TypeScript Interview Questions

Why use TypeScript for an automation framework instead of plain JavaScript?

When would you use an interface versus a type alias in a test framework codebase?

Interfaces are best for object shapes that might be extended or merged (like page object or config contracts), while type aliases are more flexible for unions, tuples, and mapped types.

How do you use generics to build a reusable page object or API client class?

Generics let a single class or function work with multiple types safely, so an API client can return a strongly-typed response for whatever endpoint's shape you pass in.

Give examples of utility types you've used in an automation framework, like Partial, Pick, or Omit.

Partial<T> makes all fields optional for building partial test fixtures, Pick<T, K> extracts a subset of fields for a lightweight DTO, and Omit<T, K> removes fields you don't want exposed, like excluding a password from a response type.

What does strictNullChecks do, and why is it valuable in test automation code?

strictNullChecks makes null and undefined distinct types that must be explicitly handled, preventing runtime 'cannot read property of undefined' errors from locator/element lookups that silently return nothing.

When would you choose a union of string literals over an enum for something like environment names or test tags?

Union literal types are lighter-weight, don't emit any runtime JS, and work more naturally with plain string values from config files, whereas enums generate actual runtime objects and are better when you need reverse-mapping or namespacing.

How does declaration merging let you extend a test runner's fixture types, like adding custom Playwright fixtures?

Declaration merging lets you add new properties to an existing interface (like Playwright's TestArgs) by re-declaring it elsewhere with the same name, so TypeScript combines both declarations into one type.

How do you type an API client so response validation failures are caught at compile time, not just at runtime?

Define explicit interfaces for request/response payloads and pair them with a runtime schema validator like Zod, then infer the TypeScript type directly from that schema so compile-time types and runtime validation never drift apart.

What is type inference, and why does it matter in TypeScript?

type inference is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply type inference in a production TypeScript project?

Apply type inference through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is interfaces versus type aliases, and why does it matter in TypeScript?

interfaces versus type aliases is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply interfaces versus type aliases in a production TypeScript project?

Apply interfaces versus type aliases through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is union types, and why does it matter in TypeScript?

union types is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply union types in a production TypeScript project?

Apply union types through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is intersection types, and why does it matter in TypeScript?

intersection types is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply intersection types in a production TypeScript project?

Apply intersection types through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is type guards, and why does it matter in TypeScript?

type guards is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply type guards in a production TypeScript project?

Apply type guards through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is generics, and why does it matter in TypeScript?

generics is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply generics in a production TypeScript project?

Apply generics through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is generic constraints, and why does it matter in TypeScript?

generic constraints is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply generic constraints in a production TypeScript project?

Apply generic constraints through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is utility types, and why does it matter in TypeScript?

utility types is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply utility types in a production TypeScript project?

Apply utility types through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is keyof operator, and why does it matter in TypeScript?

keyof operator is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply keyof operator in a production TypeScript project?

Apply keyof operator through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is typeof type queries, and why does it matter in TypeScript?

typeof type queries is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply typeof type queries in a production TypeScript project?

Apply typeof type queries through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is indexed access types, and why does it matter in TypeScript?

indexed access types is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply indexed access types in a production TypeScript project?

Apply indexed access types through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is mapped types, and why does it matter in TypeScript?

mapped types is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply mapped types in a production TypeScript project?

Apply mapped types through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is conditional types, and why does it matter in TypeScript?

conditional types is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply conditional types in a production TypeScript project?

Apply conditional types through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is discriminated unions, and why does it matter in TypeScript?

discriminated unions is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply discriminated unions in a production TypeScript project?

Apply discriminated unions through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is unknown versus any, and why does it matter in TypeScript?

unknown versus any is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply unknown versus any in a production TypeScript project?

Apply unknown versus any through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is never type, and why does it matter in TypeScript?

never type is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply never type in a production TypeScript project?

Apply never type through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is function overloads, and why does it matter in TypeScript?

function overloads is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply function overloads in a production TypeScript project?

Apply function overloads through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is readonly types, and why does it matter in TypeScript?

readonly types is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply readonly types in a production TypeScript project?

Apply readonly types through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is enums versus literal unions, and why does it matter in TypeScript?

enums versus literal unions is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply enums versus literal unions in a production TypeScript project?

Apply enums versus literal unions through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is declaration merging, and why does it matter in TypeScript?

declaration merging is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply declaration merging in a production TypeScript project?

Apply declaration merging through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is strict mode, and why does it matter in TypeScript?

strict mode is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply strict mode in a production TypeScript project?

Apply strict mode through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is module augmentation, and why does it matter in TypeScript?

module augmentation is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply module augmentation in a production TypeScript project?

Apply module augmentation through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.

What is satisfies operator, and why does it matter in TypeScript?

satisfies operator is a core TypeScript concept used to make testing decisions more accurate, repeatable, and aligned with product risk.

How would you apply satisfies operator in a production TypeScript project?

Apply satisfies operator through an explicit, repeatable design: define the expected behavior, automate the highest-risk path, capture useful diagnostics, and verify the result against a stable oracle.