Javascript Reference

console.log() → Prints output to the browser console (for debugging).

alert() → Shows a popup alert message.

prompt() → Shows a popup asking the user for input.

confirm() → Shows a popup with OK/Cancel and returns true/false.

var x = 10; → Old way to declare a variable (function-scoped).

let x = 10; → Declares a variable (block-scoped). Best for values that change.

const x = 10; → Declares a constant (block-scoped). Best for values that should not be reassigned.

"text" → String: text in quotes.

123 → Number: integer or decimal.

true / false → Boolean: true or false values.

null → Intentionally empty value.

undefined → Variable declared but not assigned a value.

[] → Array: list of values.

{} → Object: key-value pairs.

+ - * / % → Math operators (add, subtract, multiply, divide, remainder).

= → Assignment operator.

== → Loose equality (compares values, can convert types).

=== → Strict equality (compares value AND type). Recommended.

!= / !== → Not equal (loose / strict).

> < >= <= → Comparison operators.

&& → AND operator.

|| → OR operator.

! → NOT operator.

if (condition) {} → Runs code if condition is true.

else {} → Runs code if condition is false.

else if (condition) {} → Checks another condition.

switch (value) {} → Runs different code based on matching cases.

for (let i=0; i<10; i++) {} → Repeats code a set number of times.

while (condition) {} → Repeats code while condition is true.

do {} while (condition); → Runs once then repeats while condition is true.

for...of → Loops through values of an array.

for...in → Loops through keys in an object.

function name() {} → Creates a function.

return → Sends a value back from a function.

() → Used to call a function.

(param) → Parameter: input value a function receives.

const fn = () => {} → Arrow function (shorter function syntax).

let arr = [1,2,3]; → Creates an array.

arr.push(x) → Adds to the end of an array.

arr.pop() → Removes last item.

arr.shift() → Removes first item.

arr.unshift(x) → Adds to the start.

arr.length → Number of items in array.

arr.map(fn) → Creates a new array by transforming values.

arr.filter(fn) → Creates a new array with matching values.

arr.find(fn) → Finds the first matching value.

arr.forEach(fn) → Runs a function for each item.

let obj = {name:"Arseine"}; → Creates an object.

obj.key → Access object value with dot notation.

obj["key"] → Access object value with bracket notation.

delete obj.key → Removes a property from an object.

document → The whole HTML document object.

document.getElementById("id") → Selects an element by ID.

document.querySelector("css") → Selects the first matching element.

document.querySelectorAll("css") → Selects all matching elements.

element.innerText → Sets/gets visible text.

element.innerHTML → Sets/gets HTML inside an element.

element.style.color = "red" → Changes inline style of an element.

element.classList.add("class") → Adds a class.

element.classList.remove("class") → Removes a class.

element.classList.toggle("class") → Toggles a class on/off.

element.addEventListener("click", fn) → Runs code when an event happens.

click → Fires when user clicks.

input → Fires when input value changes.

submit → Fires when a form is submitted.

keydown → Fires when a key is pressed.

setTimeout(fn, 1000) → Runs code once after a delay (milliseconds).

setInterval(fn, 1000) → Runs code repeatedly every delay.

clearTimeout(id) → Stops a timeout.

clearInterval(id) → Stops an interval.

JSON.stringify(obj) → Converts object to JSON string.

JSON.parse(json) → Converts JSON string to object.

localStorage.setItem("key","value") → Saves data in browser storage.

localStorage.getItem("key") → Reads data from browser storage.

localStorage.removeItem("key") → Deletes stored data.

fetch("url") → Requests data from a server (returns a Promise).

then() → Runs after a Promise succeeds.

catch() → Runs if a Promise fails.

async function name() {} → Declares an async function.

await fetch("url") → Waits for the Promise result inside async function.