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

8 Data Types

Operators Cheat Sheet

Control Flow Patterns

Type Coercion Rules

Modern Operators

Troubleshooting

Common Issues

Debug Checklist

Production Patterns

Input Validation

Safe Property Access

Related

fundamentals

JavaScript Fundamentals Skill

Quick Reference Card

Variable Declaration