Javascript Mastery

🧠 JavaScript Mastery

Overview

This public intake copy packages plugins/antigravity-awesome-skills-claude/skills/javascript-mastery from https://github.com/sickn33/antigravity-awesome-skills into the native Omni Skills editorial shape without hiding its origin.

Use it when the operator needs the upstream workflow, support files, and repository context to stay intact while the public validator and private enhancer continue their normal downstream flow.

This intake keeps the copied upstream files intact and uses the external_source block in metadata.json plus ORIGIN.md as the provenance anchor for review.

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

Imported source sections that did not map cleanly to the public headings are still preserved below or in the support files. Notable imported sections: 1. Fundamentals, 2. Scope & Closures, 3. Functions & Execution, 4. Event Loop & Async, 5. Functional Programming, 6. Objects & Prototypes.

When to Use This Skill

Use this section as the trigger filter. It should make the activation boundary explicit before the operator loads files, runs commands, or opens a pull request.

• Explaining JavaScript concepts
• Debugging tricky JS behavior
• Teaching JavaScript fundamentals
• Reviewing code for JS best practices
• Understanding language quirks
• Use when the request clearly matches the imported source intent: 33+ essential JavaScript concepts every developer should know, inspired by 33-js-concepts.

Operating Table

Workflow

This workflow is intentionally editorial and operational at the same time. It keeps the imported source useful to the operator while still satisfying the public intake standards that feed the downstream enhancer flow.

1. Confirm the user goal, the scope of the imported workflow, and whether this skill is still the right router for the task.
2. Read the overview and provenance files before loading any copied upstream support files.
3. Load only the references, examples, prompts, or scripts that materially change the outcome for the current request.
4. Execute the upstream workflow while keeping provenance and source boundaries explicit in the working notes.
5. Validate the result against the upstream expectations and the evidence you can point to in the copied files.
6. Escalate or hand off to a related skill when the work moves out of this imported workflow's center of gravity.
7. Before merge or closure, record what was used, what changed, and what the reviewer still needs to verify.

Imported Workflow Notes

Imported: 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.

Examples

Example 1: Ask for the upstream workflow directly

Use @javascript-mastery to handle <task>. Start from the copied upstream workflow, load only the files that change the outcome, and keep provenance visible in the answer.

Explanation: This is the safest starting point when the operator needs the imported workflow, but not the entire repository.

Example 2: Ask for a provenance-grounded review

Review @javascript-mastery against metadata.json and ORIGIN.md, then explain which copied upstream files you would load first and why.

Explanation: Use this before review or troubleshooting when you need a precise, auditable explanation of origin and file selection.

Example 3: Narrow the copied support files before execution

Use @javascript-mastery for <task>. Load only the copied references, examples, or scripts that change the outcome, and name the files explicitly before proceeding.

Explanation: This keeps the skill aligned with progressive disclosure instead of loading the whole copied package by default.

Example 4: Build a reviewer packet

Review @javascript-mastery using the copied upstream files plus provenance, then summarize any gaps before merge.

Explanation: This is useful when the PR is waiting for human review and you want a repeatable audit packet.

Best Practices

Treat the generated public skill as a reviewable packaging layer around the upstream repository. The goal is to keep provenance explicit and load only the copied source material that materially improves execution.

• Keep the imported skill grounded in the upstream repository; do not invent steps that the source material cannot support.
• Prefer the smallest useful set of support files so the workflow stays auditable and fast to review.
• Keep provenance, source commit, and imported file paths visible in notes and PR descriptions.
• Point directly at the copied upstream files that justify the workflow instead of relying on generic review boilerplate.
• Treat generated examples as scaffolding; adapt them to the concrete task before execution.
• Route to a stronger native skill when architecture, debugging, design, or security concerns become dominant.

Troubleshooting

Problem: The operator skipped the imported context and answered too generically

Symptoms: The result ignores the upstream workflow in plugins/antigravity-awesome-skills-claude/skills/javascript-mastery, fails to mention provenance, or does not use any copied source files at all. Solution: Re-open metadata.json, ORIGIN.md, and the most relevant copied upstream files. Check the external_source block first, then restate the provenance before continuing.

Problem: The imported workflow feels incomplete during review

Symptoms: Reviewers can see the generated SKILL.md, but they cannot quickly tell which references, examples, or scripts matter for the current task. Solution: Point at the exact copied references, examples, scripts, or assets that justify the path you took. If the gap is still real, record it in the PR instead of hiding it.

Problem: The task drifted into a different specialization

Symptoms: The imported skill starts in the right place, but the work turns into debugging, architecture, design, security, or release orchestration that a native skill handles better. Solution: Use the related skills section to hand off deliberately. Keep the imported provenance visible so the next skill inherits the right context instead of starting blind.

Related Skills

• @00-andruia-consultant - Use when the work is better handled by that native specialization after this imported skill establishes context.
• @00-andruia-consultant-v2 - Use when the work is better handled by that native specialization after this imported skill establishes context.
• @10-andruia-skill-smith - Use when the work is better handled by that native specialization after this imported skill establishes context.
• @10-andruia-skill-smith-v2 - Use when the work is better handled by that native specialization after this imported skill establishes context.

Additional Resources

Use this support matrix and the linked files below as the operator packet for this imported skill. They should reflect real copied source material, not generic scaffolding.

Imported Reference Notes

Imported: Quick Reference Card

Imported: Resources

• 33 JS Concepts
• JavaScript.info
• MDN JavaScript Guide
• You Don't Know JS

Imported: 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

Imported: 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"

Imported: 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 };
}

Imported: 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 & Composition

// 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

Imported: 6. Objects & Prototypes

6.1 Prototypal Inheritance

// 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!");
  }
}

6.2 Object Methods

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

Imported: 7. Modern JavaScript (ES6+)

7.1 Destructuring

// 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" } };

7.2 Spread & Rest

// 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

7.3 Modules

// 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");

7.4 Optional Chaining & Nullish Coalescing

// 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)

Imported: Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.