JavaScript Core ⚡ Recently updatedUpdated on Jun 30, 2026 — this content may still be evolving.

JavaScript Core: Revision Sheet

One-page rapid review. Read this the night before your interview.

Types & Coercion

var vs let vs const

• var: function-scoped, hoisted as undefined
• let/const: block-scoped, TDZ until declaration
• const: binding constant, object contents can still mutate

Event Loop Order

1. Synchronous code (call stack)
2. Microtasks (Promises, queueMicrotask)
3. Macrotasks (setTimeout, setInterval, I/O)

Closures

Inner function + access to outer variables after outer returns. Use: data privacy, factories, debounce/throttle.

this Binding (priority)

1. new → new object
2. call/apply/bind → explicit
3. Method call → object
4. Standalone → undefined (strict) / global (sloppy)
5. Arrow → lexical (outer scope)

Array Methods

Object Copy

const shallow = { ...obj };
const deep = structuredClone(obj);

Promises

Promise.all([p1, p2])      // all resolve or first reject
Promise.race([p1, p2])     // first settle
Promise.allSettled([...])  // wait for all, never reject

async/await

async function getData() {
  try {
    const res = await fetch(url);
    if (!res.ok) throw new Error(res.status);
    return await res.json();
  } catch (e) {
    // handle
  }
}

Debounce vs Throttle

• Debounce: wait until activity stops (search input)
• Throttle: max once per interval (scroll)

Prototype Chain

obj.__proto__ === Constructor.prototype
instanceof checks prototype chain

ES6 Destructuring

const { name, age = 18 } = user;
const [first, ...rest] = arr;

Optional Chaining & Nullish

user?.address?.city
value ?? 'default'  // only null/undefined, not 0 or ''

Quick Polyfill Reminders

• bind: return function that calls original with fixed this + args
• debounce: clearTimeout + setTimeout
• Promise.all: array in order, fail fast

Interview Phrases That Impress

• “I’d use === to avoid type coercion”
• “That’s a microtask, so it runs before setTimeout”
• “Arrow function inherits this lexically”
• “I’d shallow copy with spread, deep copy with structuredClone”
• “Debounce for search, throttle for scroll”

Red Flags to Avoid

• “Closures are just nested functions”
• Using var in new code
• Ignoring promise error handling
• Arrow functions as object methods when this is needed