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How do you select by ID and class, and what is returned?
This tests basic DOM API knowledge and awareness of live collections. Use document.getElementById for IDs and document.getElementsByClassName for classes, which returns a live HTMLCollection. A red flag is calling the result a static Array or NodeList.
What happens when you declare var globally versus let or const?
Knowledge that var creates a Window property while let and const do not. A good answer notes var adds window.x, let/const do not pollute the global object, yet both are globally scoped.
What is the fundamental difference between window and document?
Window is the browser tab and global scope; document is the object-oriented DOM representation of the loaded page.
What is a type predicate? Write a custom type guard for User.
Tests if you know a type predicate is a compile-time narrowing hint requiring runtime validation. Great answers define parameter is Type, validate every User property, and demonstrate narrowing. Red flag: omitting runtime checks or using as casting.
Create a generic getProperty using generics and keyof
Whether you can constrain a generic key with keyof and return the exact property type. Use T for the object and K extends keyof T for the key, returning T[K].
Key differences: type alias vs interface for object shapes
Tests your grasp of TypeScript extensibility and declaration merging. Contrast interfaces' extends and open merging against type aliases' unions, intersections, and primitives; prefer interfaces for public APIs and types for unions or mapped types.
How do you type and guard a string-or-string-array argument?
Tests union typing and runtime narrowing. Annotate as string | string[], then branch with Array.isArray(). Strong answers note typeof returns "object" for arrays and that assertions bypass safety.
Explain the difference between any and unknown, and demonstrate type-safe narrowing
This tests your grasp of TypeScript top types: any disables checking while unknown forces narrowing. A strong answer defines both, accepts unknown, and uses typeof or a type guard before operating. Red flag: saying they are equivalent or relying on as casts.
What is noImplicitAny and why is it best practice?
State that noImplicitAny errors when inference fails, forcing explicit types instead of plain any.
Explain what TypeScript's type inference is and show an inferred variable declaration
This tests whether you understand TypeScript deduces types without annotations. A strong answer defines inference as deriving types from values and gives an example like let x = 3 inferring number. A red flag is claiming explicit types are required.
Declare a string name and an array of lucky numbers in TypeScript
Declare the name with a lowercase string type and the lucky numbers as number[] or Array<number>.
Ambient Module Declarations: Compile-Time Trust Falls
Ambient module declarations are compile-time trust falls: you tell TypeScript the shape of code it cannot see. Use them for untyped npm packages, CSS imports, or CDN scripts.
ES Modules in TypeScript: The Compile-Time Contract
TypeScript ES modules are a compile-time target, not a runtime guarantee. Configure this when targeting modern browsers or Node with native ESM. The footgun is omitting .js extensions in imports, which it requires in ESM output even for .ts source files.
Worker postMessage: The Structured Clone Pipe
Worker postMessage moves data between threads via structured cloning. Use it to offload heavy work without blocking the UI. The message argument is mandatory, so omitting it is an error and you must pass null when there is no data.
Runtime Response Validation with Schema Libraries
TypeScript types evaporate at runtime, so a fetch response typed as User[] can be null or malformed. Schema libraries like Zod give static types and runtime guards from one contract. The footgun is using as instead of parse, silently reintroducing crashes.
Kent C. Dodds Fixes Accidental Monorepo with Workspaces
Kent C. Dodds consolidated four deployable apps into a proper npm workspace, deleting three nested lockfiles and adding minimal Nx caching. The migration exposed hardcoded paths and invalid package aliases that broke production once Node enforced package…
Cloudflare Sandboxes Cut Container Heartbeat Plumbing
Kent C. Dodds replaced Cloudflare Containers with Sandboxes, deleting heartbeat and shutdown logic for his FFmpeg pipeline. PR #729 uses one-shot exec() inside the existing queue worker, cutting deploy surface and eliminating long-lived service orchestration.
Kent C. Dodds Adds SQLite FTS5 to Vector Search
Kent C. Dodds added SQLite FTS5 to Vectorize embeddings after semantic search missed exact matches like "React Testing Library." The hybrid pipeline uses Reciprocal Rank Fusion. If your search fails on API names, add BM25 backup, not bigger embedding models.
Vercel Connect replaces env tokens with runtime OIDC
Vercel Connect Public Beta replaces static env tokens with short-lived, task-scoped credentials exchanged at runtime via OIDC. Agents request least-privilege access per job instead of holding long-lived shared secrets, eliminating manual rotation when…
Vercel ships AI Gateway, Workflow SDK, and Sandbox for agents
Vercel Agent Stack unifies hundreds of models, durable workflows, and isolated microVMs for production agents. One endpoint routes across providers, checkpoints resume failed runs, and sandboxes isolate unreviewed code.