TypeScript Interview Questions and Type System Guide

TypeScript is the standard for modern web application engineering. Evaluating a developer's TypeScript skills requires assessing their mastery of the compile-time type system, type safety patterns, and generic utilities.

1. type Alias vs interface

• •interface: Best for object shapes and class contracts. Supports declaration merging (re-opening interfaces across files).
• •type: Alias for any type (primitives, unions, tuples, functions, objects). Does not support declaration merging.

interface User {
  id: string;
  name: string;
}

type UserWithRole = User & { role: "admin" | "member" };

2. Discriminated Unions (Tagged Unions)

Discriminated unions use a common literal property key to enable exhaustive type checking by the compiler.

type NetworkState =
  | { status: "loading" }
  | { status: "success"; data: string[] }
  | { status: "error"; error: Error };

function renderState(state: NetworkState) {
  switch (state.status) {
    case "loading":
      return "Loading...";
    case "success":
      return `Data items: ${state.data.join(", ")}`;
    case "error":
      return `Error: ${state.error.message}`;
  }
}

3. Built-in Utility Types

• •Partial<T>: Makes all properties optional.
• •Required<T>: Makes all properties required.
• •Readonly<T>: Makes all properties readonly.
• •Pick<T, K>: Selects a subset of properties K from T.
• •Omit<T, K>: Removes properties K from T.
• •Record<K, T>: Constructs an object type with keys K and values T.

4. Custom Type Guards (is Keyword)

Type guards provide runtime checks that narrow types within conditional blocks.

function isString(val: unknown): val is string {
  return typeof val === "string";
}

function processInput(input: unknown) {
  if (isString(input)) {
    console.log(input.toUpperCase()); // TypeScript knows input is string
  }
}

5. unknown vs any vs never

• •any: Disables all type checking. Avoid in production code.
• •unknown: Type-safe counterpart to any. Requires type checking before property access or invocation.
• •never: Represents values that can never occur (e.g. function that throws or infinite loop).