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

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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)