javascript-mastery

🧠 JavaScript Mastery

33+ essential JavaScript concepts every developer should know, inspired by 33-js-concepts.

When to Use This Skill

Use this skill when:

• Explaining JavaScript concepts
• Debugging tricky JS behavior
• Teaching JavaScript fundamentals
• Reviewing code for JS best practices
• Understanding language quirks

1. Fundamentals

1.1 Primitive Types

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:

• Primitives are immutable
• Passed by value
• typeof null === "object" is a historical bug

1.2 Type Coercion

JavaScript 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

1.3 Equality Operators

// == (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.

2. Scope & Closures

2.1 Scope Types

// 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!
  }
}

2.2 Closures

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:

• Data privacy (module pattern)
• Function factories
• Partial application
• Memoization

2.3 var vs let vs const

// 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

3. Functions & Execution

3.1 Call Stack

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

3.2 Hoisting

// 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!");
};

3.3 this Keyword

// 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"

4. Event Loop & Async

4.1 Event Loop

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:

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

4.2 Callbacks

// 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
      });
    });
  });
});

4.3 Promises

// 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

4.4 async/await

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 };
}

5. Functional Programming

5.1 Higher-Order Functions

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

5.2 Pure Functions

// 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
}

5.3 map, filter, reduce

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"

5.4 Currying & Comp