new skill web-coder (#881)

* new skill web-coder * codespellrc: add aNULL HTTPS config cipher string * Apply suggestions from code review Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com> * Apply suggestions from code review * Apply suggestion from @jhauga --------- Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
2026-03-12 20:25:11 +00:00 · 2026-03-05 05:43:04 -05:00
parent abcca71546
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+# JavaScript & Programming Reference
+
+Comprehensive reference for JavaScript, ECMAScript, programming concepts, and modern JS patterns.
+
+## Core Concepts
+
+### JavaScript
+High-level, interpreted programming language that conforms to the ECMAScript specification. Primary language for web development alongside HTML and CSS.
+
+**Key Characteristics**:
+- Dynamically typed
+- Prototype-based inheritance
+- First-class functions
+- Event-driven
+- Asynchronous execution
+
+### ECMAScript
+The standardized specification that JavaScript implements.
+
+**Major Versions**:
+- **ES5** (2009): Strict mode, JSON support
+- **ES6/ES2015**: Classes, arrow functions, promises, modules
+- **ES2016+**: Async/await, optional chaining, nullish coalescing
+
+## Data Types
+
+### Primitive Types
+
+```javascript
+// String
+let name = "John";
+let greeting = 'Hello';
+let template = `Hello, ${name}!`; // Template literal
+
+// Number
+let integer = 42;
+let float = 3.14;
+let negative = -10;
+let scientific = 1e6; // 1000000
+
+// BigInt (for very large integers)
+let big = 9007199254740991n;
+
+// Boolean
+let isTrue = true;
+let isFalse = false;
+
+// Undefined (declared but not assigned)
+let undefined_var;
+console.log(undefined_var); // undefined
+
+// Null (intentional absence of value)
+let empty = null;
+
+// Symbol (unique identifier)
+let sym = Symbol('description');
+```
+
+### Type Checking
+
+```javascript
+typeof "hello"; // "string"
+typeof 42; // "number"
+typeof true; // "boolean"
+typeof undefined; // "undefined"
+typeof null; // "object" (historical bug)
+typeof Symbol(); // "symbol"
+typeof {}; // "object"
+typeof []; // "object"
+typeof function() {}; // "function"
+
+// Better array check
+Array.isArray([]); // true
+
+// Null check
+value === null; // true if null
+```
+
+### Type Coercion and Conversion
+
+```javascript
+// Implicit coercion
+"5" + 2; // "52" (string concatenation)
+"5" - 2; // 3 (numeric subtraction)
+"5" * "2"; // 10 (numeric multiplication)
+!!"value"; // true (boolean conversion)
+
+// Explicit conversion
+String(123); // "123"
+Number("123"); // 123
+Number("abc"); // NaN
+Boolean(0); // false
+Boolean(1); // true
+parseInt("123px"); // 123
+parseFloat("3.14"); // 3.14
+```
+
+### Truthy and Falsy Values
+
+**Falsy values** (evaluate to false):
+- `false`
+- `0`, `-0`
+- `""` (empty string)
+- `null`
+- `undefined`
+- `NaN`
+
+**Everything else is truthy**, including:
+- `"0"` (string)
+- `"false"` (string)
+- `[]` (empty array)
+- `{}` (empty object)
+- `function() {}` (empty function)
+
+## Variables and Constants
+
+```javascript
+// var (function-scoped, hoisted - avoid in modern code)
+var oldStyle = "avoid this";
+
+// let (block-scoped, can be reassigned)
+let count = 0;
+count = 1; // ✓ works
+
+// const (block-scoped, cannot be reassigned)
+const MAX = 100;
+MAX = 200; // ✗ TypeError
+
+// const with objects/arrays (content can change)
+const person = { name: "John" };
+person.name = "Jane"; // ✓ works (mutating object)
+person = {}; // ✗ TypeError (reassigning variable)
+```
+
+## Functions
+
+### Function Declaration
+
+```javascript
+function greet(name) {
+  return `Hello, ${name}!`;
+}
+```
+
+### Function Expression
+
+```javascript
+const greet = function(name) {
+  return `Hello, ${name}!`;
+};
+```
+
+### Arrow Functions
+
+```javascript
+// Basic syntax
+const add = (a, b) => a + b;
+
+// With block body
+const multiply = (a, b) => {
+  const result = a * b;
+  return result;
+};
+
+// Single parameter (parentheses optional)
+const square = x => x * x;
+
+// No parameters
+const getRandom = () => Math.random();
+
+// Implicit return of object (wrap in parentheses)
+const makePerson = (name, age) => ({ name, age });
+```
+
+### First-Class Functions
+
+Functions are values that can be:
+- Assigned to variables
+- Passed as arguments
+- Returned from other functions
+
+```javascript
+// Assign to variable
+const fn = function() { return 42; };
+
+// Pass as argument
+function execute(callback) {
+  return callback();
+}
+execute(() => console.log("Hello"));
+
+// Return from function
+function createMultiplier(factor) {
+  return function(x) {
+    return x * factor;
+  };
+}
+const double = createMultiplier(2);
+double(5); // 10
+```
+
+### Closures
+
+Functions that remember their lexical scope:
+
+```javascript
+function createCounter() {
+  let count = 0; // Private variable
+  
+  return {
+    increment() {
+      count++;
+      return count;
+    },
+    decrement() {
+      count--;
+      return count;
+    },
+    getCount() {
+      return count;
+    }
+  };
+}
+
+const counter = createCounter();
+counter.increment(); // 1
+counter.increment(); // 2
+counter.decrement(); // 1
+counter.getCount(); // 1
+```
+
+### Callback Functions
+
+Function passed as an argument to be executed later:
+
+```javascript
+// Array methods use callbacks
+const numbers = [1, 2, 3, 4, 5];
+
+numbers.forEach(num => console.log(num));
+
+const doubled = numbers.map(num => num * 2);
+
+const evens = numbers.filter(num => num % 2 === 0);
+
+const sum = numbers.reduce((acc, num) => acc + num, 0);
+```
+
+### IIFE (Immediately Invoked Function Expression)
+
+```javascript
+(function() {
+  // Code here runs immediately
+  console.log("IIFE executed");
+})();
+
+// With parameters
+(function(name) {
+  console.log(`Hello, ${name}`);
+})("World");
+
+// Arrow function IIFE
+(() => {
+  console.log("Arrow IIFE");
+})();
+```
+
+## Objects
+
+### Object Creation
+
+```javascript
+// Object literal
+const person = {
+  name: "John",
+  age: 30,
+  greet() {
+    return `Hello, I'm ${this.name}`;
+  }
+};
+
+// Constructor function
+function Person(name, age) {
+  this.name = name;
+  this.age = age;
+}
+
+const john = new Person("John", 30);
+
+// Object.create
+const proto = { greet() { return "Hello"; } };
+const obj = Object.create(proto);
+```
+
+### Accessing Properties
+
+```javascript
+const obj = { name: "John", age: 30 };
+
+// Dot notation
+obj.name; // "John"
+
+// Bracket notation
+obj["age"]; // 30
+const key = "name";
+obj[key]; // "John"
+
+// Optional chaining (ES2020)
+obj.address?.city; // undefined (no error if address doesn't exist)
+obj.getName?.(); // undefined (no error if getName doesn't exist)
+```
+
+### Object Methods
+
+```javascript
+const person = { name: "John", age: 30, city: "NYC" };
+
+// Get keys
+Object.keys(person); // ["name", "age", "city"]
+
+// Get values
+Object.values(person); // ["John", 30, "NYC"]
+
+// Get entries
+Object.entries(person); // [["name", "John"], ["age", 30], ["city", "NYC"]]
+
+// Assign (merge objects)
+const extended = Object.assign({}, person, { country: "USA" });
+
+// Spread operator (modern alternative)
+const merged = { ...person, country: "USA" };
+
+// Freeze (make immutable)
+Object.freeze(person);
+person.age = 31; // Silently fails (throws in strict mode)
+
+// Seal (prevent adding/removing properties)
+Object.seal(person);
+```
+
+### Destructuring
+
+```javascript
+// Object destructuring
+const person = { name: "John", age: 30, city: "NYC" };
+const { name, age } = person;
+
+// With different variable names
+const { name: personName, age: personAge } = person;
+
+// With defaults
+const { name, country = "USA" } = person;
+
+// Nested destructuring
+const user = { profile: { email: "john@example.com" } };
+const { profile: { email } } = user;
+
+// Array destructuring
+const numbers = [1, 2, 3, 4, 5];
+const [first, second, ...rest] = numbers;
+// first = 1, second = 2, rest = [3, 4, 5]
+
+// Skip elements
+const [a, , c] = numbers;
+// a = 1, c = 3
+```
+
+## Arrays
+
+```javascript
+// Create arrays
+const arr = [1, 2, 3];
+const empty = [];
+const mixed = [1, "two", { three: 3 }, [4]];
+
+// Access elements
+arr[0]; // 1
+arr[arr.length - 1]; // Last element
+arr.at(-1); // 3 (ES2022 - negative indexing)
+
+// Modify arrays
+arr.push(4); // Add to end
+arr.pop(); // Remove from end
+arr.unshift(0); // Add to beginning
+arr.shift(); // Remove from beginning
+arr.splice(1, 2, 'a', 'b'); // Remove 2 elements at index 1, insert 'a', 'b'
+
+// Iteration
+arr.forEach(item => console.log(item));
+for (let item of arr) { console.log(item); }
+for (let i = 0; i < arr.length; i++) { console.log(arr[i]); }
+
+// Transformation
+const doubled = arr.map(x => x * 2);
+const evens = arr.filter(x => x % 2 === 0);
+const sum = arr.reduce((acc, x) => acc + x, 0);
+
+// Search
+arr.includes(2); // true
+arr.indexOf(2); // Index or -1
+arr.find(x => x > 2); // First matching element
+arr.findIndex(x => x > 2); // Index of first match
+
+// Test
+arr.some(x => x > 5); // true if any match
+arr.every(x => x > 0); // true if all match
+
+// Sort and reverse
+arr.sort((a, b) => a - b); // Ascending
+arr.reverse(); // Reverse in place
+
+// Combine
+const combined = arr.concat([4, 5]);
+const spread = [...arr, 4, 5];
+
+// Slice (copy portion)
+const portion = arr.slice(1, 3); // Index 1 to 3 (exclusive)
+
+// Flat (flatten nested arrays)
+[[1, 2], [3, 4]].flat(); // [1, 2, 3, 4]
+```
+
+## Control Flow
+
+### Conditionals
+
+```javascript
+// if/else
+if (condition) {
+  // code
+} else if (otherCondition) {
+  // code
+} else {
+  // code
+}
+
+// Ternary operator
+const result = condition ? valueIfTrue : valueIfFalse;
+
+// Switch statement
+switch (value) {
+  case 1:
+    // code
+    break;
+  case 2:
+  case 3:
+    // code for 2 or 3
+    break;
+  default:
+    // default code
+}
+
+// Nullish coalescing (ES2020)
+const value = null ?? "default"; // "default"
+const value = 0 ?? "default"; // 0 (0 is not nullish)
+
+// Logical OR for defaults (pre-ES2020)
+const value = falsy || "default";
+
+// Optional chaining
+const city = user?.address?.city;
+```
+
+### Loops
+
+```javascript
+// for loop
+for (let i = 0; i < 10; i++) {
+  console.log(i);
+}
+
+// while loop
+let i = 0;
+while (i < 10) {
+  console.log(i);
+  i++;
+}
+
+// do-while loop
+do {
+  console.log(i);
+  i++;
+} while (i < 10);
+
+// for...of (iterate values)
+for (const item of array) {
+  console.log(item);
+}
+
+// for...in (iterate keys - avoid for arrays)
+for (const key in object) {
+  console.log(key, object[key]);
+}
+
+// break and continue
+for (let i = 0; i < 10; i++) {
+  if (i === 5) break; // Exit loop
+  if (i === 3) continue; // Skip iteration
+  console.log(i);
+}
+```
+
+## Asynchronous JavaScript
+
+### Callbacks
+
+```javascript
+function fetchData(callback) {
+  setTimeout(() => {
+    callback("Data received");
+  }, 1000);
+}
+
+fetchData(data => console.log(data));
+```
+
+### Promises
+
+```javascript
+// Create promise
+const promise = new Promise((resolve, reject) => {
+  setTimeout(() => {
+    const success = true;
+    if (success) {
+      resolve("Success!");
+    } else {
+      reject("Error!");
+    }
+  }, 1000);
+});
+
+// Use promise
+promise
+  .then(result => console.log(result))
+  .catch(error => console.error(error))
+  .finally(() => console.log("Done"));
+
+// Promise utilities
+Promise.all([promise1, promise2]); // Wait for all
+Promise.race([promise1, promise2]); // First to complete
+Promise.allSettled([promise1, promise2]); // Wait for all (ES2020)
+Promise.any([promise1, promise2]); // First to succeed (ES2021)
+```
+
+### Async/Await
+
+```javascript
+// Async function
+async function fetchData() {
+  try {
+    const response = await fetch('https://api.example.com/data');
+    const data = await response.json();
+    return data;
+  } catch (error) {
+    console.error('Error:', error);
+  }
+}
+
+// Use async function
+fetchData().then(data => console.log(data));
+
+// Top-level await (ES2022, in modules)
+const data = await fetchData();
+```
+
+## Classes
+
+```javascript
+class Person {
+  // Constructor
+  constructor(name, age) {
+    this.name = name;
+    this.age = age;
+  }
+  
+  // Instance method
+  greet() {
+    return `Hello, I'm ${this.name}`;
+  }
+  
+  // Getter
+  get info() {
+    return `${this.name}, ${this.age}`;
+  }
+  
+  // Setter
+  set birthYear(year) {
+    this.age = new Date().getFullYear() - year;
+  }
+  
+  // Static method
+  static species() {
+    return "Homo sapiens";
+  }
+}
+
+// Inheritance
+class Employee extends Person {
+  constructor(name, age, jobTitle) {
+    super(name, age); // Call parent constructor
+    this.jobTitle = jobTitle;
+  }
+  
+  // Override method
+  greet() {
+    return `${super.greet()}, I'm a ${this.jobTitle}`;
+  }
+}
+
+// Usage
+const john = new Person("John", 30);
+john.greet(); // "Hello, I'm John"
+Person.species(); // "Homo sapiens"
+
+const jane = new Employee("Jane", 25, "Developer");
+jane.greet(); // "Hello, I'm Jane, I'm a Developer"
+```
+
+## Modules
+
+### ES6 Modules (ESM)
+
+```javascript
+// Export (math.js)
+export const PI = 3.14159;
+export function add(a, b) {
+  return a + b;
+}
+export default class Calculator {
+  // ...
+}
+
+// Import
+import Calculator, { PI, add } from './math.js';
+import * as math from './math.js';
+import { add as sum } from './math.js'; // Rename
+```
+
+### CommonJS (Node.js)
+
+```javascript
+// Export (math.js)
+module.exports = {
+  add(a, b) {
+    return a + b;
+  }
+};
+
+// Import
+const math = require('./math');
+```
+
+## Error Handling
+
+```javascript
+// Try/catch
+try {
+  // Code that might throw
+  throw new Error("Something went wrong");
+} catch (error) {
+  console.error(error.message);
+} finally {
+  // Always runs
+  console.log("Cleanup");
+}
+
+// Custom errors
+class ValidationError extends Error {
+  constructor(message) {
+    super(message);
+    this.name = "ValidationError";
+  }
+}
+
+throw new ValidationError("Invalid input");
+```
+
+## Best Practices
+
+### Do's
+- ✅ Use `const` by default, `let` when needed
+- ✅ Use strict mode (`'use strict';`)
+- ✅ Use arrow functions for callbacks
+- ✅ Use template literals for string interpolation
+- ✅ Use destructuring for cleaner code
+- ✅ Use async/await for asynchronous code
+- ✅ Handle errors properly
+- ✅ Use descriptive variable names
+- ✅ Keep functions small and focused
+- ✅ Use modern ES6+ features
+
+### Don'ts
+- ❌ Use `var` (use `let` or `const`)
+- ❌ Pollute global scope
+- ❌ Use `==` (use `===` for strict equality)
+- ❌ Modify function parameters
+- ❌ Use `eval()` or `with()`
+- ❌ Ignore errors silently
+- ❌ Use synchronous code for I/O operations
+- ❌ Create deeply nested callbacks (callback hell)
+
+## Glossary Terms
+
+**Key Terms Covered**:
+- Algorithm
+- Argument
+- Array
+- Asynchronous
+- Binding
+- BigInt
+- Bitwise flags
+- Block (scripting)
+- Boolean
+- Callback function
+- Camel case
+- Class
+- Closure
+- Code point
+- Code unit
+- Compile
+- Compile time
+- Conditional
+- Constant
+- Constructor
+- Control flow
+- Deep copy
+- Deserialization
+- ECMAScript
+- Encapsulation
+- Exception
+- Expando
+- First-class function
+- Function
+- Hoisting
+- IIFE
+- Identifier
+- Immutable
+- Inheritance
+- Instance
+- JavaScript
+- JSON
+- JSON type representation
+- Just-In-Time Compilation (JIT)
+- Kebab case
+- Keyword
+- Literal
+- Local scope
+- Local variable
+- Loop
+- Method
+- Mixin
+- Modularity
+- Mutable
+- Namespace
+- NaN
+- Native
+- Null
+- Nullish value
+- Number
+- Object
+- Object reference
+- OOP
+- Operand
+- Operator
+- Parameter
+- Parse
+- Polymorphism
+- Primitive
+- Promise
+- Property (JavaScript)
+- Prototype
+- Prototype-based programming
+- Pseudocode
+- Recursion
+- Regular expression
+- Scope
+- Serialization
+- Serializable object
+- Shallow copy
+- Signature (functions)
+- Sloppy mode
+- Snake case
+- Static method
+- Static typing
+- Statement
+- Strict mode
+- String
+- Stringifier
+- Symbol
+- Synchronous
+- Syntax
+- Syntax error
+- Type
+- Type coercion
+- Type conversion
+- Truthy
+- Falsy
+- Undefined
+- Value
+- Variable
+
+## Additional Resources
+
+- [MDN JavaScript Reference](https://developer.mozilla.org/en-US/docs/Web/JavaScript)
+- [ECMAScript Specification](https://tc39.es/ecma262/)
+- [JavaScript.info](https://javascript.info/)
+- [You Don't Know JS (book series)](https://github.com/getify/You-Dont-Know-JS)