Language

What JavaScript is, where it runs, the ECMAScript versions, strict mode, source structure (scripts vs modules), and the relationship to HTML/CSS in the browser.

Primitive types, the boxed object types, type coercion, the operator zoo, and the subtle equality rules that catch every newcomer.

Conditionals (`if`, `switch`, ternary), loops (`for`, `while`, `for…of`, `for…in`), labels, `break`/`continue`, and modern iteration helpers.

Function declarations vs expressions vs arrows, parameters and defaults, rest/spread, scope rules, hoisting, and closures — including why every list/loop trap eventually leads here.

Object literals, property descriptors, the prototype chain, classes, inheritance, private fields, and the built-in keyed collections (`Map`, `Set`).

The event loop, callbacks, Promises, async/await, error handling across async boundaries, microtasks vs macrotasks, and concurrency patterns (`Promise.all`, `Promise.race`, …).

The Document Object Model — querying, traversing, creating, and mutating elements; event handling and delegation; forms; and the modern alternatives to manual DOM work.

The Browser Object Model (`window`, `location`, `history`, `navigator`, timers, storage, cookies), ES modules, and the modern syntax that defines idiomatic 2026 JavaScript (destructuring, spread, optional chaining, template literals, modules, top-level await).