ReddRadar
Agent skill
functions
Advanced function patterns including declaration styles, closures, scope chains, hoisting, and this binding. Master function composition and advanced techniques.
Install this agent skill to your Project
npx add-skill https://github.com/pluginagentmarketplace/custom-plugin-javascript/tree/main/skills/functions
SKILL.md
Functions & Scope Skill
Quick Reference Card
Function Styles
// Declaration (hoisted)
function greet(name) { return `Hello, ${name}!`; }
// Expression (not hoisted)
const greet = function(name) { return `Hello, ${name}!`; };
// Arrow (lexical this)
const greet = (name) => `Hello, ${name}!`;
const greet = name => `Hello, ${name}!`; // Single param
const getUser = async (id) => await fetch(`/api/${id}`);
Scope Rules
Global Scope
└── Function Scope
└── Block Scope (let/const)
const global = 'accessible everywhere';
function outer() {
const outerVar = 'accessible in outer + inner';
function inner() {
const innerVar = 'only accessible here';
console.log(global, outerVar, innerVar); // All work
}
}
Closure Pattern
function createCounter() {
let count = 0; // Private state
return {
increment: () => ++count,
decrement: () => --count,
get: () => count
};
}
const counter = createCounter();
counter.increment(); // 1
counter.increment(); // 2
This Binding Rules
| Context | this Value |
|---|---|
| Global | window/global |
| Object method | The object |
| Arrow function | Lexical (outer) |
call/apply/bind |
Explicit value |
Constructor (new) |
New instance |
// Explicit binding
fn.call(thisArg, arg1, arg2);
fn.apply(thisArg, [args]);
const bound = fn.bind(thisArg);
Advanced Patterns
// IIFE (Immediately Invoked)
const module = (function() {
const private = 'hidden';
return { getPrivate: () => private };
})();
// Currying
const multiply = a => b => a * b;
const double = multiply(2);
double(5); // 10
// Memoization
function memoize(fn) {
const cache = new Map();
return (...args) => {
const key = JSON.stringify(args);
if (!cache.has(key)) cache.set(key, fn(...args));
return cache.get(key);
};
}
Troubleshooting
Common Issues
| Problem | Symptom | Fix |
|---|---|---|
Lost this |
undefined or wrong value |
Use arrow fn or .bind() |
| Closure loop bug | All callbacks same value | Use let not var |
| Hoisting confusion | Undefined before declaration | Declare at top |
| TDZ error | ReferenceError | Move let/const before use |
The Classic Loop Bug
// BUG: var is function-scoped
for (var i = 0; i < 3; i++) {
setTimeout(() => console.log(i), 100);
}
// Output: 3, 3, 3
// FIX: Use let (block-scoped)
for (let i = 0; i < 3; i++) {
setTimeout(() => console.log(i), 100);
}
// Output: 0, 1, 2
Debug Checklist
// 1. Check this context
console.log('this is:', this);
// 2. Verify closure captures
function test() {
let x = 1;
return () => { console.log('x:', x); };
}
// 3. Check hoisting
console.log(typeof myFunc); // 'function' or 'undefined'?
Production Patterns
Factory Pattern
function createLogger(prefix) {
return {
log: (msg) => console.log(`[${prefix}] ${msg}`),
error: (msg) => console.error(`[${prefix}] ${msg}`)
};
}
const apiLogger = createLogger('API');
apiLogger.log('Request received');
Debounce/Throttle
function debounce(fn, delay) {
let timeoutId;
return (...args) => {
clearTimeout(timeoutId);
timeoutId = setTimeout(() => fn(...args), delay);
};
}
Related
- Agent 02: Functions & Scope (detailed learning)
- Skill: fundamentals: Variables and basics
- Skill: asynchronous: Async functions
Recommended Agent Skills
Expand your agent's capabilities with these related and highly-rated skills.
pluginagentmarketplace/custom-plugin-javascript
debugging
JavaScript debugging techniques using DevTools, Node.js debugger, and advanced troubleshooting.
pluginagentmarketplace/custom-plugin-javascript
patterns
JavaScript design patterns and architectural best practices.
pluginagentmarketplace/custom-plugin-javascript
asynchronous
Master asynchronous JavaScript patterns including callbacks, promises, async/await, event loop mechanics, and real-world async patterns.
pluginagentmarketplace/custom-plugin-javascript
dom-apis
DOM manipulation and browser APIs including element selection, events, storage, fetch, and modern Web APIs.
pluginagentmarketplace/custom-plugin-javascript
testing
JavaScript testing with Jest, Vitest, and Testing Library for comprehensive test coverage.
pluginagentmarketplace/custom-plugin-javascript
ecosystem
JavaScript ecosystem including npm, build tools (Webpack, Vite), testing (Jest, Vitest), linting, and CI/CD integration.
Didn't find tool you were looking for?