JavaScript Core ⚡ Recently updated

JavaScript Core: Topics Not to Miss

Check off each topic before your interview.

Variables & Scope

var vs let vs const — hoisting, temporal dead zone
Block scope vs function scope
Global scope pollution and IIFE pattern
Shadowing variables in nested scopes

Data Types & Coercion

Primitives: string, number, boolean, null, undefined, symbol, bigint
Reference types: objects, arrays, functions
typeof quirks (typeof null === 'object')
== vs === and when coercion happens
Truthy/falsy values
NaN checks (Number.isNaN vs isNaN)

Functions

Function declarations vs expressions vs arrow functions
Arrow functions: no own this, no arguments, no constructor
Closures — what they capture and why they matter
IIFE use cases
Higher-order functions (functions taking/returning functions)
Default parameters and rest/spread (...args)

`this` Keyword

Implicit binding (method call)
Explicit binding (call, apply, bind)
new binding
Arrow functions inherit this from enclosing scope
Losing this in callbacks (classic interview trap)

Prototypes & OOP

Prototype chain and __proto__ vs prototype
Object.create(), new, constructor functions
ES6 classes — syntactic sugar over prototypes
instanceof how it works
Inheritance patterns

Arrays & Objects

Mutating vs non-mutating methods (map vs push)
map, filter, reduce, find, some, every
Destructuring arrays and objects
Spread and rest operators
Shallow vs deep copy (structuredClone, JSON trick)
Object.keys, values, entries, assign, freeze

Async JavaScript

Callbacks and callback hell
Promises: states, chaining, error handling
async/await syntax and error handling with try/catch
Promise.all, Promise.race, Promise.allSettled
Event loop: call stack, task queue, microtask queue
setTimeout vs setImmediate vs queueMicrotask (conceptual)

ES6+ Features

Template literals
Optional chaining (?.) and nullish coalescing (??)
Modules: import/export vs CommonJS
Set, Map, WeakMap, WeakSet basics
Generators and iterators (awareness level)

Error Handling

try/catch/finally
Throwing custom errors
Unhandled promise rejections

Performance & Patterns

Debounce vs throttle — when to use each
Memoization concept
Event delegation pattern
Currying (basic understanding)

Browser APIs (JS side)

fetch API basics
localStorage vs sessionStorage
JSON.parse / JSON.stringify pitfalls

Self-test: Can you explain closures, implement bind, predict the output of a setTimeout + Promise chain, and write debounce from scratch in 20 minutes? If not, start with polyfills.md and tricky-output-questions.md.