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
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