Comprehensive JavaScript reference covering 33+ essential concepts every developer should know. From fundamentals like primitives and closures to advanced patterns like async/await and functional programming. Use when explaining JS concepts, debugging JavaScript issues, or teaching JavaScript fundamentals.
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33+ essential JavaScript concepts every developer should know, inspired by 33-js-concepts.
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JavaScript has 7 primitive types:
// String
const str = "hello";
// Number (integers and floats)
const num = 42;
const float = 3.14;
// BigInt (for large integers)
const big = 9007199254740991n;
// Boolean
const bool = true;
// Undefined
let undef; // undefined
// Null
const empty = null;
// Symbol (unique identifiers)
const sym = Symbol("description");
Key points:
typeof null === "object" is a historical bugJavaScript implicitly converts types:
// String coercion
"5" + 3; // "53" (number → string)
"5" - 3; // 2 (string → number)
// Boolean coercion
Boolean(""); // false
Boolean("hello"); // true
Boolean(0); // false
Boolean([]); // true (!)
// Equality coercion
"5" == 5; // true (coerces)
"5" === 5; // false (strict)
Falsy values (8 total):
false, 0, -0, 0n, "", null, undefined, NaN
// == (loose equality) - coerces types
null == undefined; // true
"1" == 1; // true
// === (strict equality) - no coercion
null === undefined; // false
"1" === 1; // false
// Object.is() - handles edge cases
Object.is(NaN, NaN); // true (NaN === NaN is false!)
Object.is(-0, 0); // false (0 === -0 is true!)
Rule: Always use === unless you have a specific reason not to.
// Global scope
var globalVar = "global";
function outer() {
// Function scope
var functionVar = "function";
if (true) {
// Block scope (let/const only)
let blockVar = "block";
const alsoBlock = "block";
var notBlock = "function"; // var ignores blocks!
}
}
A closure is a function that remembers its lexical scope:
function createCounter() {
let count = 0; // "closed over" variable
return {
increment() {
return ++count;
},
decrement() {
return --count;
},
getCount() {
return count;
},
};
}
const counter = createCounter();
counter.increment(); // 1
counter.increment(); // 2
counter.getCount(); // 2
Common use cases:
// var - function scoped, hoisted, can redeclare
var x = 1;
var x = 2; // OK
// let - block scoped, hoisted (TDZ), no redeclare
let y = 1;
// let y = 2; // Error!
// const - like let, but can't reassign
const z = 1;
// z = 2; // Error!
// BUT: const objects are mutable
const obj = { a: 1 };
obj.a = 2; // OK
obj.b = 3; // OK
function first() {
console.log("first start");
second();
console.log("first end");
}
function second() {
console.log("second");
}
first();
// Output:
// "first start"
// "second"
// "first end"
Stack overflow example:
function infinite() {
infinite(); // No base case!
}
infinite(); // RangeError: Maximum call stack size exceeded
// Variable hoisting
console.log(a); // undefined (hoisted, not initialized)
var a = 5;
console.log(b); // ReferenceError (TDZ)
let b = 5;
// Function hoisting
sayHi(); // Works!
function sayHi() {
console.log("Hi!");
}
// Function expressions don't hoist
sayBye(); // TypeError
var sayBye = function () {
console.log("Bye!");
};
// Global context
console.log(this); // window (browser) or global (Node)
// Object method
const obj = {
name: "Alice",
greet() {
console.log(this.name); // "Alice"
},
};
// Arrow functions (lexical this)
const obj2 = {
name: "Bob",
greet: () => {
console.log(this.name); // undefined (inherits outer this)
},
};
// Explicit binding
function greet() {
console.log(this.name);
}
greet.call({ name: "Charlie" }); // "Charlie"
greet.apply({ name: "Diana" }); // "Diana"
const bound = greet.bind({ name: "Eve" });
bound(); // "Eve"
console.log("1");
setTimeout(() => console.log("2"), 0);
Promise.resolve().then(() => console.log("3"));
console.log("4");
// Output: 1, 4, 3, 2
// Why? Microtasks (Promises) run before macrotasks (setTimeout)
Execution order:
// Callback pattern
function fetchData(callback) {
setTimeout(() => {
callback(null, { data: "result" });
}, 1000);
}
// Error-first convention
fetchData((error, result) => {
if (error) {
console.error(error);
return;
}
console.log(result);
});
// Callback hell (avoid this!)
getData((data) => {
processData(data, (processed) => {
saveData(processed, (saved) => {
notify(saved, () => {
// 😱 Pyramid of doom
});
});
});
});
// Creating a Promise
const promise = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("Success!");
// or: reject(new Error("Failed!"));
}, 1000);
});
// Consuming Promises
promise
.then((result) => console.log(result))
.catch((error) => console.error(error))
.finally(() => console.log("Done"));
// Promise combinators
Promise.all([p1, p2, p3]); // All must succeed
Promise.allSettled([p1, p2]); // Wait for all, get status
Promise.race([p1, p2]); // First to settle
Promise.any([p1, p2]); // First to succeed
async function fetchUserData(userId) {
try {
const response = await fetch(`/api/users/${userId}`);
if (!response.ok) throw new Error("Failed to fetch");
const user = await response.json();
return user;
} catch (error) {
console.error("Error:", error);
throw error; // Re-throw for caller to handle
}
}
// Parallel execution
async function fetchAll() {
const [users, posts] = await Promise.all([
fetch("/api/users"),
fetch("/api/posts"),
]);
return { users, posts };
}
Functions that take or return functions:
// Takes a function
const numbers = [1, 2, 3];
const doubled = numbers.map((n) => n * 2); // [2, 4, 6]
// Returns a function
function multiply(a) {
return function (b) {
return a * b;
};
}
const double = multiply(2);
double(5); // 10
// Pure: same input → same output, no side effects
function add(a, b) {
return a + b;
}
// Impure: modifies external state
let total = 0;
function addToTotal(value) {
total += value; // Side effect!
return total;
}
// Impure: depends on external state
function getDiscount(price) {
return price * globalDiscountRate; // External dependency
}
const users = [
{ name: "Alice", age: 25 },
{ name: "Bob", age: 30 },
{ name: "Charlie", age: 35 },
];
// map: transform each element
const names = users.map((u) => u.name);
// ["Alice", "Bob", "Charlie"]
// filter: keep elements matching condition
const adults = users.filter((u) => u.age >= 30);
// [{ name: "Bob", ... }, { name: "Charlie", ... }]
// reduce: accumulate into single value
const totalAge = users.reduce((sum, u) => sum + u.age, 0);
// 90
// Chaining
const result = users
.filter((u) => u.age >= 30)
.map((u) => u.name)
.join(", ");
// "Bob, Charlie"
// Currying: transform f(a, b, c) into f(a)(b)(c)
const curry = (fn) => {
return function curried(...args) {
if (args.length >= fn.length) {
return fn.apply(this, args);
}
return (...moreArgs) => curried(...args, ...moreArgs);
};
};
const add = curry((a, b, c) => a + b + c);
add(1)(2)(3); // 6
add(1, 2)(3); // 6
add(1)(2, 3); // 6
// Composition: combine functions
const compose =
(...fns) =>
(x) =>
fns.reduceRight((acc, fn) => fn(acc), x);
const pipe =
(...fns) =>
(x) =>
fns.reduce((acc, fn) => fn(acc), x);
const addOne = (x) => x + 1;
const double = (x) => x * 2;
const addThenDouble = compose(double, addOne);
addThenDouble(5); // 12 = (5 + 1) * 2
const doubleThenAdd = pipe(double, addOne);
doubleThenAdd(5); // 11 = (5 * 2) + 1
// Prototype chain
const animal = {
speak() {
console.log("Some sound");
},
};
const dog = Object.create(animal);
dog.bark = function () {
console.log("Woof!");
};
dog.speak(); // "Some sound" (inherited)
dog.bark(); // "Woof!" (own method)
// ES6 Classes (syntactic sugar)
class Animal {
speak() {
console.log("Some sound");
}
}
class Dog extends Animal {
bark() {
console.log("Woof!");
}
}
const obj = { a: 1, b: 2 };
// Keys, values, entries
Object.keys(obj); // ["a", "b"]
Object.values(obj); // [1, 2]
Object.entries(obj); // [["a", 1], ["b", 2]]
// Shallow copy
const copy = { ...obj };
const copy2 = Object.assign({}, obj);
// Freeze (immutable)
const frozen = Object.freeze({ x: 1 });
frozen.x = 2; // Silently fails (or throws in strict mode)
// Seal (no add/delete, can modify)
const sealed = Object.seal({ x: 1 });
sealed.x = 2; // OK
sealed.y = 3; // Fails
delete sealed.x; // Fails
// Array destructuring
const [first, second, ...rest] = [1, 2, 3, 4, 5];
// first = 1, second = 2, rest = [3, 4, 5]
// Object destructuring
const { name, age, city = "Unknown" } = { name: "Alice", age: 25 };
// name = "Alice", age = 25, city = "Unknown"
// Renaming
const { name: userName } = { name: "Bob" };
// userName = "Bob"
// Nested
const {
address: { street },
} = { address: { street: "123 Main" } };
// Spread: expand iterable
const arr1 = [1, 2, 3];
const arr2 = [...arr1, 4, 5]; // [1, 2, 3, 4, 5]
const obj1 = { a: 1 };
const obj2 = { ...obj1, b: 2 }; // { a: 1, b: 2 }
// Rest: collect remaining
function sum(...numbers) {
return numbers.reduce((a, b) => a + b, 0);
}
sum(1, 2, 3, 4); // 10
// Named exports
export const PI = 3.14159;
export function square(x) {
return x * x;
}
// Default export
export default class Calculator {}
// Importing
import Calculator, { PI, square } from "./math.js";
import * as math from "./math.js";
// Dynamic import
const module = await import("./dynamic.js");
// Optional chaining (?.)
const user = { address: { city: "NYC" } };
const city = user?.address?.city; // "NYC"
const zip = user?.address?.zip; // undefined (no error)
const fn = user?.getName?.(); // undefined if no method
// Nullish coalescing (??)
const value = null ?? "default"; // "default"
const zero = 0 ?? "default"; // 0 (not nullish!)
const empty = "" ?? "default"; // "" (not nullish!)
// Compare with ||
const value2 = 0 || "default"; // "default" (0 is falsy)
| Concept | Key Point |
|---|---|
== vs === | Always use === |
var vs let | Prefer let/const |
| Closures | Function + lexical scope |
this | Depends on how function is called |
| Event loop | Microtasks before macrotasks |
| Pure functions | Same input → same output |
| Prototypes | __proto__ → prototype chain |
?? vs || | ?? only checks null/undefined |
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