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How would you use tsconfig.json to enable shorter path aliases?
This tests TypeScript module resolution. A strong answer sets compilerOptions.baseUrl, maps paths like @/* to src/*, and warns bundlers need matching aliases. A red flag is omitting baseUrl or claiming paths alone changes runtime resolution.
What problem does esModuleInterop solve for CommonJS imports?
Tests CommonJS-to-ESM interop. A strong answer covers: default imports for CommonJS modules; runtime helpers checking __esModule and wrapping exports; and implicit allowSyntheticDefaultImports. Red flag: claiming it is purely type-level with no emit impact.
How do modern build tools handle TypeScript vs tsc?
Separating transpilation from type checking for speed. Great answer: fast native tooling strips TS per-file on demand via native ESM; browser loads only visited modules; tsc/ts-loader check/bundle the whole graph upfront.
How would you structure TypeScript code and exports for effective tree-shaking?
Tests ES module semantics and sideEffects for library authors. Great answers: emit ESM, set sideEffects:false, avoid stateful barrel files, and preserve import syntax. Red flag: saying ESM alone guarantees tree-shaking without mentioning side effects.
What isolatedModules enforces and why bundlers need it
IsolatedModules forbids features needing cross-file type info because Babel and esbuild compile each file alone; const enums, certain re-exports, and namespace tricks are flagged.
How do you configure a dual ESM/CommonJS package?
Use exports.require for .cjs and exports.import for .mjs, keep type unset or explicit, and compile separate artifacts.
Explain TypeScript Project References in a monorepo and their configuration
This tests monorepo build scaling. Good answers: split into composite projects with references arrays, use tsc --build for incremental compilation, and consume .d.ts outputs for boundaries. Red flag: calling it path mapping or omitting --build.
How do you add TypeScript types for a library without bundled types?
This tests the DefinitelyTyped @types workflow. Install @types/package as a dev dependency via npm or yarn, skipping it when the library already bundles .d.ts files. A red flag is suggesting manual declarations or extra tooling before checking DefinitelyTyped.
How do you write a minimal dom-utils.d.ts for a JS function?
Tests bridging untyped JavaScript into TypeScript via ambient module declarations. A strong answer declares a module matching the file path and exports the function with a typed signature. Red flag: suggesting rename to .ts or using any.
How do you globally add a property to a third-party ButtonProps interface?
Tests TypeScript declaration merging and module augmentation. A strong answer uses a .d.ts file that declares the same module and interface name to merge the missing property globally. Red flag: suggesting edits inside node_modules or local wrapper types.
How do you type a custom property on global window in TypeScript?
Tests declaration merging and global scope augmentation in TypeScript. Strong answer: declare global in a .ts file, a .d.ts with interface Window, or scoped declare const window. Red flag: any or ts-ignore instead of interface merging.
How do you derive a PATCH payload type from UserProfile?
Tests knowledge of TypeScript utility types for APIs. A great answer uses Partial<UserProfile> to make all fields optional in one line and connects it to PATCH subset semantics. Red flag: manually rewriting properties as optional or confusing PUT with PATCH.
What is the exact output and event sequence of this code?
Tests event loop priority between sync, microtasks, and macrotasks. A strong answer gives Start, End, Promise, Timeout; explains Promise.then is a microtask and setTimeout is a macrotask, microtasks draining before the next macrotask.
Explain the difference between microtask and macrotask queues
Macrotasks like setTimeout run one per tick; microtasks like Promises drain fully after each task before rendering.
Optimize a canvas with thousands of moving objects
Tests GPU-bound vs CPU-bound bottleneck diagnosis. Strong answers: batch draw calls to cut JS-to-GPU overhead, and render to a smaller back buffer to reduce fill-rate cost.
Type a compose function using generics and overloads
It tests your ability to type higher-order pipelines where each function's output feeds the next input. Use function overloads with chained generics for each arity so TypeScript infers the composed parameter and return types.
Create a CustomEvent with typed detail and a type-safe listener
Tests TypeScript generics with DOM events. Strong answer: generic CustomEvent wrapper, typed detail, listener typed via generic param. Red flag: using any for detail or casting event.target instead of parameterizing the event type.
How does export = differ from export default in TypeScript declarations?
It tests CommonJS to ES module interop in TypeScript. export = maps to the entire module.exports object and must be imported with import = require() or ES interop, while export default creates a named binding. A red flag is claiming they are interchangeable.
How do you write a declaration file for a JavaScript class?
Declare a class in a .d.ts, type the constructor, add instance members, and attach statics on the class.
Type a UMD library for script tags and CommonJS/ESM
Tests TypeScript UMD declarations for dual global and module usage. Use export as namespace MyLib for the global, export = MyLib for require, and declare the API inside a namespace. Red flag: default exports or declare global instead of export as namespace.