Claude-skill-registry fundamentals

Core JavaScript fundamentals including variables, data types, operators, control flow, and basic syntax. Essential foundation for all JavaScript development.

install

source · Clone the upstream repo

git clone https://github.com/majiayu000/claude-skill-registry

Claude Code · Install into ~/.claude/skills/

T=$(mktemp -d) && git clone --depth=1 https://github.com/majiayu000/claude-skill-registry "$T" && mkdir -p ~/.claude/skills && cp -r "$T/skills/data/fundamentals" ~/.claude/skills/majiayu000-claude-skill-registry-fundamentals && rm -rf "$T"

manifest: skills/data/fundamentals/SKILL.md

JavaScript Fundamentals Skill

Quick Reference Card

Variable Declaration

const PI = 3.14159;     // Immutable binding (preferred)
let count = 0;          // Reassignable
var legacy = "avoid";   // Function-scoped (avoid)

8 Data Types

Type	Example	typeof
String	`"hello"`	`"string"`
Number	`42` , `3.14` , `NaN`	`"number"`
Boolean	`true` , `false`	`"boolean"`
Null	`null`	`"object"`
Undefined	`undefined`	`"undefined"`
Symbol	`Symbol('id')`	`"symbol"`
BigInt	`9007199254740991n`	`"bigint"`
Object	`{}` , `[]` , `fn`	`"object"`

Operators Cheat Sheet

// Arithmetic
+ - * / % **

// Comparison (always use strict)
=== !== > < >= <=

// Logical
&& || !

// Nullish
?? ?.

// Assignment
= += -= *= /= ??= ||= &&=

Control Flow Patterns

// Early return (preferred)
function validate(input) {
  if (!input) return { error: 'Required' };
  if (input.length < 3) return { error: 'Too short' };
  return { valid: true };
}

// Switch with exhaustive handling
function getColor(status) {
  switch (status) {
    case 'success': return 'green';
    case 'warning': return 'yellow';
    case 'error': return 'red';
    default: return 'gray';
  }
}

Type Coercion Rules

// Explicit conversion (preferred)
Number('42')     // 42
String(42)       // "42"
Boolean(1)       // true

// Truthy values: non-zero numbers, non-empty strings, objects
// Falsy values: 0, "", null, undefined, NaN, false

Modern Operators

// Nullish coalescing
const name = input ?? 'Guest';     // Only null/undefined

// Optional chaining
const city = user?.address?.city;  // Safe navigation

// Logical assignment
config.debug ??= false;            // Assign if nullish

Troubleshooting

Common Issues

Error	Cause	Fix
`ReferenceError: x is not defined`	Variable not declared	Check spelling, scope
`TypeError: Cannot read property`	Accessing null/undefined	Use optional chaining `?.`
`NaN` result	Invalid number operation	Validate input types
Unexpected `true` / `false`	Loose equality `==`	Use strict `===`

Debug Checklist

// 1. Check type
console.log(typeof variable);

// 2. Check value
console.log(JSON.stringify(variable));

// 3. Check for null/undefined
console.log(variable === null, variable === undefined);

// 4. Use debugger
debugger;

Production Patterns

Input Validation

function processNumber(value) {
  if (typeof value !== 'number' || Number.isNaN(value)) {
    throw new TypeError('Expected a valid number');
  }
  return value * 2;
}

Safe Property Access

const city = user?.address?.city ?? 'Unknown';
const callback = options.onComplete?.();

Agent 01: JavaScript Fundamentals (detailed learning)
Skill: functions: Function patterns
Skill: data-structures: Objects and arrays