Pick, Omit, Record, Readonly, ReturnType and Awaited are the most common. They derive new types from existing ones instead of duplicating definitions.

Question 10

What are type guards and narrowing?

Accepted Answer

Narrowing refines a broad type to a specific one using checks like typeof, instanceof, in, or equality on a discriminant. Custom type guards are functions returning value is Type.

Question 11

What does strict mode enable in tsconfig?

Accepted Answer

strict: true turns on a family of checks including strictNullChecks, noImplicitAny, strictFunctionTypes and useUnknownInCatchVariables. It should be on for any new automation project.

Question 12

Walk through a typical tsconfig.json for a Playwright project.

Accepted Answer

Key options are target, module, moduleResolution, strict, baseUrl/paths for aliases, and types. Playwright transpiles tests itself, so tsconfig mainly drives editor checks and tsc --noEmit in CI.

Question 13

How do you type a page object in Playwright?

Accepted Answer

Import Page and Locator types, declare locators as readonly properties, and initialise them in the constructor. Methods return Promise<void> or typed values so tests get autocompletion.

Question 14

How do you create typed custom fixtures in Playwright?

Accepted Answer

Call test.extend with a generic describing your fixture types, then provide each fixture with an async function that calls use(). Tests then receive fully typed fixtures through destructuring.

Question 15

Explain keyof, typeof and mapped types.

Accepted Answer

keyof produces a union of an object type's keys, typeof extracts a type from a value, and mapped types iterate over keys to build new types. Together they derive types from runtime config.

Question 16

What does the satisfies operator do?

Accepted Answer

satisfies checks that a value conforms to a type without widening the value's inferred type. You get validation plus precise literal types, which is ideal for config objects.

Question 17

Coding: write a typed groupBy function for test results.

Accepted Answer

Accept an array of T and a key selector returning a string key, then reduce into a Record<K, T[]>. Generics keep the item type intact in the output.

Question 18

Coding: implement a typed waitUntil polling helper.

Accepted Answer

Poll an async condition at an interval until it returns a truthy value or a timeout elapses, then return the value or throw. Generics let the caller get the resolved value typed.

Question 19

Coding: validate an unknown API response with a type guard.

Accepted Answer

Accept unknown, check it is a non-null object, then verify each field's runtime type and return value is T. This bridges compile-time types and runtime data in API tests.

Question 20

How do you add types for a custom Cypress command?

Accepted Answer

Augment the global Cypress namespace's Chainable interface in a .d.ts or support file using declare global. This uses interface declaration merging so cy.login() is type-checked.

Question 21

How is TypeScript compiled and run, and what happens to types at runtime?

Accepted Answer

The TypeScript compiler (tsc) type-checks the code and emits plain JavaScript; all type annotations are erased, so they don't exist at runtime. Test runners like Playwright, Vitest or ts-jest compile TypeScript for you on the fly.

Question 22

What is type inference in TypeScript?

Accepted Answer

TypeScript infers types from initial values, return statements and context, so you don't have to annotate everything. Annotate function parameters and public APIs, and let inference handle local variables.

Question 23

How do you type arrays and tuples in TypeScript?

Accepted Answer

Arrays use string[] or Array<string> for any length of one type. Tuples have a fixed length with a known type per position, like [string, number], and can have labels and optional elements.

Question 24

How do you type functions, including optional, default and rest parameters?

Accepted Answer

Annotate each parameter and optionally the return type. Optional parameters use ?, default parameters take a value and are optional automatically, and rest parameters collect extra arguments into a typed array.

Question 25

How do classes and access modifiers work in TypeScript?

Accepted Answer

TypeScript classes support public (default), private and protected modifiers plus readonly, all checked at compile time. Parameter properties let you declare and assign a field directly in the constructor signature.

Question 26

How do you extend interfaces and implement them in classes?

Accepted Answer

An interface can extend one or more other interfaces to combine their members, and a class uses implements to promise it satisfies an interface's shape. implements only checks the class; it doesn't add any code.

Question 27

What are literal types and how do they help in test code?

Accepted Answer

Literal types restrict a value to exact strings, numbers or booleans, like 'chromium' | 'firefox' | 'webkit'. Combined in unions, they catch typos at compile time and power autocomplete.

Question 28

What is the difference between void and never?

Accepted Answer

void means a function returns no meaningful value; it finishes normally. never means a function never returns at all, because it always throws or loops forever, and it's also the type of impossible values after exhaustive narrowing.

Question 29

What are type assertions (as) and when are they risky?

Accepted Answer

A type assertion tells the compiler to treat a value as a specific type, like response as User. It performs no runtime check, so a wrong assertion hides bugs; prefer type guards or schema validation for external data.

Question 30

How do modules and import type work in TypeScript?

Accepted Answer

Each file with an import or export is a module; you share code with named or default exports. import type imports only types, which are erased at compile time and never load the module at runtime.

Question 31

Coding: write a typed TypeScript function that counts vowels and reverses each word.

Accepted Answer

Lowercase the input and count characters found in a Set of vowels. To reverse each word, split on spaces, reverse each word's characters and join back, keeping the word order.

Question 32

How do abstract classes work in TypeScript, and when do you use one over an interface?

Accepted Answer

An abstract class can't be instantiated and may contain both implemented methods and abstract ones that subclasses must implement. Use it when page objects share real code like waits and navigation; use an interface when you only need a shape.

Question 33

What are function overloads in TypeScript?

Accepted Answer

Overloads declare multiple call signatures for one function, followed by a single implementation that handles all cases. Callers see precise return types depending on the arguments they pass.

Question 34

How do generic constraints and default type parameters work?

Accepted Answer

A constraint like limits which types can be passed, so you can safely use their properties inside the function. Default type parameters, like , apply when the caller doesn't specify or infer one.

Question 35

What are discriminated unions and how do you use them?

Accepted Answer

A discriminated union is a union of object types that share a literal property, such as status or kind. Checking that property narrows the value to the matching member, giving safe access to its specific fields.

Question 36

What are index signatures, Record and indexed access types?

Accepted Answer

An index signature like { [key: string]: number } describes objects with arbitrary keys of one type, and Record<K, V> is the utility version. Indexed access types such as User['address'] pull out the type of a property.

Question 37

What does as const do?

Accepted Answer

as const makes a literal deeply readonly and keeps the narrowest literal types instead of widening to string or number. It's commonly used to derive union types from a constant array or object.

Question 38

How do you type async functions and Promises in TypeScript?

Accepted Answer

An async function always returns Promise, where T is the type of the returned value. Awaited unwraps nested promise types, and Promise.all keeps each element's type in the resulting tuple.

Question 39

Why is the catch variable unknown, and how do you handle errors safely?

Accepted Answer

Anything can be thrown in JavaScript, not only Error objects, so with useUnknownInCatchVariables (part of strict) the catch variable is unknown. Narrow it with instanceof Error before reading message or stack.

Question 40

How do you set up Jest or Vitest with TypeScript?

Accepted Answer

For Jest, use ts-jest (type-checks while transforming) or babel/swc (faster, no type check) and add @types/jest. Vitest supports TypeScript out of the box via Vite and ships its own types.

Question 41

What are .d.ts declaration files and @types packages?

Accepted Answer

A .d.ts file contains only type declarations describing JavaScript code, so TypeScript can type-check its usage. @types/* packages from DefinitelyTyped supply declarations for libraries that don't ship their own.

Question 42

Coding: write a typed anagram checker and group anagrams in TypeScript.

Accepted Answer

Normalize each word and use its sorted characters as a key; two words are anagrams if their keys match. Grouping uses a Map<string, string[]> keyed by that sorted form.

Question 43

Coding: find the missing number and the second largest value in a typed number array.

Accepted Answer

For the missing number, subtract the array's sum from n * (n + 1) / 2. For the second largest, track the top two distinct values in a single pass and return undefined if there isn't a second one.

Question 44

Coding: write a generic chunk function that splits an array into batches.

Accepted Answer

Loop with a step equal to the batch size and slice the array at each step. Making it generic (<T>) keeps the element type, so chunk(users, 10) returns User[][].

Question 45

What are conditional types and the infer keyword?

Accepted Answer

A conditional type picks a type based on a check: T extends U ? X : Y. Inside the extends clause, infer declares a type variable that TypeScript fills in, which is how utilities like ReturnType and Awaited are built.

Question 46

What are template literal types?

Accepted Answer

Template literal types build string types from other types using backtick syntax, like `data-test-${string}`. They can combine unions into every permutation and are useful for typing event names, routes or test IDs.

Question 47

How do decorators work in TypeScript 5, and how could you use one in tests?

Accepted Answer

TypeScript 5.0 supports standard ECMAScript decorators, functions that receive a class member and a context object and can wrap or replace it. A method decorator can, for example, log each page-object step or wrap it in a Playwright test.step.

Question 48

What does structural typing mean in TypeScript, and what are excess property checks?

Accepted Answer

TypeScript compares types by their shape, not by their declared names, so any object with the required properties is compatible. Excess property checks are an exception: object literals assigned directly to a type are rejected if they have unknown properties.

Question 49

Which TypeScript 5.x features are useful in automation projects?

Accepted Answer

TypeScript 5.0 added standard decorators and const type parameters, 5.2 added the using keyword for explicit resource management, 5.4 added NoInfer, and 5.5 infers type predicates from filter callbacks automatically. Combined with satisfies from 4.9, they make test helpers safer.

Question 50

Coding: implement a generic typed sort without Array.prototype.sort.

Accepted Answer

Write a generic insertion sort that accepts a comparator (a, b) => number, so it works for numbers, strings or objects. Copy the input to keep it immutable; insertion sort is O(n^2) worst case and stable.

Interview Questions

01Why use TypeScript for test automation?BeginnerType Basics

02What are the basic types in TypeScript?BeginnerType Basics

03What is the difference between type and interface?BeginnerType Basics

04What is the difference between any and unknown?BeginnerType Basics

05Explain union and intersection types.BeginnerType Basics

06What are enums, and should you use them?BeginnerType Basics

07What do optional (?), readonly and non-null assertion (!) mean?BeginnerType Basics

08What are generics and how do you use them in automation?IntermediateAdvanced Types

09Which utility types do you use most?IntermediateAdvanced Types

10What are type guards and narrowing?IntermediateAdvanced Types

11What does strict mode enable in tsconfig?IntermediateConfiguration

12Walk through a typical tsconfig.json for a Playwright project.IntermediateConfiguration

13How do you type a page object in Playwright?IntermediateFramework Typing

14How do you create typed custom fixtures in Playwright?IntermediateFramework Typing

15Explain keyof, typeof and mapped types.AdvancedAdvanced Types