Frameworks change; the language underneath them is what a senior engineer is expected to know cold. Interviewers reach for JavaScript questions because they are framework-neutral and because the answers reveal mental models: whether you can predict the order of console.logs around promises and timers, explain why a closure leaks, say what a module's live binding is, or format a currency without a library. This section is the language at the depth a senior frontend or fullstack candidate needs, pinned to the September 2026 snapshot: ES2026 was ratified in June 2026, Node 24 is the active LTS and Node 26 becomes LTS in October.

Know

• The runtime model is small: one call stack, a heap, a task queue, a microtask queue, and a rendering step between tasks in browsers. Everything about async, timers, await ordering and responsiveness follows from it (event loop).
• Scope is lexical and closures capture variables, not values. this is decided at the call site except for arrows, and prototypes are the object model classes sugar over (closures, scope and this).
• Modules are static, live and singletons. ESM versus CommonJS still causes real bugs in 2026, and bundlers build on ESM semantics (modules and bundling).
• Collections and iteration protocols are where modern JavaScript grew most: Map/Set methods, iterator helpers, generators, streams (collections, iterators and generators).
• Memory and performance questions are about reachability, leaks from listeners and closures, and measuring before optimising (memory and performance).
• The language keeps moving: ES2024 to ES2026 features and the ES2027 pipeline, and the Baseline vocabulary for deciding what to use unpolyfilled (modern JavaScript).
• Dates, numbers and Intl are the practical interview trap: time zones, floating point, and formatting that must agree between server and client (dates, numbers and Intl).

How it works

A warm-up that tests four of the pages at once. Predict the output before running it:

console.log('1 sync')

setTimeout(() => console.log('6 timeout'), 0)

Promise.resolve().then(() => console.log('3 microtask'))

queueMicrotask(() => console.log('4 microtask (queued second)'))

;(async () => {
  console.log('2 async body runs synchronously until the first await')
  await null
  console.log('5 after await: a microtask queued after the two above')
})()

// 1, 2, 3, 4, 5, 6: synchronous code first, then the whole microtask queue in order, then the timer task.

const counter = {
  count: 0,
  inc() { this.count++ },
  incLater() { setTimeout(() => this.count++, 0) }, // arrow: lexical this -> counter
  incBroken() { setTimeout(function () { this.count++ }, 0) }, // this is undefined in strict mode (modules)
}
const { inc } = counter
inc() // TypeError in strict mode: `this` is undefined, not counter

Exercise

Be able to say