Agent skill
fundamentals
Core JavaScript fundamentals including variables, data types, operators, control flow, and basic syntax. Essential foundation for all JavaScript development.
Stars
163
Forks
31
Install this agent skill to your Project
npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/development/fundamentals
SKILL.md
JavaScript Fundamentals Skill
Quick Reference Card
Variable Declaration
javascript
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
javascript
// Arithmetic
+ - * / % **
// Comparison (always use strict)
=== !== > < >= <=
// Logical
&& || !
// Nullish
?? ?.
// Assignment
= += -= *= /= ??= ||= &&=
Control Flow Patterns
javascript
// 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
javascript
// 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
javascript
// 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
javascript
// 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
javascript
function processNumber(value) {
if (typeof value !== 'number' || Number.isNaN(value)) {
throw new TypeError('Expected a valid number');
}
return value * 2;
}
Safe Property Access
javascript
const city = user?.address?.city ?? 'Unknown';
const callback = options.onComplete?.();
Related
- Agent 01: JavaScript Fundamentals (detailed learning)
- Skill: functions: Function patterns
- Skill: data-structures: Objects and arrays
Didn't find tool you were looking for?