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?.();

Related

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