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
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