JS.08.03 Mid-level

Newer built-ins (ES2022–ES2026) + Proxy and Reflect

Comes up in: JavaScript, Recruiter screen

This concept is being written. It gets the full five steps — explanation, predict, interview angle, practice with tests, and review cards. Here is exactly what it will cover, so you can study it elsewhere in the meantime.

What it covers

at, findLast, Object.hasOwn, Object.groupBy/Map.groupBy, Array.fromAsync, Promise.withResolvers, Error.cause, structuredClone, change-array-by-copy, Set methods, Iterator helpers, RegExp v flag
ES2026 features from facts
how to check support (Baseline, Node version)
traps (get, set, has, deleteProperty, apply)
invariants
Reflect for default behaviour
uses: validation, reactivity (Vue 3), observable objects
performance cost

Words you will know

BaselinepolyfilltranspileproxytrapReflectinvariant

What you will practise

rewrite old code using new APIs
observable object that logs changes

Interview questions this answers

What are Proxies used for? How does Vue use them?
Name some JavaScript features added in the last three years.