Cheat Sheet

Basics

Include scripts, comments, and simple examples.

<!-- Inline -->
<script>
  // JS goes here
  console.log('Hello world');
</script>

<!-- External file -->
<script src="app.js"></script>

Variables & Types

Use let/const for block scope; var is function-scoped.

// Declaration examples
var legacy = 'hello';
let counter = 0;
const PI = 3.14159;

// Common types
let name = "Jane";           // string
let isActive = true;         // boolean
let scores = [10, 20, 30];   // array
let user = { id: 1, name: 'J' }; // object

Arrays

Common methods for manipulating arrays.

const fruits = ["Banana","Apple","Pear"];
fruits.push("Orange");          // add
fruits.pop();                   // remove last
fruits.includes("Apple");       // true
const part = fruits.slice(1,3); // shallow copy portion
fruits.splice(2,1, "Kiwi");     // remove & replace

Operators

Arithmetic, comparison, logical.

5 + 3;   // 8
10 % 3;    // 1 remainder
x === y;   // strict equality
x !== y;   // not equal
a && b;    // logical AND
a || b;    // logical OR
!flag;     // logical NOT

Functions

Classic, arrow, default params, IIFE.

// Classic
function greet(name){ return `Hello ${name}`; }

// Arrow
const add = (a,b) => a + b;

// Default param
function welcome(user = 'Guest'){ console.log(user); }

// IIFE (immediately-invoked)
(function(){ console.log('Init'); })();

Loops

for, while, for…of, break/continue.

// for
for(let i=0;i<5;i++){ console.log(i); }

// while
let n=3; while(n--){console.log(n);}

// for...of for arrays
for(const v of [1,2,3]) { console.log(v); }

// break & continue
for(let i=0;i<5;i++){
  if(i===2) continue;
  if(i===4) break;
}

If / Else

Conditional logic with ternary shortcuts.

if(age >= 18){
  console.log('Adult');
} else if(age >= 13){
  console.log('Teen');
} else {
  console.log('Child');
}

// ternary
const status = age >= 18 ? 'Adult' : 'Minor';

Strings

Common methods & escape sequences.

const s = "JavaScript";
s.toLowerCase();
s.toUpperCase();
s.includes("Script");
s.replace("Java","ECMA");
s.slice(0,4);   // "Java"
// escapes: \n, \t, \', \"

Regular Expressions

Patterns, flags, common usage.

// flags: i (ignore), g (global), m (multiline)
const rx = /\\d+/g;
"abc123".match(rx);       // ["123"]
/^[A-Z]/.test("Hello");   // true if starts with capital

Numbers & Math

Parsing, Math helper methods.

Number("42");      // 42
parseInt("10px");    // 10
parseFloat("3.14");  // 3.14

Math.random();       // 0..1
Math.round(2.7);     // 3
Math.max(1,5,3);     // 5
Math.sqrt(16);       // 4

Dates

Create and manipulate dates.

const now = new Date();
const custom = new Date(2025, 11, 25); // year, monthIndex(0-11), day
now.getFullYear();
now.getMonth();    // 0-11
now.getTime();     // ms since 1970

DOM (Document)

Select elements, update content, create nodes.

// Select
const el = document.getElementById('title');
const firstBtn = document.querySelector('.btn');

// Update
el.textContent = 'New Title';
el.innerHTML = '<strong>Bold</strong>';

// Create / append
const div = document.createElement('div');
div.textContent = 'Hi';
document.body.appendChild(div);

Browser & Window

Window methods & properties.

alert('Hi');
console.log(window.location.href);
setTimeout(()=>console.log('later'), 1000);
localStorage.setItem('k','v');

Events

Listening patterns and common event names.

// Click
button.addEventListener('click', (e) => {
  console.log('clicked', e.target);
});

// Keyboard
window.addEventListener('keydown', (e) => {
  if(e.key === 'Enter') console.log('enter pressed');
});

Errors & Debugging

try/catch and useful console tools.

try {
  mightFail();
} catch(err) {
  console.error('Error:', err.message);
} finally {
  console.log('cleanup');
}

// console methods: log, warn, error, table, group

Skills – Learn Online

JavaScript Master Cheat Sheet