Javascript Mastery

# 🧠 JavaScript Mastery

> 33+ essential JavaScript concepts every developer should know, inspired by [33-js-concepts](https://github.com/leonardomso/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:

```javascript
// 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:

```javascript
// 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

```javascript
// == (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

```javascript
// 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:

```javascript
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

```javascript
// 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

```javascript
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:

```javascript
function infinite() {
  infinite(); // No base case!
}
infinite(); // RangeError: Maximum call stack size exceeded
```

### 3.2 Hoisting

```javascript
// 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()